Internet DRAFT - draft-ietf-ospf-nssa-update
draft-ietf-ospf-nssa-update
Network Working Group R. Coltun
Internet Draft FORE Systems
Expiration Date: June 2001 V. Fuller
File name: draft-ietf-ospf-nssa-update-10.txt BBN Planet
P. Murphy
US Geological Survey
December 2000
The OSPF NSSA Option
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Coltun, Fuller, Murphy [Page i]
Internet Draft OSPF NSSA Option December 2000
Table Of Contents
1.0 Abstract ................................................. 1
2.0 Overview ................................................. 2
2.1 Motivation - transit networks ............................ 2
2.2 Motivation - corporate networks .......................... 3
2.3 Proposed Solution ........................................ 4
3.0 NSSA Intra-area Implementation Details ................... 6
3.1 The N-bit ................................................ 6
3.2 Type-7 Address Ranges .................................... 7
3.3 Type-7 LSAs .............................................. 7
3.4 Originating Type-7 LSAs .................................. 9
3.5 Calculating Type-7 AS External Routes .................... 10
3.6 Incremental Updates ...................................... 13
3.7 Optionally Importing Summary LSAs ........................ 13
4.0 Intra-AS implementation Details .......................... 14
4.1 Type-7 Translator Election ............................... 14
4.2 Translating Type-7 LSAs into Type-5 LSAs ................. 15
4.3 Flushing Translated Type-7 LSAs .......................... 18
5.0 Security Considerations .................................. 18
6.0 Acknowledgments .......................................... 20
7.0 References ............................................... 20
8.0 Authors' Addresses ....................................... 21
Appendix A: The Options Field ................................ 22
Appendix B: Router-LSAs ...................................... 23
Appendix C: Type-7 LSA Packet Format ......................... 25
Appendix D: Configuration Parameters ......................... 26
Appendix E: The P-bit Policy Paradox ......................... 27
Appendix F: Differences from RFC 1587 ........................ 29
1.0 Abstract
This memo documents an optional type of OSPF area which is somewhat
humorously referred to as a "not-so-stubby" area (or NSSA). NSSAs
are similar to the existing OSPF stub area configuration option but
have the additional capability of importing AS external routes in a
limited fashion.
The OSPF NSSA Option was originally defined in RFC 1587. The
functional differences between this memo and RFC 1587 are explained
in Appendix F. All differences, while expanding capability, are
backward-compatible in nature. Implementations of this memo and of
RFC 1587 will interoperate.
Please send comments to ospf@discuss.microsoft.com.
Coltun, Fuller, Murphy [Page 1]
Internet Draft OSPF NSSA Option December 2000
2.0 Overview
2.1 Motivation - transit networks
Wide-area transit networks often have connections to moderately-
complex "leaf" sites. A leaf site may have multiple IP network
numbers assigned to it. Typically, one of the leaf site's networks
is directly connected to a router provided and administered by the
transit network while the others are distributed throughout and
administered by the site. From the transit network's perspective,
all of the network numbers associated with the site make up a single
"stub" entity. For example, BBN Planet has one site composed of a
class-B network, 130.57.0.0, and a class-C network, 192.31.114.0.
From BBN Planet's perspective, this configuration looks something
like this:
192.31.114
|
(cloud)
-------------- 130.57.4
|
|
------ 131.119.13 ------
|BR18|------------|BR10|
------ ------
|
V
to BBN Planet "core" OSPF system
where the "cloud" consists of the subnets of 130.57 and network
192.31.114, all of which are learned by RIP on router BR18.
Topologically, this cloud looks very much like an OSPF stub area.
The advantages of running the cloud as an OSPF stub area are:
1. Type-5 routes (OSPF external link state advertisements (LSAs))
are not advertised beyond the router labeled "BR10". This is
advantageous because the link between BR10 and BR18 may be a
low-speed link or the router BR18 may have limited resources.
2. The transit network is abstracted to the "leaf" router BR18 by
advertising only a default route across the link between BR10
and BR18.
3. The cloud becomes a single, manageable "leaf" with respect to
the transit network.
4. The cloud can become, logically, a part of the transit
network's OSPF routing system.
Coltun, Fuller, Murphy [Page 2]
Internet Draft OSPF NSSA Option December 2000
However, the current definition of the OSPF protocol [OSPF] imposes
topological limitations which restrict simple cloud topologies from
becoming OSPF stub areas. In particular, it is illegal for a stub
area to import routes external to OSPF; it is not possible for
routers BR18 and BR10 to both be members of the stub area and to
import the routes learned from RIP or other IP routing protocols as
Type-5 (OSPF external LSAs) into the OSPF routing domain. In order
to run OSPF out to BR18, BR18 must be a member of a non-stub area or
the OSPF backbone before it can import routes other than its
directly-connected network(s). Since it is not acceptable for BR18
to maintain all of BBN Planet's external (Type-5) routes, BBN Planet
is forced by OSPF's topological limitations to only run OSPF out to
BR10 and to run RIP between BR18 and BR10.
2.2 Motivation - corporate networks
In a corporate network which supports a large corporate
infrastructure it is not uncommon for OSPF area 0 to have a large
non-zero area infrastructure which injects large routing tables into
area 0. Organizations within the corporate infrastructure may
routinely multi-home their non-0 OSPF areas to strategically located
backbone area 0 routers, either to provide backbone redundancy or to
increase backbone connectivity or both. Because of these large
routing tables, OSPF aggregation via summarization is routinely used
and recommended. Stub areas are also recommended to keep the size of
the routing tables of non-backbone routers small. Organizations
within the corporation are administratively autonomous and compete
for corporate backbone resources. They also want isolation from each
other in order to protect their own network resources within the
organization.
Consider a typical backbone connection, as shown on the next page,
where routers A1, B1 and A2, B2 are connected to area 1 and area 2
respectively, and where routers A0 and B0 are border routers which
connect to both area 1 and area 2. Assume the 192.168.192/20 and
192.168.208/22 clouds are subnetted with a protocol different from
the corporate OSPF instance. These other protocols could be RIP,
IGRP, or second and third OSPF instances separate from the corporate
OSPF backbone instance.
Area 1 and Area 2 would like to be stub areas to minimize the size of
their link state data base. It is also desirable to aggregate the
subnets of 192.168.192/20 and 192.168.208/22 each in a single
advertisement to the backbone in such a way that the preferred path
to 192.168.192/20 is through A0 and the preferred path to
192.168.208/22 is through B0.
Coltun, Fuller, Murphy [Page 3]
Internet Draft OSPF NSSA Option December 2000
+---A0------Area 0 cloud------B0---+
| | | |
| | | |
| |T1 56kbs| |
56kbs| | | |T1
| | | |
| | Area 1 cloud | |
| A1-----192.168.192/20-----B1 |
| |
+---A2 B2---+
| |
| Area 2 cloud |
+-----192.168.208/22------+
The current standard OSPF stub area has no mechanism to support the
redistribution of routes for the subnets of 192.168.192/20 and
192.168.208/22 within a stub area or the ability to aggregate a range
of external routes in any OSPF area. Any solution to this dilemma
must also honor Area 1's path of choice to 192.168.192/20 through A0
with redundancy through B0 while at the same time honoring Area 2's
path of choice to 192.168.208/20 through B0 with redundancy through
A0.
2.3 Proposed Solution
This document describes a new optional type of OSPF area, somewhat
humorously referred to as a "not-so-stubby" area (or NSSA), which has
the capability of importing external routes in a limited fashion.
The OSPF specification defines two general classes of area
configuration. The first allows Type-5 LSAs to be flooded throughout
the area. In this configuration, Type-5 LSAs may be originated by
routers internal to the area or flooded into the area by area border
routers. These areas, referred to herein as Type-5 capable areas (or
just plain areas in the OSPF specification), are distinguished by the
fact that they can carry transit traffic. The backbone is always a
Type-5 capable area. The second type of area configuration, called
stub, allows no Type-5 LSAs to be propagated into/throughout the area
and instead depends on default routing to external destinations.
NSSAs are defined in much the same manner as existing stub areas. To
support NSSAs, a new option bit (the "N" bit) and a new type of LSA
(Type-7) are defined. The "N" bit ensures that routers belonging to
an NSSA agree on its configuration. Similar to the stub area's use
of the "E" bit, both NSSA neighbors must agree on the setting of the
"N" bit or the OSPF neighbor adjacency will not form.
Coltun, Fuller, Murphy [Page 4]
Internet Draft OSPF NSSA Option December 2000
Type-7 LSAs provide for carrying external route information within an
NSSA. Type-7 AS external LSAs have virtually the same syntax as the
Type-5 AS external LSAs with the obvious exception of the link-state
type (see section 3.3 for more details). There are two major
semantic differences between Type-5 and Type-7 LSAs.
o Type-7 LSAs may be originated by and advertised throughout an
NSSA; as with stub areas, Type-5 LSAs are not flooded into NSSAs
and do not originate there.
o Type-7 LSAs are advertised only within a single NSSA; they are
not flooded into the backbone area or any other area by border
routers, though the information which they contain may be
propagated into the backbone area (see section 4.2).
In order to allow limited exchange of external information across an
NSSA border, NSSA border routers will translate selected Type-7 LSAs
received from the NSSA into Type-5 LSAs. These Type-5 LSAs will be
flooded to all Type-5 capable areas. NSSA border routers may be
configured with address ranges so that several Type-7 LSAs may be
aggregated into a single Type-5 LSA. The NSSA border routers which
perform translation are configurable. In the absence of a configured
translator one is elected.
In addition, an NSSA border router should originate a default LSA (IP
address of 0.0.0.0) into the NSSA. Default routes are necessary
because NSSAs do not receive full routing information and must have a
default route in order to route to AS-external destinations. Like
stub areas, NSSAs may be connected to the backbone at more than one
area border router, but may not be used as a transit area. Note that
a Type-7 default route originated by an NSSA border router is never
translated into a Type-5 LSA, however, a Type-7 default route
originated by an NSSA internal AS boundary router (one that is not
also an area border router) may be translated into a Type-5 LSA.
Like stub areas, the importing of OSPF summary routes (Type-3 LSAs)
into NSSAs is a configuration option. However particular care should
be taken to ensure that OSPF internal routes are not obscured by OSPF
external (Type-7) routes. This may happen when other IGPs, like RIP
and ISIS, leak routing information between an OSPF NSSA and another
OSPF area. In these cases, all OSPF summary routes should be
imported into the effected NSSAs. The recommended default behavior
is to import OSPF summary routes into NSSAs. Since AS-external-LSAs
(Type-5) are not imported into NSSAs, NSSA border routers should not
originate Type-4 summary-LSAs into their NSSAs.
When summary routes are not imported, the default LSA originated by a
NSSA border router into the NSSA should be a Type-3 summary LSA. The
Coltun, Fuller, Murphy [Page 5]
Internet Draft OSPF NSSA Option December 2000
Type-3 summary default route insures intra-AS connectivity to the
rest of the OSPF domain, as it takes precedence over any Type-7
external default route which might originate on an NSSA internal
router. This Type-3 summary default route prevents the OSPF domain's
internal traffic, which is normally forwarded by OSPF summary routes,
from exiting the AS via any NSSA Type-7 external default route
originated by an NSSA internal router. Type-7 external defaults
generated on NSSA internal routers and the no-summary option are
mutually exclusive. When summary routes are imported into the NSSA,
the default LSA originated by a NSSA border router into the NSSA
should be a Type-7 LSA.
One final note, NSSA border routers never generate Type-4 summary
LSAs for their NSSA ASBRs as their Type-7 external advertisements are
never flooded beyond the NSSA's borders.
In our transit topology examples the subnets of 130.57 and network
192.31.114 will still be learned by RIP on router BR18 but now both
BR10 and BR18 can be in an NSSA and all of BBN Planet's external
routes are hidden from BR18; BR10 becomes an NSSA border router and
BR18 becomes an AS boundary router internal to the NSSA. BR18 will
import the subnets of 130.57 and network 192.31.114 as Type-7 LSAs
into the NSSA. BR10 then translates these routes into Type-5 LSAs
and floods them into BBN Planet's backbone.
In our corporate example, the subnets of 192.168.192/20 and
192.168.208/22 are learned via their respective routing protocols,
redistributed throughout NSSAs 1 and 2, and then aggregated during
the translation process into a single Type-5 LSA which is flooded
into Area 0. Area 1 may configure A0 to perform translation, while
Area 2 configures B0 as its translator.
3.0 NSSA Intra-area Implementation Details
3.1 The N-bit
The N-bit ensures that all members of an NSSA agree on the area's
configuration. Together, the N-bit and E-bit reflect an interface's
(and consequently the interface's associated area) external LSA
flooding capability. As explained in section 10.5 of the OSPF
specification, if Type-5 LSAs are not flooded into/throughout the
area, the E-bit must be clear in the option field of the received
Hello packets. Interfaces associated with an NSSA will not send or
receive Type-5 LSAs on that interface but may send and receive Type-7
LSAs. Therefore, if the N-bit is set in the options field, the E-bit
must be clear.
To support the NSSA option an additional check must be made in the
Coltun, Fuller, Murphy [Page 6]
Internet Draft OSPF NSSA Option December 2000
function that handles the receiving of the Hello packet to verify
that both the N-bit and the E-bit found in the Hello packet's option
field match the value of the options that have been configured for
the receiving interface. A mismatch in the options causes processing
of the received Hello packet to stop and the packet to be dropped.
3.2 Type-7 Address Ranges
NSSA border routers may be configured with Type-7 address ranges.
Each address range is defined as an [address,mask] pair. Many
separate Type-7 networks may fall into a single address range, just
as a subnetted network is composed of many separate subnets. NSSA
border routers may aggregate Type-7 routes by advertising a single
Type-5 LSA for each Type-7 address range. The Type-5 LSA resulting
from a Type-7 address range match will be distributed to all Type-5
capable areas. Section 4.2 gives the details of generating Type-5
LSAs from Type-7 address ranges.
A Type-7 address range includes the following configurable items.
o An [address,mask] pair.
o A status indication of either Advertise or DoNotAdvertise.
o An external route tag.
3.3 Type-7 LSAs
External routes are imported into NSSAs as Type-7 LSAs by NSSA AS
boundary routers. An NSSA AS boundary router (ASBR) is a router
which has an interface associated with the NSSA and is exchanging
routing information with routers belonging to another AS. Like OSPF
ASBRs, an NSSA router indicates it is an NSSA ASBR by setting the E-
bit in its router-LSA. As with Type-5 LSAs a separate Type-7 LSA is
originated for each destination network. To support NSSAs, the
link-state database must therefore be expanded to contain Type-7
LSAs.
Type-7 LSAs are identical to Type-5 LSAs except for the following
(see section 12.4.4 "AS external links" in the OSPF specification).
1. The type field in the LSA header is 7.
2. Type-7 LSAs are only flooded within the originating NSSA. The
flooding of Type-7 LSAs follows the same rules as the flooding
of Type-1 and Type-2 LSAs.
3. Type-7 LSAs are only listed within the OSPF area data
Coltun, Fuller, Murphy [Page 7]
Internet Draft OSPF NSSA Option December 2000
structures of their respective NSSAs, making them area
specific. Type-5 LSAs, which are flooded to all Type-5 capable
areas, have global scope and are listed in the OSPF protocol
data structure.
4. At the NSSA border router, selected Type-7 LSAs are translated
into type 5-LSAs and flooded into the OSPF domain's transit
topology.
5. Type-7 LSAs have a propagate (P) bit which, when set, tells an
NSSA border router to translate the Type-7 LSA into a Type-5
LSA. Examples of how the P-bit is used for loop avoidance are
described in section 4.2.
6. Those Type-7 LSAs that are to be translated into Type-5 LSAs
must have their forwarding address set.
Type-5 LSAs which are translations of Type-7 LSAs normally contain a
forwarding address. The exception to this is when the Type-5 LSA is
an aggregation of Type-7 LSAs, in which case the Type-5 LSA may not
contain a forwarding address (see section 4.2 for details). The
forwarding address contained in Type-5 LSAs results in more efficient
routing to Type-7 AS external networks when there are multiple NSSA
border routers. Moreover the forwarding address in a Type-7 LSA
eases its translation into a Type-5 LSA, as the NSSA border router
will not be required to compute the forwarding address.
If the network between the NSSA AS boundary router and the adjacent
AS is advertised into OSPF as an internal OSPF route, the forwarding
address should be the next hop address as is currently done with
Type-5 LSAs. If the intervening network is not advertised into OSPF
as an internal OSPF route and the Type-7 LSA's P-bit is set, a
forwarding address should be selected from one of the router's active
OSPF interface addresses which belong to the NSSA. If no such
addresses exist, then no Type-7 LSAs with the P-bit set should
originate from this router.
When a router is forced to pick a forwarding address for a Type-7
LSA, precedence should be given first to the router's loopback
addresses (provided internal addressing is supported). If a loopback
address is not used and the selected forwarding address's interface
transitions to a Down state (see OSPF Section 9.3), one must select a
new forwarding address for any Type-7 LSAs which reference the
previously selected forwarding address and then re-originate these
Type-7 LSAs. If internal addresses are not available, preference
should be given to the router's active OSPF stub network addresses to
avoid the possible extra hop of a transit network's address.
Coltun, Fuller, Murphy [Page 8]
Internet Draft OSPF NSSA Option December 2000
Type-5 and Type-7 metrics and path types are directly comparable.
3.4 Originating Type-7 LSAs
NSSA AS boundary routers may only originate Type-7 LSAs. All NSSA
border routers must have the capability of translating Type-7 LSAs
into Type-5 LSAs as described in Section 4.2. NSSA border routers
must set bit E (external bit) in their router (Type-1) LSAs
originated for directly attached non-stub areas.
An NSSA internal AS boundary router must set the P-bit in the LSA
header's option field of any Type-7 LSA whose path it wants
advertised into the OSPF domain's full transit topology. The LSAs of
these networks must have a valid non-zero forwarding address. If the
P-bit is clear the LSA is not translated into a Type-5 LSA by NSSA
border routers.
When an NSSA border router originates both a Type-5 and a Type-7 LSA
for the same network, the P-bit must be clear in the Type-7 NSSA so
that the Type-7 LSA isn't again translated into a Type-5 LSA by
another NSSA border router. If the border router only originates a
Type-7 LSA, it may set the P-bit, thus allowing the network to be
aggregated/propagated during Type-7 translation. If an NSSA
originates a Type-5 LSA with a forwarding address which is part of
the NSSA, it should also originate a Type-7 LSA into the NSSA. If two
NSSA routers, both reachable from one another over the NSSA,
originate functionally equivalent AS-external-LSAs (i.e., same
destination, cost and non-zero forwarding address), then the router
having the least preferred LSA should flush its LSA (See [OSPF]
Section 12.4.4.1). Preference between two Type-7 LSAs is determined
by the following tie breaker rules:
1) An LSA with the p-bit set is preferred over one with the p-bit
clear.
2) If the p-bit settings are the same, the LSA with the higher
router ID is preferred.
A Type-7 default route (network 0.0.0.0) may be originated into the
NSSA by any NSSA router. The Type-7 default route originated by the
NSSA border router must have the P-bit clear. The Type-7 default
route originated by an NSSA ASBR which is not an NSSA border router
may have the P-bit set. A Type-7 default route may be installed by
NSSA border routers if and only if its P-bit is set (see Appendix E).
An LSA for the default destination must be originated by all the
NSSA's border routers in order to support intra-AS routing and
inter-AS routing. This default destination is advertised in either a
Coltun, Fuller, Murphy [Page 9]
Internet Draft OSPF NSSA Option December 2000
Type-3 or Type-7 LSA, as described in the Section 3.7.
3.5 Calculating Type-7 AS External Routes
This calculation must be run when Type-7 LSAs are processed during
the AS external route calculation. This calculation may process
Type-5 LSAs, but it is written that way only for comparison sake.
Non-default Type-7 LSAs with the P-bit clear may be installed in the
OSPF routing table of NSSA border routers. Since these LSAs are not
propagated throughout the OSPF domain, traffic which originates
external to an NSSA and which passes through one of the NSSA's border
routers may be unexpectedly diverted into the NSSA (See Appendix E).
An NSSA border router should examine both Type-5 LSAs and Type-7 LSAs
if either Type-5 or Type-7 routes need to be updated or recalculated.
This is done as part of the AS external route calculation. An NSSA
internal router should examine Type-7 LSAs when Type-7 routes need to
be recalculated.
What follows is only a modest modification of the OSPF Version 2
Specification Section 16.4. Original text is suffixed with [OSPF].
NSSA specific text is suffixed with [NSSA].
AS external routes are calculated by examining AS-external-LSAs, be
they Type-5 or Type-7. Each of the AS-external-LSAs is considered in
turn. Most AS-external-LSAs describe routes to specific IP
destinations. An AS-external-LSA can also describe a default route
for the Autonomous System (Destination ID = DefaultDestination,
network/subnet mask = 0x00000000). For each AS-external-LSA [~OSPF]:
(1) If the metric specified by the LSA is LSInfinity, or if the
age of the LSA equals MaxAge, then examine the next LSA.
[OSPF]
(2) If the LSA was originated by the calculating router itself,
examine the next LSA. [OSPF]
(3) Call the destination described by the LSA N. N's address is
obtained by masking the LSA's Link State ID with the
network/subnet mask contained in the body of the LSA. Look up
the routing table entries which match the LSA's type for the
AS boundary router (ASBR) that originated the LSA. For a
Type-5 LSA, routing table entries may only be selected from
each attached non-NSSA/non-stub area. Since the flooding
scope of a Type-7 LSA is restricted to the originating NSSA,
the routing table entry of its ASBR must be found in the
originating NSSA. If no entries exist for the ASBR (i.e. the
Coltun, Fuller, Murphy [Page 10]
Internet Draft OSPF NSSA Option December 2000
ASBR is unreachable over the transit topology for a Type-5
LSA, or, for a Type-7 LSA, it is unreachable over its
originating NSSA), do nothing with this LSA and consider the
next in the list. [NSSA]
Else if the destination is a Type-7 default route (destination
ID = DefaultDestination) and one of the following is true,
then do nothing with this LSA and consider the next in the
list:
o The calculating router is a border router and the LSA
has its P-bit clear. Appendix E describes technique for
border router Type-7 default installation without
propagation. [NSSA]
o The calculating router is suppressing the import of
summary (Type-3) LSAs.
Else, this LSA describes an AS external path to destination N.
Examine the forwarding address specified in the AS-external-
LSA. This indicates the IP address to which packets for the
destination should be forwarded. [OSPF]
If the forwarding address is set to 0.0.0.0 then packets
should be sent to the ASBR itself. If the LSA is Type-5, from
among the multiple non-NSSA routing table entries for the ASBR
(both NSSA and non-NSSA ASBR entries might exists on an NSSA
border router), select the preferred entry as follows [~OSPF]:
If RFC1583Compatibility is set to "disabled", prune the
set of routing table entries for the ASBR as described
in OSPF Section 16.4.1. In any case, among the
remaining routing table entries, select the routing
table entry with the least cost; when there are multiple
least cost routing table entries the entry whose
associated area has the largest OSPF Area ID (when
considered as an unsigned 32-bit integer) is chosen.
[OSPF]
Since a Type-7 LSA only has area-wide flooding scope, when its
forwarding address is set to 0.0.0.0, its ASBR's routing table
entry must be chosen from the originating NSSA. Here no
pruning is necessary since this entry always contains non-
backbone intra-area paths. [NSSA]
If the forwarding address is non-zero look up the forwarding
address in the routing table. For Type-5 LSAs the matching
routing table entry must specify an intra-area or inter-area
Coltun, Fuller, Murphy [Page 11]
Internet Draft OSPF NSSA Option December 2000
path. For Type-7 LSAs the matching routing table entry must
specify an intra-area path through the originating NSSA. If no
such path exists then do nothing with this LSA and consider
the next in the list. [OSPF]
(4) Let X be the cost specified by the preferred routing table
entry for the ASBR/forwarding address, and Y the cost
specified in the LSA. X is in terms of the link state metric,
and Y is a type 1 or 2 external metric. [OSPF]
(5) Now, look up the routing table entry for the destination N.
If no entry exists for N, install the AS external path to N,
with the next hop equal to the list of next hops to the
ASBR/forwarding address, and advertising router equal to the
ASBR. If the external metric type is 1, then the path-type is
set to Type-1 external and the cost is equal to X + Y. If the
external metric type is 2, the path-type is set to Type-2
external, the link-state component of the route's cost is X,
and the type 2 cost is Y. [OSPF]
(6) Otherwise compare the AS external path described by the LSA
with the existing paths in N's routing table entry. If the
new path is preferred, it replaces the present paths in N's
routing table entry. If the new path is of equal preference,
it is added to N's routing table entry's list of paths. [OSPF]
Preference is defined as follows:
(a) Intra-area and inter-area paths are always preferred over
AS external paths. [OSPF]
(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external paths,
the paths with the smallest advertised type 2 metric are
always preferred. [OSPF]
(c) If the new AS external path is still indistinguishable
from the current paths in N's routing table entry, and
RFC1583Compatibility is set to "disabled", select the
preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section 16.4.1.
Here intra-NSSA paths are equivalent to the intra-area
paths of non-backbone regular OSPF areas. [NSSA]
(d) If the new AS external path is still indistinguishable
from the current paths in N's routing table entry, select
the preferred path based on a least cost comparison. Type
1 external paths are compared by looking at the sum of the
Coltun, Fuller, Murphy [Page 12]
Internet Draft OSPF NSSA Option December 2000
distance to the forwarding address and the advertised type
1 metric (X+Y). Type 2 external paths advertising equal
type 2 metrics are compared by looking at the distance to
the forwarding addresses. [OSPF]
(e) If the current LSA is functionally the same as an
installed LSA (i.e., same destination, cost and non-zero
forwarding address) then apply the following priorities in
deciding which LSA is preferred:
a. A Type-7 LSA with the P-bit set.
b. A Type-5 LSA.
c. The LSA with the higher router ID. [NSSA]
3.6 Incremental Updates
Incremental updates for Type-7 LSAs should be treated the same as
incremental updates for Type-5 LSAs (see section 16.6 of the OSPF
specification). That is, if a new instance of a Type-7 LSA is
received it is not necessary to recalculate the entire routing table.
If there is already an OSPF internal route to the destination
represented by the Type-7 LSA, no recalculation is necessary.
Otherwise, the procedure in the proceeding section will have to be
performed but only for the external routes (Type-5 and Type-7) whose
networks describe the same networks as the newly received LSA.
3.7 Optionally Importing Summary LSAs
In order for backbone summary internal routes to be preferred over
external Type-7 routes, all implementations must support the optional
import of summary LSAs from the backbone into an NSSA. The default
behavior is to import Type-3 summary LSAs. A new area configuration
parameter, ImportSummaries, has been added. When set to enabled,
Type-3 summary routes are imported. When set to disabled, summary
routes are not imported. The default setting is enabled.
When summary routes are not imported, the default LSA originated by a
NSSA border router into the NSSA should be a Type-3 summary LSA.
This protects the NSSA from routing intra-AS traffic out the AS via a
Type-7 external default route originating from an internal NSSA
router. Unlike the stub area case, when summary routes are imported
into the NSSA, a Type-3 summary default route must not be injected
into the NSSA, otherwise the Type-3 summary default route would be
chosen over potentially more preferred Type-7 default routes.
Coltun, Fuller, Murphy [Page 13]
Internet Draft OSPF NSSA Option December 2000
4.0 Intra-AS implementation Details
4.1 Type-7 Translator Election
It is not recommended that multiple NSSA border routers perform the
translation unless the efficient routing of packets through area 0 to
an NSSA partitioned by aggregation requires it. It is normally
sufficient to have only one NSSA border router perform the
translation. Excessive numbers of Type-7 translators unnecessarily
increase the size of the OSPF link state data base.
A new area configuration parameter, NSSATranslatorRole, is defined in
Appendix D. It specifies whether or not an NSSA router will
unconditionally translate Type-7 LSAs to Type-5 LSAs when acting as
an NSSA border router. When set to Always, Type-7 LSAs are always
translated regardless of the translator state of other NSSA border
routers. When set to Candidate and acting as an NSSA border router,
an NSSA router will participate in the translator election process
described below.
A new bit called Nt is added to the router-LSA. NSSA border routers,
which are configured to unconditionally translate Type-7 LSAs into
Type-5 LSAs, set bit Nt in their NSSA router-LSA. All other routers
clear bit Nt in their NSSA router-LSAs.
A new area parameter called the NSSATranslatorState is maintained in
the OSPF area data structure. By default all NSSA routers initialize
NSSATranslatorState to disabled. When an NSSA router attains border
router status and has its NSSATranslatorRole set to Always, it sets
NSSATranslatorState to enabled and begins the unconditional
translation of Type-7 LSAs into Type-5 LSAs for the NSSA. When an
NSSA border router loses its border router status,
NSSATranslatorState is always reset to disabled and the Nt bit is
cleared in a new router LSA.
If an NSSA border router has its NSSATranslatorState set to disabled
and, from the subset of NSSA border routers which are reachable over
the NSSA and reachable as ASBRs over the AS's transit topology, no
such router exists either with bit Nt set in its router-LSA or with
higher router ID, then this router begins to perform the translation
of Type-7 LSAs into Type-5 LSAs for the NSSA and it sets
NSSATranslatorState to elected. The Nt bit of an elected translator
is always clear. These conditions may result in more than one
elected translator for the NSSA, should one of the NSSA border
routers lose connectivity to area 0.
All NSSA border routers must set the E-bit in their router-LSA to
directly attached non-stub areas even when they are not translating.
Coltun, Fuller, Murphy [Page 14]
Internet Draft OSPF NSSA Option December 2000
This allows other NSSA border routers to see their ASBR status across
the AS's transit topology. Failure to do so may cause the election
algorithm to elect unnecessary translators. Every NSSA border router
is a potential translator.
An elected translator will continue to perform translation duties
until supplanted by a reachable NSSA border router whose Nt bit is
set to true or whose router ID is greater. Such an event might be
triggered by the manual setting of the NSSATranslatorState to enabled
in one of the NSSA border routers or a topological rejoining of a
partitioned NSSA. Any change in the membership of the reachable NSSA
border router set, both over the NSSA and as ASBRs over the AS's
transit topology, or a change in a router-LSA's Nt bit setting should
force an NSSA border router to recheck its Type-7 translation status.
If an elected translator determines its services are no longer
required, it should continue to perform its translation duties for
the additional time interval defined by a new area configuration
parameter, TranslatorStabilityInterval. This minimizes excessive
flushing of translated Type-7 LSAs and provides for a more stable
translator transition. The default value for the
TranslatorStabilityInterval parameter has been defined as 40 seconds
(see Appendix D).
Configuring the identity of the translator can be used to bias the
routing to aggregated destinations.
4.2 Translating Type-7 LSAs into Type-5 LSAs
This step is performed as part of the NSSA's Dijkstra calculation
after Type-5 and Type-7 routes have been calculated. If the
calculating router is currently not an NSSA border router translator,
then this translation algorithm should be skipped. Only installed
Type-7 LSAs and those non-default Type-7 LSAs originated by the
router itself should be examined. Locally sourced Type-7 LSAs should
be processed first.
Note that it is possible for a Type-5 LSA generated by translation to
supplant a Type-5 LSA originating from a local OSPF external source.
Future reoriginations of the locally source Type-5 LSA should be
suppressed until the Type-5 LSA generated by translation is flushed.
A Type-7 LSA and a Type-7 range best match one another if there does
not exist a more specific Type-7 range which contains the Type-7 LSA.
For each translation eligible Type-7 LSA perform the following:
(1) If the Type-7 LSA has the P-bit clear, or its forwarding
address is set to 0.0.0.0, or the most specific Type-7 range
which subsumes the network has DoNotAdvertise status, then do
Coltun, Fuller, Murphy [Page 15]
Internet Draft OSPF NSSA Option December 2000
nothing with this Type-7 LSA and consider the next one in the
list. Otherwise term the LSA as translatable and proceed with
step (2).
(2) If the Type-7 LSA is not contained in any explicitly
configured Type-7 address range and the calculating router has
the highest router ID amongst NSSA translators which have
originated a functionally equivalent Type-5 LSA (i.e. same
destination, cost and non-zero forwarding address) and which
are reachable over area 0 and the NSSA, then a Type-5 LSA
should be generated if there currently is no Type-5 LSA
originating from this router corresponding to the Type-7 LSA's
network or there is an existing Type-5 LSA and either it
corresponds to a local OSPF external source whose path type
and metric is less preferred (see Section 3.5 step (6), parts
(b) and (d)) or it doesn't and the Type-5 LSA's path type or
cost(s) have changed (See Section 3.5 step (5)) or the
forwarding address no longer maps to a translatable Type-7
LSA.
The newly originated Type-5 LSA will describe the same network
and have the same network mask, path type, metric, forwarding
address and external route tag as the Type-7 LSA. The
advertising router field will be the router ID of this area
border router. The link-state ID is equal to the LSA's
network address (in the case of multiple originations of
Type-5 LSAs with the same network address but different mask,
the link-state ID can also have one or more of the range's
"host" bits set).
(3) Else the Type-7 LSA must be aggregated by the most specific
Type-7 range which subsumes it. If this Type-7 range has the
same [address,mask] pair as the Type-7 LSA's network and the
LSA is the only Type-7 LSA which best matches this range, then
flag the Type-7 LSA as not contained in any explicitly
configured Type-7 address range and continue processing the
LSA as described in step (2). Otherwise compute the path type
and metric for this Type-7 range as described below.
The path type and metric of the Type-7 range is determined
from the path types and metrics of those translatable Type-7
LSAs which best match the range plus any locally sourced
Type-5 LSAs whose network has the same [address,mask] pair.
If any of these LSAs have a path type of 2 the range's path
type is 2, otherwise it is 1. If the range's path type is 1
its metric is the highest cost amongst its LSAs; if the
range's path type is 2 its metric is the highest Type-2 cost +
1 amongst its LSAs (See Section 3.5 step (5)). 1 is added to
Coltun, Fuller, Murphy [Page 16]
Internet Draft OSPF NSSA Option December 2000
the Type-2 cost to ensure that the translated Type-5 does not
appear closer on the NSSA border than a translatable Type-7
LSA whose network has the same [address,mask] pair and Type-2
cost.
A Type-5 LSA is generated from the Type-7 range when there
currently is no Type-5 LSA originated by this router whose
network has the same [address,mask] pair as the range or there
is but either its path type or metric has changed or its
forwarding address is non-zero.
The newly generated Type-5 LSA will have link-state ID equal
to the Type-7 range's address (in the case of multiple
originations of Type-5 LSAs with the same network address but
different mask, the link-state ID can also have one or more of
the range's "host" bits set). The advertising router field
will be the router ID of this area border router. The network
mask and the external route tag are set to the Type-7 range's
configured values. The forwarding address is set to 0.0.0.0.
The path type and metric are set to the Type-7 range's path
type and metric as defined above.
The pending processing of other translation eligible Type-7
LSAs which best match this Type-7 range is suppressed. Thus
at most a single Type-5 LSA is originated for each Type-7
range.
For example, given a Type-7 range of [10.0.0.0, 255.0.0.0] which
subsumes the following Type-7 routes:
10.1.0.0 path type 1, metric 10
10.2.0.0 path type 1, metric 11
10.3.0.0 path type 2, metric 5
a Type-5 LSA would be generated with a path type of 2 and a metric 6.
Given a Type-7 range of [10.0.0.0, 255.0.0.0] which subsumes the
following Type-7 routes:
10.1.0.0 path type 1, metric 10
10.2.0.0 path type 1, metric 11
10.3.0.0 path type 1, metric 5
a Type-5 LSA will be generated with a path type of 1 and a metric 11.
These Type-7 range metric and path type rules will avoid routing
loops in the event that path types 1 and 2 are both used within the
area.
Coltun, Fuller, Murphy [Page 17]
Internet Draft OSPF NSSA Option December 2000
As with all newly originated Type-5 LSAs, a Type-5 LSA that is the
result of a Type-7 LSA translation or aggregation is flooded to all
attached Type-5 capable areas.
Like Type-3 ranges, a Type-7 range performs the dual function of
setting propagation policy via its Advertise/DoNotAdvertise parameter
and aggregation via its network address and mask pair. Unlike Type-3
summary links, Type-5 translation may result in more efficient
routing when the forwarding address is set, as is done during step
(2) of the translation procedure.
Another important feature of this translation process is that it
allows a Type-7 range to apply different properties (aggregation,
forwarding address, and Advertise/DoNotAdvertise status) for the
Type-7 routes it subsumes, versus those Type-7 routes subsumed by
other more specific Type-7 ranges contained by the Type-7 range.
4.3 Flushing Translated Type-7 LSAs
If an NSSA border router has either translated or aggregated an
installed Type-7 LSA into a Type-5 LSA which should no longer be
translated or aggregated, then the Type-5 LSA should either be
flushed or reoriginated as an aggregation of other Type-7 LSAs.
If an NSSA border router is translating Type-7 LSA's into Type-5
LSA's with
NSSATranslatorState = elected
and the NSSA border router has determined that its translator
election status has been deposed by another NSSA border router, then,
as soon as the TranslatorStabilityInterval has expired without the
router reelecting itself as a translator, Type-5 LSAs generated by
translating Type-7 address ranges are flushed. Conversely Type-5
LSAs generated by translating Type-7 LSAs are not immediately
flushed, but are allowed to remain in the OSPF routing domain as if
the originator is still an elected translator. This minimizes the
impact of an NSSA which changes its translator frequently.
5.0 Security Considerations
There are two types of issues that need be addressed when looking at
protecting routing protocols from misconfigurations and malicious
attacks. The first is authentication and certification of routing
protocol information. The second is denial of service attacks
resulting from repetitive origination of the same router
advertisement or origination of a large number of distinct
advertisements resulting in database overflow. Note that both of
Coltun, Fuller, Murphy [Page 18]
Internet Draft OSPF NSSA Option December 2000
these concerns exist independently of a router's support for the NSSA
option.
The OSPF protocol addresses authentication concerns by authenticating
OSPF protocol exchanges. OSPF supports multiple types of
authentication; the type of authentication in use can be configured
on a per network segment basis. One of OSPF's authentication types,
namely the Cryptographic authentication option, is believed to be
secure against passive attacks and provides significant protection
against active attacks. When using the Cryptographic authentication
option, each router appends a "message digest" to its transmitted
OSPF packets. Receivers then use the shared secret key and the
received digest to verify that each received OSPF packet is
authentic.
The quality of the security provided by the Cryptographic
authentication option depends completely on the strength of the
message digest algorithm (MD5 is currently the only message digest
algorithm specified), the strength of the key being used, and the
correct implementation of the security mechanism in all communicating
OSPF implementations. It also requires that all parties maintain the
secrecy of the shared secret key. None of the standard OSPF
authentication types provide confidentiality. Nor do they protect
against traffic analysis. For more information on the standard OSPF
security mechanisms, see Sections 8.1, 8.2, and Appendix D of [OSPF].
[DIGI] describes the extensions to OSPF required to add digital
signature authentication to Link State data and to provide a
certification mechanism for router data. [DIGI] also describes the
added LSA processing and key management as well as a method for
migration from or co-existence with standard OSPF V2.
Repetitive origination of advertisements are addressed by OSPF by
mandating a limit on the frequency that new instances of any
particular LSA can be originated and accepted during the flooding
procedure. The frequency at which new LSA instances may be
originated is set to once every MinLSInterval seconds, whose value is
5 seconds (see Section 12.4 of [OSPF]). The frequency at which new
LSA instances are accepted during flooding is once every MinLSArrival
seconds, whose value is set to 1 (see Section 13, Appendix B and G.1
of [OSPF]).
Proper operation of the OSPF protocol requires that all OSPF routers
maintain an identical copy of the OSPF link state database. However,
when the size of the link state database becomes very large, some
routers may be unable to keep the entire database due to resource
shortages; this is termed "database overflow". When database
overflow is anticipated, the routers with limited resources can be
Coltun, Fuller, Murphy [Page 19]
Internet Draft OSPF NSSA Option December 2000
accommodated by configuring OSPF stub areas and NSSAs. [OVERFLOW]
details a way of gracefully handling unanticipated database
overflows.
6.0 Acknowledgments
This document was produced by the OSPF Working Group, chaired by John
Moy.
In addition, the comments of the following individuals are also
acknowledged:
Phani Jajjarvarpu cisco
Dino Farinacci cisco
Jeff Honig Cornell University
Acee Lindem IBM
John Moy Sycamore Networks, Inc.
Antoni Przygienda Redback Networks, Inc
Doug Williams IBM
Alex Zinin cisco
7.0 References
[DIGI] S. Murphy, M. Badger, B. Wellington, "OSPF with Digital
Signatures", RFC 2154, Trusted Information Systems, June 1997.
[MUEX] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon,
Inc., March 1994.
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, Cascade Communications
Corp., April 1998.
[OPAQUE] Coltun, Rob, "The OSPF Opaque LSA Option", RFC 2370, FORE
Systems, July 1998.
[OVERFLOW] Moy, J., "OSPF Database Overflow", RFC 1765, Cascade,
March 1995.
Coltun, Fuller, Murphy [Page 20]
Internet Draft OSPF NSSA Option December 2000
8.0 Authors' Addresses
This update uses much of the original text from RFC 1587 authored by
Rob Coltun
Redback Networks, Inc.
1195 Borregas Avenue
Sunnyvale, CA 94089
Phone: (408) 548-3947
EMail: rcoltun@redback.com
Vince Fuller
GTE Internetworking
3801 East Bayshore Road
Palo Alto, California 94303
Phone: (415) 528-7227
EMail: vaf@BBNPlanet.com
New sections, edits and revisions have been added by
Pat Murphy
US Geological Survey
345 Middlefield Road
Menlo Park, California 94560
Phone: (415) 329-4044
EMail: pmurphy@noc.doi.net
Coltun, Fuller, Murphy [Page 21]
Internet Draft OSPF NSSA Option December 2000
Appendix A: The Options Field
The OSPF options field is present in OSPF Hello packets, Database
Description packets and all link state advertisements. See appendix A.2 in
[OSPF] and [OPAQUE] for a description of the options field. Six bits
are assigned but only two (the E-bit and the N/P bit) are described
completely in this section.
--------------------------------------
| * | O | DC | EA | N/P | MC | E | * |
--------------------------------------
The Type-7 LSA options field
E-bit: Type-5 AS external LSAs are not flooded into/through OSPF
stub areas and NSSAs. The E-bit ensures that all members
of a stub area agree on that area configuration. The E-
bit is meaningful only in OSPF Hello and Database
Description packets. When the E-bit is clear in the Hello
packet sent out a particular interface, it means that the
router will neither send nor receive Type-5 AS external
LSAs on that interface (in other words, the interface
connects to a stub area or NSSA). Two routers will not
become neighbors unless they agree on the state of the E-
bit.
N-bit: The N-bit describes the router's NSSA capability. The N-
bit is used only in Hello packets and ensures that all
members of an NSSA agree on that area's configuration.
When the N-bit is set in the Hello packet and sent out a
particular interface, it means that the router will send
and receive Type-7 LSAs on that interface. Two routers
will not form an adjacency unless they agree on the state
of the N-bit. If the N-bit is set in the options field,
the E-bit must be clear.
P-bit: The P-bit is used only in the Type-7 LSA header. It flags
the NSSA border router to translate the Type-7 LSA into a
Type-5 LSA. The default setting for the P-bit is clear.
Coltun, Fuller, Murphy [Page 22]
Internet Draft OSPF NSSA Option December 2000
Appendix B: Router-LSAs
Router-LSAs are the Type-1 LSAs. Each router in an area originates a
router-LSA. The LSA describes the state and cost of the router's
links (i.e., interfaces) to the area. All of the router's links to
the area must be described in a single router-LSA. For details
concerning the construction of router-LSAs, see the OSPF
Specification, Section 12.4.1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 Nt|W|V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | TOS 0 metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
In router-LSAs, the Link State ID field is set to the router's OSPF
Router ID. The T-bit is set in the LSA's Option field if and only if
the router is able to calculate a separate set of routes for each IP
TOS. Router-LSAs are flooded throughout a single area only.
Coltun, Fuller, Murphy [Page 23]
Internet Draft OSPF NSSA Option December 2000
bit V
When set, the router is an endpoint of one or more fully
adjacent virtual links having the described area as Transit
area (V is for virtual link endpoint).
bit E
When set, the router is an AS boundary router (E is for
external). ALL NSSA border routers must set bit E in the
router-LSAs to directly attached standard areas and NSSAs.
(See Section 4.1 for details).
bit B
When set, the router is an area border router (B is for
border).
bit W
When set, the router is a wild-card multicast receiver (W is
for wild).
bit Nt
When set, the router is an NSSA border router which is
unconditionally translating Type-7 LSAs into Type-5 LSAs (Nt
is for NSSA translation). Note that such routers have their
NSSATranslatorRole area configuration parameter set to Always
(See Appendix D and Section 4.1).
The remainder of the router links specification is as defined in the
OSPF Specification, Section A.4.2.
Coltun, Fuller, Murphy [Page 24]
Internet Draft OSPF NSSA Option December 2000
Appendix C: Type-7 LSA Packet Format
0 32
------------------------------------
| | Options | 7 |
| -------------------
| Link-State Header |
| |
------------------------------------
| Network Mask |
------------------------------------ ______
|E| TOS | metric | .
------------------------------------ . repeated for each TOS
| Forwarding Address | .
------------------------------------ .
| External Route Tag | ______
------------------------------------
The definitions of the link-state ID, network mask, metrics and
external route tag are the same as the definitions for Type-5 LSAs
(see Appendix A.4.5 in the [OSPF]), except for the forwarding address
and the N/P-bit. The Options field must have the N/P bit set as
described in Appendix A when the originating router desires that the
external route be propagated throughout the OSPF domain.
Forwarding address
Data traffic for the advertised destination will be forwarded to
this address. If the forwarding address is set to 0.0.0.0, data
traffic will be forwarded to the LSA's originator (i.e., the
responsible NSSA AS boundary router). If the P-bit is set the
forwarding address must be non-zero. If the network between the
NSSA AS boundary router and the adjacent AS is advertised into the
NSSA as an internal OSPF route, the forwarding address should be
the next hop address. If the intervening network is not
advertised into the NSSA as an internal OSPF route, the forwarding
address should be any one of the router's active OSPF interface
addresses (see Section 3.3 for details).
Coltun, Fuller, Murphy [Page 25]
Internet Draft OSPF NSSA Option December 2000
Appendix D: Configuration Parameters
Appendix C.2 in the OSPF specification lists the area
configuration parameters. The area ID and the list of address
ranges for Type-3 summary routes remain unchanged. Section 3.2 of
this document lists the configuration parameters for Type-7
address ranges. The following area configuration parameters have
been added:
NSSATranslatorRole
Specifies whether or not the router will unconditionally
translate Type-7 LSAs to Type-5 LSAs when acting as an NSSA
border router. When set to Always, Type-7 LSAs are always
translated regardless of the translator state of other NSSA
border routers. When set to Candidate and acting as an NSSA
border router, it participates in the translator election
process described in Section 4.1. The default setting is
Candidate.
TranslatorStabilityInterval
Defines the length of time an elected Type-7 translator will
continue to perform its translator duties once it has
determined that translator status has been deposed by another
NSSA border router translator as described in Section 4.1 and
4.3. The default setting is 40 seconds.
ImportSummaries
When set to enabled, Type-3 summary LSAs are imported into the
NSSA. When set to disabled, Type-3 summary LSAs are not
imported into the NSSA. The default setting is enabled.
Implementations must provide a vehicle for setting the P-bit of
external routes imported into the NSSA as Type-7 LSAs. Without
configuration, the default setting of the P-bit is clear (see Section
3.3 and Appendix B).
For NSSAs the ExternalRoutingCapability area configuration parameter
must be set to accept Type-7 external routes. Additionally there
must be a way of configuring an NSSA border router to advertise a
default route into the NSSA with configurable metric type (1 or 2)
and cost.
Coltun, Fuller, Murphy [Page 26]
Internet Draft OSPF NSSA Option December 2000
Appendix E: The P-bit Policy Paradox.
Non-default Type-7 LSAs with the P-bit clear may be installed in the
OSPF routing table of NSSA border routers (see Section 3.5). These
LSAs are not propagated throughout the OSPF domain as translated
Type-5 LSAs (see Section 4.2). Thus traffic which is external to an
NSSA and which passes through one of the NSSA's border routers may be
hijacked into the NSSA by a route installed from a Type-7 LSA with
the P-bit clear. This may be contrary to the expected path at the
source of the traffic. It may also violate the routing policy
intended by the Type-7 LSA's clear P-bit. A Type-7 range configured
with DoNotAdvertise exhibits the same paradox for any installed
Type-7 LSAs it subsumes.
This paradox is best illustrated by the following example. Consider
an OSPF domain (AS 1842) with connections for default Internet
routing and to external AS 4156. NSSA 1 and OSPF Area 2 are
partially defined in the following diagram:
AS 4156
|
Area 2 |
|
A2 A0 Area 0 C0-----Internet
| | | Default
| | |
| | |
+-----------------B0---------------+
/\
/ \
/ \
Internet------------A1 B1------AS 4156 (p-bit clear)
Default (p-bit set)
NSSA 1
Here A0, B0, and C0 are Area 0 routers, A1 and B1 are NSSA 1 routers,
and A2 is an Area 2 router. B0 is a border router for both NSSA 1
and Area 2.
If NSSA 1 routes for AS 4156 are installed on B0 so that the NSSA 1
tree below A1 can take advantage of it, then A2's traffic to AS 4156
is hijacked through B0 by B1, rather than its computed path through
A0.
The P-bit paradox can also appear with default routes. By setting
the P-bit on a Type-7 default LSA, and configuring DoNotAdvertise for
[0,0] in an NSSA's border routers, the default route can be installed
but not propagated. In the example above, if A1's default is
Coltun, Fuller, Murphy [Page 27]
Internet Draft OSPF NSSA Option December 2000
installed on B0 and the range [0,0] has DoNotAdvertise set, then A2's
default bound traffic is hijacked through B0 by A1 rather than the
computed path through C0.
Coltun, Fuller, Murphy [Page 28]
Internet Draft OSPF NSSA Option December 2000
Appendix F: Differences from RFC 1587
This section documents the differences between this memo and RFC
1587. All differences are backward-compatible. Implementations of
this memo and of RFC 1587 will interoperate.
F.1 Enhancements to OSPF summary LSAs. .
The flooding of backbone summary LSAs (Type-3 LSAs) into the NSSA
is now optional. In RFC 1587 the flooding of backbone summary
LSAs was mandated in order to guarantee inter-area routes are
preferred over external routes. The current recommended default
behavior is to import summary LSAs. When summary routes are not
imported, the default LSA originated by a NSSA border router into
the NSSA should be a Type-3 summary LSA.
See Sections 2.2 and 3.4 for details.
F.2 Changes to Type-7 LSAs.
The setting of the forwarding address in Type-7 LSAs has been
further refined.
See Section 3.3 for details.
F.3 Changes to the Type-7 AS external routing calculation.
The Type-7 external route calculation has been revised. Most
notably:
o The path preference defined in OSPF Section 16.4.1 has been
included.
o A Type-7 default route with the P-bit clear will not be
installed on an NSSA border router. This protects the
default routing of other OSPF Areas. (See Appendix E.)
See Section 3.5 for details.
F.4 Changes to translating Type-7 LSAs into Type-5 LSAs
The translator election algorithm of RFC 1587 has been updated to
close a bug which results when the translator with the highest
router ID loses connectivity to the AS's transit topology. The
default translator election process occurs only in the absence of
an existing translator.
The identity of the translator is optionally configurable, with
Coltun, Fuller, Murphy [Page 29]
Internet Draft OSPF NSSA Option December 2000
more than one allowed. This allows the network designer to
choose the most cost effective intra-AS route for NSSA originated
Type-5 LSA aggregations of Type-7 LSAs.
Self-originated non-default Type-7 LSAs are now included in the
translation process.
The translation process has been strengthened to close some of
the weak points of RFC 1587.
See Sections 4.1 and 4.2 for details.
F.5 Changes to flushing translated Type-7 LSAs
An NSSA border router, which was elected by the augmented RFC
1587 translator selection process defined in Section 4.1 and has
been deposed from translation duties by another NSSA border
router, flushes its self-originated Type-5 LSAs that resulted
from the aggregation of Type-7 LSAs. This prevents these
obsolete aggregations from short circuiting the preferred path
through the new translator(s). A deposed translator continues to
maintain its self-originated Type-5 LSAs resulting from
translation until they age out normally.
See Section 4.3 for details.
F.6 P-bit additions
The P-bit default has been defined as clear. RFC 1587 had no
default setting. ( See Appendix C)
A discussion on the packet forwarding impact of installing Type-7
LSAs with the P-bit clear on NSSA border routers has been added
as Appendix E.
Coltun, Fuller, Murphy [Page 30]
