Internet DRAFT - draft-bashandy-bgp-frr-vector-label
draft-bashandy-bgp-frr-vector-label
Network Working Group A. Bashandy
Internet Draft N. Kumar
Intended status: Standards Track M. Konstantynowicz
Expires: January 2013 Cisco Systems
July 7, 2012
BGP FRR Protection against Edge Node Failure Using Vector Labels
draft-bashandy-bgp-frr-vector-label-00.txt
Abstract
Consider a BGP free core scenario. Suppose the edge BGP speakers PE1,
PE2,..., PEn know about a prefix P/m via the external routers CE1,
CE2,..., CEm. If the edge router PEi crashes or becomes totally
disconnected from the core, it is desirable for a core router "P"
carrying traffic to the failed edge router PEi to immediately restore
traffic by re-tunneling packets originally tunneled to PEi and
destined to the prefix P/m to one of the other edge routers that
advertised P/m, say PEj, until BGP re-converges. In doing so, it is
highly desirable to keep the core BGP-free while not imposing
restrictions on external connectivity or complicating provisioning
effort. Thus (1) a core router should not be required to learn any
BGP prefix, (2) the size of the forwarding and routing tables in the
core routers should be independent of the number of BGP prefixes, (3)
re-routing traffic without waiting for re-convergence must not cause
loops, (4) provisioning effort should be kept at minimum, and (5)
there should be no restrictions on what edge routers advertise what
prefixes. For labeled prefixes, (6) the label stack on the packet
must allow the repair PEj to correctly forward the packet and (7)
there must not be any need to perform more than one label lookup on
any edge or core router during steady state
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Table of Contents
1. Introduction...................................................3
1.1. Conventions used in this document.........................5
1.2. Terminology...............................................5
1.3. Problem definition........................................7
2. Overview of BGP FRR in an MPLS Core............................8
2.1. Control Plane operation...................................8
2.2. Forwarding behavior at Steady State (while pPE is reachable)
..............................................................12
2.3. Forwarding behavior when pPE Fails.......................13
3. Overview of the BGP FRR using Vector Labels in an IP Core.....14
3.1. Pure IP Core.............................................15
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3.1.1. Control Plane.......................................15
3.1.2. Forwarding plane during Steady State (when pPE is
reachable).................................................15
3.1.3. Forwarding plane at Failure (when pPE is unreachable)15
3.2. Hybrid IP core...........................................16
3.2.1. Control Plane.......................................16
3.2.2. Forwarding Plane during Steady State (when pPE is
reachable).................................................17
3.2.3. Forwarding plane at Failure (when pPE is unreachable)17
4. Rules for Choosing and Managing the Repair path...............17
4.1. General Rules for Managing the Repair Path...............18
4.2. Rules for Choosing the Repair Path for Labeled Prefixes..19
5. Inter-operability with Existing IP FRR Mechanisms.............19
6. Example.......................................................20
6.1. Control Plane............................................21
6.2. Forwarding Plane at Steady State (When PE0 is reachable).22
6.3. Forwarding Plane at Failure (When PE0 is not reachable)..24
7. Security Considerations.......................................25
8. IANA Considerations...........................................25
9. Conclusions...................................................25
10. References...................................................26
10.1. Normative References....................................26
10.2. Informative References..................................27
11. Acknowledgments..............................................27
Appendix A. Other Algorithms to Allocate and Disseminate Vector
labels...........................................................28
A.1. iPE chooses the repair path..............................28
A.1.1. Allocating Vector Labels using a Hash Function......28
A.1.1.1.1. Calculating and distributing the mapping rNH-
>vL to different routers.............................28
A.1.1.1.2. Risk of Mis-configuration leading to Mismatch
in rNH-->vL Mapping..................................29
A.1.1.1.3. Risk of forwarding to Incorrect VRF during
convergence only.....................................29
A.1.2. pPE Allocates and advertises vL with protected prefixes
...........................................................29
A.1.2.1.1. Risk of forward to Incorrect VRF during
Convergence Only.....................................30
A.2. pPE chooses rPE and distributes the mapping of vL-->rNH..30
A.3. Combination of iPE and pPE Choosing rPE.................31
1. Introduction
In a BGP free core, where traffic is tunneled between edge routers,
BGP speakers advertise reachability information about prefixes to
other edge routers but not to core routers. For labeled address
families, namely AFI/SAFI 1/4, 2/4, 1/128, and 2/128, an edge
router assigns local labels to prefixes and associates the local
label with each advertised prefix such as L3VPN [10], 6PE [11], and
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Softwire [9]. Suppose that a given edge router is chosen as the
best next-hop for a prefix P/m. An ingress router that receives a
packet from an external router and destined to the prefix P/m
"tunnels" the packet across the core to that egress router. If the
prefix P/m is a labeled prefix, the ingress router pushes the label
advertised by the egress router before tunneling the packet to the
egress router. Upon receiving the packet from the core, the egress
router takes the appropriate forwarding decision based on the
content of the packet or the label pushed on the packet.
In modern networks, it is not uncommon to have a prefix reachable
via multiple edge routers. One example is the best external path
[8]. Another more common and widely deployed scenario is L3VPN [10]
with multi-homed VPN sites. As an example, consider the L3VPN
topology depicted in Figure 1.
PE1 .............+
|
+--------+---------------+
| |
| VPN 1 Network |
| |
| VPN prefix |
| (10.0.0.0/8) |
| |
+---+--------------------+
|
/------CE1
/
/
BGP-free core P--------PE0
\
\
\------CE2
|
+---+--------------------+
| |
| VPN 2 Network |
| |
| VPN prefix |
| (20.0.0.0/8) |
| |
+--------+---------------+
|
PE2 .............+
Figure 1 VPN prefix reachable via multiple PEs
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As illustrated in Figure 1, the edge router PE0 is the primary NH
for both 10.0.0.0/8 and 20.0.0.0/8. At the same time, both
10.0.0.0/8 and 20.0.0.0/8 are reachable through the other edge
routers PE1 and PE2, respectively.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [1].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
1.2. Terminology
This section defines the terms used in this document. For ease of
use, we will use terms similar to those used by L3VPN [10]
o BGP-Free core: A network where BGP prefixes are only known to
the edge routers and traffic is tunneled between edge routers
o External prefix: It is a prefix P/m (of any AFI/SAFI) that a BGP
speaker has an external path. The BGP speaker may learn about
the prefix from an external peer through BGP, some other
protocol, or manual configuration. The protected prefix is
advertised to some or all of the internal peers.
o Protectable prefix: It is an external prefix P/m of any
AFI/SAFI) that a BGP speaker has an external path to and is
eligible to have a repair path.
o Protected prefix: It is an external prefix P/m (of any AFI/SAFI)
that a BGP speaker has an external path to and also has a repair
path to.
o Primary Egress PE, "ePE": It is an IBGP peer that can reach the
prefix P/m through an external path and advertised the prefix to
the other IBGP peers. The primary egress PE was chosen as the
best path by one or more internal peers. In other words, the
primary egress PE is an egress PE that will normally be used by
some ingress PEs when there is no failure. Referring to Figure
1, PE0 is an egress PE.
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o Protected egress PE, "pPE" (Protected PE for simplicity): It is
an egress PE for which there exists a repair path for some or
all of the prefixes to which it has an external path. Referring
to Figure 1, PE0 is a protected egress PE.
o Protected edge router: Any protected egress PE.
o Protected next-hop (pNH): It is an IPv4 or IPv6 host address
belonging to the protected egress PE. Traffic tunneled to this
IP address will be protected via the mechanism proposed in this
document. Note that, in most cases, the protected next-hop will
be different from the next-hop attribute in the BGP update
message [2][3].
o CE: It is an external router through which an egress PE can
reach a prefix P/m. The routers "CE1" and "CE2" in Figure 1 are
examples of such CEs.
o Ingress PE, "iPE": It is a BGP speaker that learns about a
prefix through another IBGP peer and chooses that IBGP peer as
the next-hop for the prefix.
o Repairing P router "rP" (Also "Repairing core router" and
"repairing router"): A core router that attempts to restore
traffic when the primary egress PE is no longer reachable
without waiting for IGP or BGP to re-converge. The repairing P
router restores the traffic by rerouting the traffic (through a
tunnel) towards the pre-calculated repair PE when it detects
that the primary egress PE is no longer reachable. Referring to
Figure 1, the router "P" is the repairing P router.
o Repair egress PE "rPE" (Repair PE for simplicity): It is an
egress PE other than the primary egress PE that can reach the
protected prefix P/m through an external neighbor. The repair PE
is pre-calculated via other PEs prior to any failure. Referring
to Figure 1, PE1 is the repair PE for 10.0.0.0/8 while PE2 is
the repair PE for 20.0.0.0/8.
o Underlying Repair label (rL): The underlying repair label is the
label that is advertised by rPE and is used by rPE to forward
repaired traffic, which is traffic re-tunneled by the rP after
detecting that the pPE is no longer reachable. A repair label is
defined for labeled protected prefixes only.
o Repair next-hop (rNH): It is an IPv4 or IPv6 host address
belonging to the repair egress PE. If the protected prefix is
advertised via BGP, then the repair next-hop MAY be the next-hop
attribute in the BGP update message [2][3].
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o BGP nexthop (bgpNH): This is the usual next-hop attribute for
route advertisements as specified in [2]in [3]. In most case,
bgpNH is different from pNH
o Vector Label (vL): It is a label that identifies the repair PE
within a certain label context. Every distinct rPE must have a
distinct vector label the aforementioned label context. Vector
labels in different label contexts may overlap
o Repair path (Also Repair Egress Path): It is the repair next-
hop. If an underlying repair label exists, the repair path is
the repair next-hop together with the underlying repair label.
o Primary tunnel: It is the tunnel from the ingress PE to the
primary egress PE
o Repair tunnel: It is the tunnel from the repairing P router to
the repair egress PE
1.3. Problem definition
The problem that we are trying to solve is as follows
o Even though multiple prefixes may share the same egress router,
they have different repair edge router. In Figure 1 above, both
10.0.0.0/8 and 20.0.0.0/8 share the same primary next hop PE0,
the routing protocol(s) must identify that the node protecting
repair node for 10.0.0.0/8 is PE1 while the node protecting
repair node for 11.0.0.0/8 is PE2
o On loosing connection to the edge router, the core router "P"
MUST reroute traffic towards the *correct* repair edge router
that can reach prefixes that were reachable via the failed edge
router without waiting for IGP or BGP to re-converge and update
the routing tables. On the failure of PE0 illustrated in Figure
1, the core router P needs to reroute traffic for 10.0.0.0/8
towards PE1 and traffic for 11.0.0.0/8 towards PE2
o The repairing core router P MUST NOT be forced to learn about
the BGP prefixes on any of the edge router. The same applies for
all core routers.
o There SHOULD NOT be a need for a special router or group of
routers to handle rerouting traffic on edge node failure.
o The size of the routing table on any core router MUST be
independent of the number of BGP prefixes in the network.
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o Rerouting traffic without waiting for IGP and BGP to re-converge
after a failure MUST NOT cause loops.
o For labeled prefixes, when a packet gets re-routed to the repair
PE, the label stack on the packet MUST ensure correct
forwarding.
o Provisioning and maintenance overhead must be kept at minimum
o At steady state, when pPE is reachable, paths taken by traffic
must not be impacted by deploying the solution proposed in this
document unless desired by the operator.
o The solution must be incrementally deployable
2. Overview of BGP FRR in an MPLS Core
The solution proposed in this document relies on the collaboration of
egress PE, ingress PE, penultimate hop routers, and repairing core
router. This section gives an overview of how to the solution works
for both labeled (AFI/SAFI 1/4, 2/4, 1/128, and 2/128) and unlabeled
(AFI/SAFI 1/1, 2/1, 1/2, and 2/2) protected prefixes in an MPLS core.
Specifications of the solution in IP core are provided in Section 3.
2.1. Control Plane operation
1. Each egress router that is capable of handling repaired traffic
assigns each protectable labeled prefix a repair label: "rL". "rL"
is advertised as optional path attribute. "rL" MUST be Per-CE or
per-VRF for good BGP attribute packing and forwarding simplicity.
For unlabeled prefix, no repair label is needed. A router that is
capable of handling repaired traffic is called a repair PE "rPE".
a. The semantics of the repair label "rL" is:
i. If "rL" is per-CE, then pop *two* labels and send the
packet to the appropriate CE
ii. If "rL" is per-VRF, then pop *two* labels and forward the
packet based on the contents under the two popped labels
2. Each protectable egress PE (pPE) is assigned a unique protectable
IP address "pNH". Traffic tunneled to pNH is protected by the BGP
FRR proposed in this document
a. Only a single pNH is needed per pPE
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b. If all iPE's support the BGP FRR scheme proposed in this
document, then pNH may be the usual BGP next-hop attribute.
Otherwise, pNH MUST NOT be identical to the BGP next-hop
attribute
c. pPE advertises pNH as a prefix into IGP
d. pPE advertises an explicit label for pNH (instead of the usual
implicit NULL). This way if the penultimate hop does not
understand the BGP FRR scheme proposed in this document, pPE
can handle the special popping behavior for protected traffic
tunneled to pNH
e. "pPE" advertises the protected next-hop "pNH" to the
penultimate hops to indicate that traffic flowing through the
tunnel to the tail end "pNH" is protected against the failure
of the node "pPE" and requires special processing by the
penultimate hop as will be described in the next few steps
f. For every BGP next-hop (bgpNH) that pPE advertises with its
routes, pPE separately advertises the mapping (bgpNH,pNH) to
all ingress PE. A method analogous to how tunnel information
is advertised [4] can be used to advertise this mapping to
ingress PE's. The mapping "(bgpNH,pNH)" means: if the ingress
PE wants to protect traffic normally tunneled to "bgpNH"
against the failure of "pPE", the iPE MUST tunnel the traffic
to "pNH" instead of bgpNH.
3. If a pPE knows that a P/m to which it has an external path is also
reachable via another PE,
a. pPE chooses one of the other PEs as a repair PE "rPE". The pPE
chooses, as a repair next-hop, an IP address "rNH" local to or
advertised by rPE. Rules governing rNH are
i. "rNH" SHOULD be the next-hop attribute advertised by rPE
when it announces reachability to the protected prefix
P/m to minimize the number of prefixes advertised into
IGP and BGP.
ii. if rPE also advertised a protected next-hop (pNH) for any
BGP prefix that rPE can protect, then rNH MUST NOT be any
protected next-hop (pNH) advertised by rP
b. pPE assigns a vector label "vL" for "rNH". A distinct "vL" is
needed for every distinct "rNH" within the context of a pPE
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c. pPE advertises the mapping (pNH,rNH,vL) to all ingress PE's.
The mapping (pNH,rNH,vL) means: "Within the context of the
protected next-hop pNH, the repair next-hop rNH is assigned
the vector label vL"
d. "pPE" advertises the triplet (pNH,rNH,vL) to candidate
repairing core routers. For example, an LDP optional TLV can
be used for this purpose
4. An ingress PE "iPE" receives route updates from pPE with "bgpNH"
as the next-hop attribute. Suppose an ingress PE "iPE" chooses
"bgpNH" as the best path for one or more protectable PE. If iPE
wants to protect traffic tunneled to "bgpNH" against pPE failure,
"iPE" performs the following steps
a. iPE receives the mapping (bgpNH,pNH) from pPE to indicate that
the protected next-hop for traffic tunneled to bgpNH is pNH
b. iPE receives the mapping (pNH,rNH,vL) from "pPE" to indicate
that the vector label pointing to the repair next-hop "rNH"
for traffic tunneled to pNH is "vL"
c. iPE receives an advertisement for the protectable route from
rPE with "rNH" as the next-hop
d. If the above 3 conditions are satisfied, then iPE chooses rPE
as the repair PE with rNH as the repair next-hop and the
vector label "vL"
As a result of the above steps, the following nodes store the
following information
o Ingress PE (iPE)
o Receives from pPE NLRI advertisement for the protected labeled
prefix P/m containing the usual BGP next-hop attribute "bgpNH"
o Receives from pPE the mapping (bgpNH,pNH). This means that if
iPE wants to protect traffic normally tunneled to "bgpNH"
against pPE failure, the iPE MUST tunnel the traffic to "pNH"
instead of "bgpNH"
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o Receives the triplet (pNH,rNH,vL). The triplet (pNH,rNH,vL)
means that if iPE chooses rNH as the repair next-hop for the
traffic tunneled to the protected next-hop pNH, then iPE has
to use the vector label "vL" while tunneling traffic. The
method of using the vector label "vL" is described in the
forwarding behavior in Section 2.2 and 2.3.
o Penultimate Hop
o Receives the "pNH" from pPE
o As such, it knows the pNH needs certain special treatment as
described in the forwarding behavior in Section 2.2 and 2.3.
o Penultimate hop advertises "pNH" as its own prefix into IGP.
The penultimate hop advertises pNH so that when pPE is lost,
nodes continue to forward the traffic towards the original pPE
and hence get protected by the rP. This behavior is required
until BGP on the iPE's recalculate and start forwarding
traffic towards an alternative PE.
o Penultimate hop advertises "pNH" as its own prefix into IGP
but with one of the following conditions
. For link-state IGPs, "pNH" MAY be advertised with
*maximum metric* so as not to affect the path taken by
the traffic flowing from iPE's to pPE's
. For distance vector IGPs, the penultimate hop advertises
metric of "pNH" as follows
PHP-metric(pNH) =
pPE-metric(pNH) + metric-From-PHP-to-pPE
That is, the metric advertised by the penultimate hop for
pNH equals the metric advertised by pPE for pNH plus the
metric from the penultimate hop to pPE
. This way the advertisement of pNH by the penultimate hop
into IGP does not impact the path taken by the traffic
from iPE's to pPE's
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. When does the penultimate hop stop advertising pNH as its
own prefix? The penultimate hop should continue to
advertise pNH long enough for iPE's to re-converge.
Advertising pNH longer than necessary is harmless because
iPE's would have already re-converged to a new BGP next-
hop and hence no traffic will be attracted to the non-
existing pNH. The specific period length can be subject
to configuration but the default value may be in the
order of 2-3 minutes
o Repairing core router "rP" (which may also be the penultimate hop)
o Receives the triplet (pNH,rNH,vL) from pPE
o Creates a distinct label context for "pNH"
. In LDP core, the context is identified by the IGP label
of pNH
. In an IP core, the context is identified by the "pNH"
address itself.
o Inserts the label vL in the label context identified by pNH.
. The forwarding entry for vL in the label context of pNH
is
. Swap vL with the IGP label of rNH
. Forward the packet towards rNH
o Installs the following forwarding entry for pNH
. If pNH is not reachable, pop the label for pNH and lookup
the label underneath the label of pNH in the label
context of pNH
. Otherwise, forward the packet to pNH as usual
What is left it to outline the forwarding behavior before and after
the failure of "pNH".
2.2. Forwarding behavior at Steady State (while pPE is reachable)
This section outlines the packet forwarding procedure when pPE is
still reachable.
1. Ingress PE (iPE) receives a packet matching P/m from an external
neighbor and reachable via pPE
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2. Ingress PE: Pushes *four* labels
o Bottom label: VPN label advertised by pPE
o Second label: rL
o Third label: vL corresponding to chosen rNH
o Top label: IGP label towards pNH (not the bgpNH attribute)
o In pushing the labels "vL" following by "rL", iPE practically
encodes the chosen repair path into the packet.
3. Penultimate Hop
a. Receives a packet with top label bound to pNH
b. Pops *three* labels *all the time*.
c. Sends packet to pNH
4. Protected Egress PE (pPE)
a. Receives a packet with top label as VPN label
b. Forwards the packet as usual
Thus the packet can be delivered correctly to its destination.
2.3. Forwarding behavior when pPE Fails
The repairing router "rP" directly connected to a failure detects
that pNH is no longer reachable. The following steps are applied.
1. Repairing router "rP"
a. Receives packet with top label bound to pNH
b. pNH is not reachable
c. Pop the label of pNH. The vector label "vL" is right under the
label of pNH
d. Lookup "vL" in the label context identified by the label of
"pNH". The lookup yields a rewrite label corresponding to the
chosen rNH
e. Swap the top label with the label of rNH
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f. Send packet towards rNH
g. In effect, the repairing router uses the vector label to find
the repair PE chosen by the ingress PE
2. Penultimate hop of rPE
a. rNH is not a protected NH for rPE
b. Thus the penultimate hop employs the usual penultimate (single
label) hop popping and then forwards the packet to rPE
3. Repair PE (rPE)
a. Receives packet with top label rL (which rPE advertised) and
the bottom label is the regular VPN label advertised by the
primary PE "pPE"
b. Make a lookup on "rL"
c. rL per CE
i. Pop *two* labels.
ii. Send to correct CE
d. rL per VRF
i. Pop *two* labels.
ii. Make IP lookup in appropriate VRF
iii. Send to the CE
To protect unlabeled traffic there is no need for dual label popping
or "rL". Instead, all the repairing router needs to do when it
detects that "pNH" is no longer reachable is to re-tunnel the packet
towards "rNH" in a regular LSP
The next section presents the solution in an IP core.
3. Overview of the BGP FRR using Vector Labels in an IP Core
This section describes the BGP FRR using vector labels solution in an
IP core for both labeled (AFI/SAFI 1/4, 2/4, 1/128, and 2/128) and
unlabeled (AFI/SAFI 1/1, 2/1, 1/2, and 2/2) protected prefixes.
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The primary difference between a MPLS core and an IP core is that the
tunnels between edge routers are IP based such as [5][6][7]. In this
section, we propose two alternatives: A completely pure IP core and a
hybrid IP/MPLS core
3.1. Pure IP Core
In this section, we propose a scheme by which core routers are
incapable of handling any kind of MPLS labels.
3.1.1. Control Plane
The pPE still needs to advertise the mapping (bgpNH,pNH) as in
Section 2 but it does not allocate or advertise a vector label.
The rPE advertises rL with protected prefixes to all its iBGP peer as
in MPLS core solution in Section 2.
Assume iPE decides that rPE is the repair PE for a protected prefix.
o iPE pushes the usual VPN label for labeled prefixes
o iPE pushes the repair label "rL" advertised by the chosen rPE
o iPE pushes *two* IP tunnel headers on the packet
o Repair tunnel header. This will be the inner tunnel header with
destination address rNH towards the rPE
o Protected tunnel header: This will be the outer tunnel header
with destination address pNH towards the pPE
3.1.2. Forwarding plane during Steady State (when pPE is reachable)
1. iPE pushes the VPN label and the repair label followed by the two
tunnel headers described in the previous section
2. rP: No special behavior necessary
3. pPE
a. Decapsulates *two* tunnel headers and the repair label "rL"
b. Uses the contents of the packet underneath
3.1.3. Forwarding plane at Failure (when pPE is unreachable)
1. iPE is not yet aware of the failure so its behavior remains the
unchanged.
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2. rP
a. Decapsulates the outer tunnel header towards pNH
b. Uses the repair tunnel header to forward the packet towards
rPE
3. rPE
a. Decapsulates the tunnel header
b. Uses the repair label "rL" to forward the packet to the
correct CE
i. Pop rL and the VPN label under it
ii. Use the forward the packet to the correct CE
3.2. Hybrid IP core
In this section, we assume that rP is capable of handling MPLS labels
3.2.1. Control Plane
The pPE needs to advertise the mapping (bgpNH,pNH). iPE also needs to
allocate a vector label for each known rPE and advertise the mapping
(pNH,rNH,vL) to all its iBGP peers and to candidate repair core
routers. This behavior is identical to iPE behavior in MPLS core in
Section 2.
The rPE advertises rL with protected prefixes to all its iBGP peer as
in the case of MPLS core described in Section 2.
Assume iPE decides that rPE is the repair PE for a given prefix:
o iPE pushes the usual VPN label for labeled prefix
o iPE pushes the repair label "rL" advertised by the chosen rPE
o Pushes the vector label of the chosen rPE
o iPE pushes two the protected tunnel header: This will be the outer
IP tunnel header with destination address pNH towards the pPE
rP behavior is identical to its behavior in an MPLS core in Section
2.
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3.2.2. Forwarding Plane during Steady State (when pPE is reachable)
4. iPE pushes the two tunnel headers described in the previous
section
5. rP: No special behavior necessary
6. pPE
a. Decapsulates the outer tunnel headers plus *two* labels (vL
and rL
b. Uses the contents of the packet after decapsulation to forward
the packet
3.2.3. Forwarding plane at Failure (when pPE is unreachable)
7. iPE is not yet aware of the failure so its behavior remains the
same
8. rP
a. Decapsulates the tunnel header towards pNH
b. Pops the vector label "vL"
c. Looks up the vector label "vL" in the label context identified
by pNH. The lookup should yield the rNH
d. Encapsulates the packet into a tunnel header with destination
address rNH and forwards the packet towards rPE
9. rPE
a. Decapsulates the tunnel header
b. Uses the repair label "rL" to forward the packet to the
correct CE
i. Pop *two* labels for labeled traffic
ii. Forward the packet to the correct CE
4. Rules for Choosing and Managing the Repair path
This section specifies rules governing how a protectable edge
router pPE chooses and advertises the repair path. Other than the
rules in this section, the method of choosing the repair path is
beyond the scope of this document.
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4.1. General Rules for Managing the Repair Path
This section specifies general rules for choosing the repair path
for both labeled and unlabeled prefixes.
1. A repair PE MUST be another edge router that advertises the same
prefix to the protected edge router pPE via IBGP peering.
2. If a repairing P router "rP" determines that the path taken by
the repair tunnel to a repair edge router rPE passes through the
protected edge router pPE, then the repairing router "P" MUST
NOT install this repair path in its forwarding plane. Instead,
the repairing "p" router MAY use other paths that do not pass
through pPE or use existing core FRR mechanisms such as [12],
[13], and [14].
3. Let the protected next-hop pNH match the IGP route pR. If the
"rP" determines that the repair tunnel to a repair edge router
passes through a next-hop of the IGP route pR, then the
repairing router SHOULD NOT install this repair path in its
forwarding plane.
4. A protected next-hop uniquely identifies an protected PE within
a BGP-free core. Thus a protected next-hop NH MUST NOT be
advertised by two different pPEs.
5. At any point in time, for the same primary and repair next-hops
pNH and rNH, only one advertisement is valid. Thus for the same
value of pNH and rNH, an advertisement of the pair (pNH,rNH)
MUST override or be preceded by the withdrawal of any previously
advertised pair (pNH,rNH).
6. If the repair PE "rPE" advertises one or more protected next-
hops, then the repair next-hop "rNH" MUST be different from any
protected next-hop "pNH" advertised by rPE
If rules (1), (2), and(3), then the tunnel to the repair edge router
rPE does not provide protection against the failure of the edge node
ePE. Instead it provides core protection against the failure of the
path through the core leading to the protected edge node pPE. Thus
existing core FRR protection mechanisms such as those specified in
[12], [13], and [14] can be used instead.
Rules (4), (5), and (5) ensures that there is no ambiguity about the
primary and repair next-hops
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4.2. Rules for Choosing the Repair Path for Labeled Prefixes
This section specifies rules in additions to those mentioned in
Section 4.1 by which an edge router iPE chooses and advertises the
repair path for a protected labeled prefix P/m.
A edge router iPE MUST only choose the edge router rPE and the
underlying repair label rL as a repair path for the prefix P/m
if the "rL" allocated on per-VPN or per-CE/per-next-hop basis.
The reason for this rule is that "rL" is advertised as path
attributes in MP/BGP updates. If "rL" is allocated on per-prefix
basis, then attribute packing will be severely impacted
5. Inter-operability with Existing IP FRR Mechanisms
Current existing IP FRR mechanisms can be divided into two
categories: core protection and edge protection. Core protection
techniques, such as [12], [13], and [14], provide protection against
internal node and/or link failure. Thus the technique proposed in
this document is not related to existing IP FRR mechanisms. If the
failure of an internal node or link results in completely
disconnecting a protectable edge node, then an administrator MAY
configure the repairing router to prefer the technique proposed in
this document over existing IP FRR mechanisms.
Edge protection techniques, such as [16] provide protection against
the failure of the link between PE and CE routers. Thus existing PE-
CE link protection can co-exist with the techniques proposed in this
document because the two techniques are independent of each other.
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6. Example
We will use and LDP core as an example. Consider the diagram
depicted in Figure 2 below. We assume that the PEs advertise repair
labels as specified in [15]
+-----------------------------------+
| |
| LDP Core |
| |
| PE1 Lo = 9.9.9.1
| |\
| | \
| | \
| | \
| | CE1.......VRF "Blue"
| | / (10.0.0.0/8)
| | / (11.0.0.0/8)
| | /
| |/
PE11 P--------PE0 Lo1 = 1.1.1.1/32
| |\ Lo2 = 1.1.1.2/32
| | \
| | \
| | \
| | CE2.......VRF "Red"
| | / (20.0.0.0/8)
| | / (21.0.0.0/8)
| | /
| |/
| PE2 Lo = 9.9.9.2
| |
| |
+-----------------------------------+
Figure 2 : Edge node BGP FRR in LDP core
o In Figure 2, PE0 is the pPE for VRFs "Blue" and "Red". PE1 and PE2
are the rPEs for VRFs "Blue" and "Red", respectively. VRF Blue has
10.0.0.0/8 and 11.0.0.0/8 and VRF Red has 20.0.0.0/8 and
21.0.0.0/8
o Assuming PE0 uses per prefix label allocation, PE0 assigns the VPN
labels 4100, 4200, 4300, and 4400 to 10.0.0.0/8, 11.0.0.0/8,
20.0.0.0/8, and 21.0.0.0/8 respectively. PE0 advertises the
prefixes 10.0.0.0/8, 11.0.0.0/8, 20.0.0.0/8, and 21.0.0.0/8 using
MP/BGP as usual
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6.1. Control Plane
1. rPEs Allocate and advertise Repair labels
a. Acting as a rPE, PE1 allocates (on per-CE basis) and
advertises a repair label rL1=3100 with the prefixes
10.0.0.0/8 and 11.0.0.0/8 to all iBGP peers
b. Similarly, PE2 allocates and advertises the repair label
rL2=3200 with the prefixes 20.0.0.0/8 and 21.0.0.0/8
2. pPE calculates and advertises the pNH
a. Assume that PE0 uses "Loopback0" as the BGP next-hop, PE0
automatically picks Loopback2 as the pNH. As such PE0
advertises (bgpNH,pNH)=(1.1.1.1,1.1.1.2) to all iBGP peers
including the iPE PE11.
b. When the iPE "PE11" receives (bgpNH,pNH)=(1.1.1.1,1.1.1.2),
PE11 understands that if it wants to protect traffic whose
bgpNH=1.1.1.1 against the failure of the node 1.1.1.1, PE11
has to tunnel the traffic to 1.1.1.2 instead of 1.1.1.1
3. pPE allocates and advertizes vector labels
a. On receiving the repair labels 3100 and 3200 from PE1 and
PE2, respectively, PE0 detects that there are two rPEs: PE1
and PE2. AS such PE0 assigns two vector labels vL1 = 1100
and vL2 = 1200 to PE1 and PE2, respectively
b. PE0 advertises (1.1.1.2, 9.9.9.1, 1100) and (1.1.1.2,
9.9.9.2, 1200) to all iBGP peers, including the ingress PE
PE11
c. On receiving (1.1.1.2, 9.9.9.1, 1100) and (1.1.1.2, 9.9.9.2,
1200), the ingress PE PE11 understand that if it were to
pick 9.9.9.1 as the rPE for packet tunneled to 1.1.1.2, then
it has to push the vector lagbel 1100. Similarly, to protect
a packet tunneled to 1.1.1.2 using the rPE 9.9.9.2, then it
has to push the vector label 1200.
d. PE0 also advertises (1.1.1.2, 9.9.9.1, 1100) and (1.1.1.2,
9.9.9.2, 1200) to all candidate repairing core routes,
including the core router "P".
4. The repairing core router creates the repair state
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a. Acting as a rP, the core router "P" receives the
advertisements (1.1.1.2, 9.9.9.1, 1100)and (1.1.1.2,
9.9.9.2, 1200) from PE0.
b. rP understands that it has to pop *3* labels when it
receives a packet whose top label is the LDP label for
1.1.1.2
c. rP creates a label context identified by the LDP label of
1.1.1.2/32
d. rP inserts the following two label entries in the created
label context
i. 1100-->9.9.9.1
ii. 1200-->9.9.9.2
5. The ingress PE calculates the rPEs
a. PE11 receives an advertisement for 10.0.0.0/8, 11.0.0.0/8,
20.0.0.0/8, and 21.0.0.0/8 from PE0 with the BGP next-
hop=1.1.1.1. Because PE11 received
(bgpNH,pNH)=(1.1.1.1,1.1.1.2) from PE0, then PE11 knows that
to protect traffic tunneled to PE0, it has to tunnel the
traffic to 1.1.1.2 instead of 1.1.1.1
b. PE11 receives an advertisement from PE1 for 10.0.0.0/8 and
11.0.0.0/8 with the repair label 3100
c. Hence PE11 picks PE1 as the rPE for the prefixes 10.0.0.0/8
and 11.0.0.0/8 with rNH=9.9.9.1 and rL=3100. Remember that
the vector label for 9.9.9.1 is 1100.
d. Similarly, PE11 receives an advertisement from PE2 for
20.0.0.0/8 and 21.0.0.0/8 with the repair label 3200
e. Hence PE11 picks PE2 as the rPE for the prefixes 10.0.0.0/8
and 11.0.0.0/8 with rNH=9.9.9.2 and rL=1200. Remember that
the vector label for 9.9.9.1 is 1100.
6.2. Forwarding Plane at Steady State (When PE0 is reachable)
1. Ingress PE PE11
a. Traffic for VRF "Blue"
i. PE11 receives a packet for VRF Blue with destination
address 10.1.1.1 from an external router.
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ii. PE11 pushes the following labels
1. The VPN label 4100
2. The Repair label 3100
3. The vector label 1100
4. The LDP label for 1.1.1.2
b. Traffic for VRF "Red"
i. PE11 receives a packet for VRF Red with destination
address 20.1.1.1 from an external router
ii. PE11 pushes the following labels
1. The VPN label 4300
2. The Repair label 3200
3. The vector label 1200
4. The LDP label for 1.1.1.2
2. Penultimate Hop of PE0 (Which is also the rP "P")
a. Receives a packet with top label for the protected next-hop
1.1.1.2
b. Pops *3* labels
c. Forwards the packet to 1.1.1.2
3. Protected PE PE0
a. Traffic for VRF "Blue"
i. PE0 receives traffic with the top label 4100.
ii. 4100 is the VPN label for VRF "Blue"
iii. PE0 pops the label 4100 and forwards the packet to CE1
b. Traffic for VRF "Red"
i. PE0 receives traffic with the top label 4300.
ii. 4300 is the VPN label for VRF "Red"
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iii. PE0 pops the label 4300 and forwards the packet to CE2
6.3. Forwarding Plane at Failure (When PE0 is not reachable)
1. The ingress PE PE11
Does not know about the failure yet and hence it does not
change its behavior.
2. Repair PE rP
a. Traffic for VRF "Blue"
i. Receives a packet with the top label being the LDP label
for 1.1.1.2
ii. 1.1.1.2 is not reachable
iii. Pop the LDP label of 1.1.1.2. The vector label 1100 is
under it
iv. Lookup the vector 1100 in the label context of 1.1.1.2.
The lookup yields the LDP label of the rNH 9.9.9.1
v. Swap the vector label 1100 with the LDP label of the of
9.9.9.1 and forward the packet towards PE1
b. Traffic for VRF "Red"
i. Receives a packet with the top label being the LDP label
for 1.1.1.2
ii. 1.1.1.2 is not reachable
iii. Pop the LDP label of 1.1.1.2. The vector label 1200 is
under it
iv. Lookup the vector 1200 in the label context of 1.1.1.2.
The lookup yields the LDP label of the rNH 9.9.9.2
v. Swap the vector label 1200 with the LDP label of the of
9.9.9.2 and forward the packet towards PE2
3. The repair Router "PE1"
a. The penultimate hop of PE1 performs the usual penultimate hop
popping
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b. PE1 receives a packet with the top label equals the repair
label 3100, which was allocated on per-CE basis and points to
CE1
c. PE1 pops *2* labels and forwards the packet to CE1
4. The repair Router "PE2"
a. The penultimate hop of PE2 performs the usual penultimate hop
popping
b. PE1 receives a packet with the top label equals the repair
label 3200, which was allocated on per-CE basis and points to
CE2
c. PE2 pops *2* labels and forwards the packet to CE2
7. Security Considerations
No additional security risk is introduced by using the mechanisms
proposed in this document
8. IANA Considerations
No requirements for IANA
9. Conclusions
This document proposes a method that allows fast re-route
protection against edge node failure or complete disconnected from
the core in a BGP-free core. The method proposed has the following
advantages
o Very scalable:
o No router has to copy the routing table of another router
o Minimum additional prefixes injected in the core. In fact, at
most one additional prefix per pPE is injected and only if
there is no spare IP address on the pPE
o Minimal provisioning overhead:
o If there is a spare IP address on the pPE, then the
provisioning effort is just enablement. If not, then the
provisioning effort is just to configure a distinct IP address
on each pPE to act as the pNH.
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o Absolutely no restriction on which PE is connected to which
VRF.
o On a PE where BGP FRR is already configured, moving,
connecting, or disconnecting a CE to/from the PE requires zero
operator intervention to protect prefixes.
o Immunity to misconfigation: the only configuration that may be
required is a distinct pNH on each pPE. The mapping (bgpnh,pPE)
and (pNH,rNH,vL) is advertised to all BGP peers. If the operator
configures the same pNH on two different pPE, then the
misconfiguration will be detected almost immediately
o No Need for IP or TE FRR: Because the exit point of the repair
tunnel from rP to rPE is different from the primary tunnel exit
point
o Works in both MPLS core and IP core
o Works with per-CE, per-VRF and per-prefix label allocation
o Can be incrementally deployed. There is no flag day. Different
routers can be upgraded at different times
o Zero impact on the paths taken by traffic: Enabling/deploying the
feature described in this document has no effect on the paths
taken by traffic at steady state
10. References
10.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4
(BGP-4), RFC 4271, January 2006
[3] Bates, T., Chandra, R., Katz, D., and Rekhter Y.,
"Multiprotocol Extensions for BGP", RFC 4760, January 2007
[4] Malhotra, P. and Rosen, E., " The BGP Encapsulation Subsequent
Address Family Identifier (SAFI) and the BGP Tunnel
Encapsulation Attribute", RFC 5512, April 2009
[5] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed., "Layer Two
Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
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[6] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[7] Perkins, C., "IP Encapsulation within IP", RFC 2003, October
1996.
10.2. Informative References
[8] Marques,P., Fernando, R., Chen, E, Mohapatra, P., Gredler, H.,
"Advertisement of the best external route in BGP", draft-ietf-
idr-best-external-04.txt, April 2011.
[9] Wu, J., Cui, Y., Metz, C., and E. Rosen, "Softwire Mesh
Framework", RFC 5565, June 2009.
[10] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC 4364, February 2006.
[11] De Clercq, J. , Ooms, D., Prevost, S., Le Faucheur, F.,
"Connecting IPv6 Islands over IPv4 MPLS Using IPv6 Provider
Edge Routers (6PE)", RFC 4798, February 2007
[12] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[13] Shand, S., and Bryant, S., "IP Fast Reroute", RFC5714, January
2010
[14] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[15] Bashandy, A., Pithawala, P., and Heitz, J., "Scalable, Loop-
Free BGP FRR using Repair Label", draft-bashandy-idr-bgp-
repair-label-02.txt", July 2011
[16] O. Bonaventure, C. Filsfils, and P. Francois. "Achieving sub-50
milliseconds recovery upon bgp peering link failures," IEEE/ACM
Transactions on Networking, 15(5):1123-1135, 2007
11. Acknowledgments
Special thanks to Clarence Filsfils, Eric Rosen, Stewart Bryant,
and Pradosh Malhotra for the valuable comments
This document was prepared using 2-Word-v2.0.template.dot.
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Appendix A. Other Algorithms to Allocate and Disseminate Vector labels
This section outlines two alternate algorithms for Allocating and
distributing vector label "vL" to "rPE" mapping. The alternate
algorithms can be divided into two categories, iPE chooses the repair
path and pPE chooses the repair path
A.1. iPE chooses the repair path
A.1.1. Allocating Vector Labels using a Hash Function
In the method of allocating and advertising vector labels outlined in
Sections 2 and 3 each pPE allocates and binds a vector label to
each known rPE. As a result, the same rPE may be bound to multiple
vector labels by multiple pPEs and thus requiring additional storage
on the rP. In this section, we propose a method by which a vector
label is computed using a hash function based on the numerical value
of rNH
A.1.1.1.1. Calculating and distributing the mapping rNH->vL to
different routers
1. We assume that all routers in a BGP free core, including edge
router, agree on the set of candidate repair next-hops. This can
be achieved via default behavior (e.g. all host routes) or some
sort of configuration, such as ISIS administrative tags
2. No need for pPE to advertise the (pNH,rNH,vL) to iPE or rP
3. Each candidate rP and iPE calculates the vL for each candidate
repair next-hop rNH
4. The rP inserts the calculated mapping vL-->rNH in a "repair label
context" that is common for all protected PEs instead of having
separate label context for each pPE.
5. If iPE chooses rNH as the repair next-hop for traffic tunneled to
pNH, iPE calculates the vL corresponding to the chosen rNH and
pushes vL as described in Sections 2.1.
6. On pPE failure, the lookup for vL occurs in the common "repair
label context"
IN the next subsections, we outline two risks of using the hash
function for rNH-->vL mapping.
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A.1.1.1.2. Risk of Mis-configuration leading to Mismatch in rNH-->vL
Mapping
1. Due to misconfiguration, some routers may not have the identical
sets of candidate repair next-hops "rNH's" or use the same hash
function to calculate vL. For example, an upgraded router may have
a new hash function enabled or the ISIS administrative tags may
not be associated with all candidate rNHs
2. To alleviate this risk, we propose that each rPE associates the
calculated value of vL for each rNH in an optional TLV in IGP
3. If a router finds that its calculated value for rNH-->vL is
different from the value received from the corresponding rPE, then
the router can raise an alarm,
A.1.1.1.3. Risk of forwarding to Incorrect VRF during convergence only
Identical mapping of rNH-->vL is only guaranteed if the set of
candidate rNH is the same on all routers. Because each router
calculates rNH-->vL independently, there is a minor risk of
forwarding to incorrect VRF. Consider the following example
1. The risk exists even if every rPE advertises the vL of its own rNH
2. Two rNH's, say rNH1 and rNH2, map to the same vL, even if rNH1,
and rNH2 protect different prefixes
3. rPE1 and rPE2 have not yet heard each other mappings
4. iPE learns about vL-->rNH1 before vL-->rNH2
5. rP/PLR learns about vL-->rNH2 before vL-->rNH1
6. If pPE fails during the short period before iPE and rP can detect
the vL collision, rP re-routes traffic to rNH2 but the repair
label pushed by iPE is for rNH1.
A.1.2. pPE Allocates and advertises vL with protected prefixes
1. pPE allocates a single vL for all prefixes reachable via the same
CE. If two prefixes bound to the same vL are protected by
different rPE's, then pPE MUST re-advertise the second protectable
prefix with a different vL to all ingress PEs
2. pPE always advertises (pNH, vL) with protected prefixes as
optional attributes all the time even if there is no rPE
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3. iPEs and the pPE agree on the way to pick the rPE. E.g. if there
are multiple rPEs, choose the one with lowest router ID
4. When rPE advertises rL for a protected prefix
a. Both pPE and iPE will get the update
b. Both pPE and iPE will choose the same rPE for the protected
prefix
5. iPE associate the correct triplet (pNH, vL, rL) with protected
prefixes without getting a re-advertisement for the prefix from
pPE
6. pPE Informs about (pNH, rNH, vL)
7. Hence rP/PLR knows that the vector label vL maps to rNH in the
label context of pNH.
8. rP/PLR inserts vL-->rNH in the context of pNH
A.1.2.1.1. Risk of forward to Incorrect VRF during Convergence Only
The conditions for the risk to exist
o More than one rNH, say rNH1 and rNH2, protect the same prefix
o iPE learns about rNH1 and has not yet learnt about rNH2
o pPE learns about rNH2 and has not yet learnt about rNH1
o pPE fails during this time period
How incorrect forwarding can occur
o pPE maps vL to rPE2 on rP/PLR while iPE maps vL to rPE1
o pPE fails during this short period,
o rP/PLR re-routes the packet to rPE2 but the repair label pushed by
iPE belongs to rPE1 (say rL1)
A.2. pPE chooses rPE and distributes the mapping of vL-->rNH
In Sections 2, 3, and A.1 the ingress PE chooses the rPE for every
protectable prefix. While it causes less churn because there is never
a need to re-advertise protected prefixes, it is difficult to
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configure a policy to control the choice of the rPE if the policy has
to be applied to all iPEs. In this Section, we propose an algorithm
to select rPE and advertise vL-->rNH via pPE instead of iPE
1. pPE allocates a single vL for all prefixes reachable via the same
CE
a. We assume that prefixes reachable via the same CE or belong to
the same VRF are protectable by the same rPE
b. If two prefixes bound to the same vL are protected by
different rPE's, then pPE MUST re-advertise the second
protectable prefix with a different vL to all ingress PEs
2. pPE always advertises (pNH, vL) with protected prefixes as
optional attributes all the time even if there is no rPE. Remember
that pNH,vL) means the vector label for protected traffic tunneled
to pNH is vL
3. Based on rPE advertisement, pPE decides that the repair next-hop
for a given protected prefix P/m is rNH. pPE sends the mapping
(vL,rNH) similar to [4] as a separate advertisement to iPEs
4. Suppose two prefixes prefixes P1/m1 and P2/m2 are associated with
the same vector label vL1 but are protected by two different
repair PEs: rNH1 and rNH2
a. Re-advertise P2/m2 with a new vector label vL2
b. pPE sends the mapping( vL1,rNH1) and (vL2,rNH) in a separate
advertisement to iPEs
c. The re-advertisement of the prefix p2/m2 with the new vector
label vL2 must be done BEFORE sending the vector label mapping
to guarantee correct forwarding
Unlike the schemes in Sections A.1.1 and A.1.2 there is no risk of
forwarding to incorrect VRF because pPE is the only source of mapping
vL-->rNH
A.3. Combination of iPE and pPE Choosing rPE
o pPE can choose the rPE by specifying the mapping of vL to rNH, re-
advertising/advertising the protected prefix with rNH, or a
combination of both
o pPE decides the prefixes for which it chooses the rPE based on
various factors. For example
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o Option 1: The operator can configure the prefixes for which the
pPE can choose the rPE.
o Option 2: If there is more than one rPE, then pPE chooses the
rPE. Otherwise, it is left to iPE
o There are probably other options
o As long as the pPE does not specify the rPE for a prefix, then the
iPE is free to choose the rPE, otherwise, the iPE has to abide by
pPE choice
A combination of iPE and pPE choosing the rPE reduces the
provisioning overhead when configuring a policy to choose the rPE at
the expense of increasing the churn.
Authors' Addresses
Ahmed Bashandy
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: bashandy@cisco.com
Nagendra Kumar
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: naikumar@cisco.com
Maciek Konstantynowicz
Cisco Systems
170 West Tasman Dr, San Jose, CA 95134
Email: mkonstan@cisco.com
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