Internet DRAFT - draft-bowers-rtgwg-mrt-applicability-to-8021qca
draft-bowers-rtgwg-mrt-applicability-to-8021qca
Routing Area Working Group C. Bowers
Internet-Draft Juniper Networks
Intended status: Informational J. Farkas
Expires: January 4, 2016 Ericsson
July 3, 2015
Applicability of Maximally Redundant Trees to IEEE 802.1Qca Path Control
and Reservation
draft-bowers-rtgwg-mrt-applicability-to-8021qca-01
Abstract
IEEE 802.1Qca Path Control and Reservation (PCR) [IEEE8021Qca] uses
the algorithm specified in [I-D.ietf-rtgwg-mrt-frr-algorithm] to
compute Maximally Redundant Trees (MRTs) to be used for the
protection of data traffic in bridged networks. This document
discusses the applicability of the MRT algorithm to 802.1Qca.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. How 802.1Qca uses the MRT algorithm . . . . . . . . . . . . . 3
2.1. MRT Explicit Trees in 802.1Qca . . . . . . . . . . . . . 3
2.2. 'MRT with GADAG' Explicit Trees in 802.1Qca . . . . . . . 3
2.3. MRT Explicit Trees as Strict Trees . . . . . . . . . . . 4
3. Other considerations . . . . . . . . . . . . . . . . . . . . 4
3.1. Unequal link metrics . . . . . . . . . . . . . . . . . . 4
3.2. Computation of MRT-Blue and MRT-Red next-hops from the
point of view of other nodes . . . . . . . . . . . . . . 5
3.3. Recalculation of MRTs . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. Informative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
IEEE 802.1Qaq Shortest Path Bridging (SPB) [IEEE8021aq] is an
amendment to IEEE Std 802.1Q that allows bridged frames to travel on
the shortest path between their source and destination(s), as opposed
to traveling along paths determined by shared spanning trees.
[IEEE8021aq] and [RFC6329] specify extensions to IS-IS that allow
bridges to share the topology information needed to construct
shortest path trees. These extensions are referred to here as ISIS-
SPB. [IEEE8021aq] has been already incorporated in [IEEE8021Q].
[IEEE8021Qca] is an amendment to [IEEE8021Q] that specifies explicit
path control, bandwidth assignment, and protection mechanisms for
data flows for bridged networks. [IEEE8021Qca] is an extension to
IS-IS that builds upon the ISIS-SPB extensions and extends them
further as described in [I-D.ietf-isis-pcr]. These extensions
(referred to here as ISIS-PCR) allow bridges to share the information
needed to construct explicit trees.
[IEEE8021Qca] specifies five different methods for the construction
of explicit trees as well as how to share the information needed to
construct these trees. These five different methods of explicit
trees are referred to as Strict Tree, Loose Tree, Loose Tree Set,
MRT, and MRT with GADAG. This document is concerned with the MRT and
'MRT with GAGAG' explicit tree methods (algorithms). Both methods
produce Maximally Redundant Trees (MRTs)
[I-D.ietf-rtgwg-mrt-frr-architecture].
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This document is intended to explain the relationship between
[IEEE8021Qca] and [I-D.ietf-rtgwg-mrt-frr-algorithm]. The text
should not be interpreted as normative with respect to either.
2. How 802.1Qca uses the MRT algorithm
The algorithm for computing Maximally Redundant Trees in
[I-D.ietf-rtgwg-mrt-frr-algorithm] has been specified with a focus on
supporting fast-reroute for the protection of unicast IP and LDP
traffic, as described in [I-D.ietf-rtgwg-mrt-frr-architecture]. The
computation described in [I-D.ietf-rtgwg-mrt-frr-algorithm] starts
with the topology of an IGP area, prunes it to form an MRT Island
topology, constructs a GADAG, and then uses that GADAG to construct a
complete set of destination-based MRT-Blue and MRT-Red trees rooted
at each node in the MRT Island, where each tree spans all nodes in
the MRT Island. [IEEE8021Qca] supports this mode of operation, but
it also supports other modes of operation as described below.
2.1. MRT Explicit Trees in 802.1Qca
In [IEEE8021Qca], the flooding of an SPB Instance sub-TLV ([RFC6329])
with the MRT ECT Algorithm value in the absence of a Topology sub-TLV
([I-D.ietf-isis-pcr]) results in the creation of an MRT-Blue and an
MRT-Red tree rooted at each node in the domain, and each tree spans
all nodes in the domain. This is equivalent to the behavior
described in [I-D.ietf-rtgwg-mrt-frr-algorithm].
In addition, [IEEE8021Qca] allows one to affect both the number and
structure of the MRT-Blue and Red trees by including the Topology
sub-TLV in the MT-Capability TLV (type 144 [RFC6329]). In the
context of the MRT ECT Algorithm, Hop sub-TLVs in the Topology sub-
TLV specify nodes with flags that can indicate Root, Exclude, or Edge
Bridge. In the context of the MRT algorithm, all nodes with the
Exclude flag set are excluded from the MRT Island. The GADAG is then
computed for the MRT Island. For each node with the Root flag set,
an MRT-Blue and an MRT-Red tree rooted at that node is constructed.
Note that the Edge Bridge flag does not affect the construction of
the MRTs. It is used to determine which filtering database (FDB)
entries to install once the trees have been determined.
2.2. 'MRT with GADAG' Explicit Trees in 802.1Qca
In the previous section, each node will compute the same GADAG based
on the same topology information using the algorithm steps in
sections 5.4 and 5.5 of [I-D.ietf-rtgwg-mrt-frr-algorithm]. Each
node then applies the algorithm steps in section 5.6.5 of
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[I-D.ietf-rtgwg-mrt-frr-algorithm] to that GADAG, in order to compute
the MRT-Blue and MRT-Red next-hops for the trees of interest.
[IEEE8021Qca] can operate in a mode where each node is supplied with
a common GADAG (communicated via ISIS-PCR), from which each node then
determines the MRT-Blue and MRT-Red next-hops for the trees of
interest. That is, this mode of operation bypasses the algorithm
steps in section 5.4 and 5.5 of [I-D.ietf-rtgwg-mrt-frr-algorithm]
and only applies the algorithm steps in section 5.6.5 to the GADAG
communicated directly via ISIS-PCR.
This behavior is controlled in [IEEE8021Qca] by flooding an SPB
Instance sub-TLV with the 'MRT with GADAG' ECT Algorithm value as
well as a Topology sub-TLV. Hop sub-TLVs in this Topology sub-TLV
are used to describe the common GADAG using an ear decomposition.
The first Hop sub-TLV is the GADAG root followed by a sequence of Hop
sub-TLVs describing an ordered ear that terminates on the GADAG root.
Subsequent ears are described as sequences of Hop sub-TLVs. Setting
the Root flag for a given Hop sub-TLV indicates that MRT-Blue and Red
trees rooted at that node should be constructed.
2.3. MRT Explicit Trees as Strict Trees
A Path Computation Element can also implement each computation step
of [I-D.ietf-rtgwg-mrt-frr-algorithm] and compute the MRTs. The MRTs
can be then specified by Topology sub-TLVs, one for each. The SPB
Instance sub-TLV then conveys the ST ECT Algorithm value.
3. Other considerations
3.1. Unequal link metrics
[IEEE8021aq] specifies that if two SPB Link Metrics are different at
each end of a link, the maximum of the two values is used in SPB
calculations. In order to provide symmetry and maintain consistency
with [IEEE8021aq], [IEEE8021Qca] places the same requirement on the
Link Metrics for the topology graph that is used in the MRT
algorithm. In order to accomplish this, [IEEE8021Qca] makes the same
modification to link metrics before applying the MRT algorithm. This
change of metric values can affect the ordering of interfaces on a
given node through the Interface_Compare function in section 5 of
[I-D.ietf-rtgwg-mrt-frr-algorithm], which in turn can affect the
GADAG computed. The change of metric values can also affect the
results of the SPF_No_Traverse_Root function used in determining MRT
next-hops, since the SPF traversal depends on metric values.
This modification of link metrics applies ONLY to [IEEE8021Qca].
When the MRT lowpoint inheritance algorithm in
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[I-D.ietf-rtgwg-mrt-frr-algorithm] is applied to IP/LDP FRR, the
topology graph with the link metrics advertised by the IGP are used
without modification by the MRT algorithm.
3.2. Computation of MRT-Blue and MRT-Red next-hops from the point of
view of other nodes
In the algorithm described in [I-D.ietf-rtgwg-mrt-frr-algorithm] a
given node computes and installs its own MRT-Blue and MRT-Red next-
hops for all destinations. This computation is all that is required
for the IP/LDP FRR application to function properly. An individual
node does not need to compute the MRT-Blue and MRT-Red next-hops used
by other nodes. Instead the complete MRT tree structure is created
in the network as the result of each node computing and installing
the appropriate MRT next-hops. This is analogous to the way that
shortest path trees are instantiated in shortest path routing, with
each node needing to compute and install only its own shortest path
next-hops.
In some scenarios, a bridge using [IEEE8021Qca] may need to know more
than just its own MRT-Blue and MRT-Red next-hops. This can be
accomplished by having a bridge perform the MRT next-hop computation
specified in section 5.6.5 of [I-D.ietf-rtgwg-mrt-frr-algorithm] from
the point of view of one or more other bridges. The result of
computing an MRT next-hop from the point of view of another bridge is
the normative result. An implementation may use another method to
compute MRT next-hops from the point of view of remote bridges as
long as it produces the same result.
This does not modify the MRT algorithm with respect to its use for
the IP/LDP FRR application as described in
[I-D.ietf-rtgwg-mrt-frr-architecture].
3.3. Recalculation of MRTs
MRTs can be used for the protection of SPTs in a bridged network
similarly to IP/LDP FRR application of MRTs. A pair of MRT-Blue and
MRT-Red then protect the SPT rooted at the MRT Root. The MRTs are
only used for protection, i.e. MRTs do not carry traffic during
normal operation, similarly to IP/LDP FRR operations. The Point of
Local Repair (PLR) is responsible for redirecting traffic from SPTs
to MRTs upon detection of a failure event.
In 802.1Qca, recalculation of MRTs after a topology change follows
the general method specified in Section 12.2 of
[I-D.ietf-rtgwg-mrt-frr-architecture], with an additional
requirement. Immediately after a failure, the PLR or PLRs redirect
some traffic onto MRTs. In the meantime, all nodes receive
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notification of the failure, recompute SPTs, and install them in
their FIBs. The PLRs take the traffic off of the MRTs, and put the
traffic on the new SPTs. Based on
[I-D.ietf-rtgwg-mrt-frr-architecture], at this point it is safe
recompute and install the new MRTs corresponding to the new topology.
802.1Qca places an additional requirement on when it is safe to
install the new MRTs. The new MRTs should not be recomputed and
installed if there is any reason to suspect that the nodes of the
domain do not share a common view on the network topology. This is
in order to prevent loops in the MRT paths that may be used by PLRs
at the next failure event. The loop prevention method to be used for
MRTs in 802.1Qca is the Agreement Protocol, which is specified in
[IEEE8021aq] and also described in [AP].
Note that while 802.1Qca requires that the Agreement Protocol be used
to avoid loops on MRTs, it does not mandate the use of the Agreement
Protocol for the shortest path trees, where other loop mitigation
techniques can be used.
4. IANA Considerations
This document introduces no new IANA Considerations.
5. Security Considerations
The ISIS-PCR extensions for the use of the MRT algorithm are not
believed to introduce new security concerns.
6. Acknowledgements
The authors would like to thank Alvaro Retana for his suggestions and
review.
7. Informative References
[AP] Seaman, M., "Agreement Protocol", September 7, 2010,
<http://www.ieee802.org/1/files/public/docs2010/
aq-seaman-agreement-protocol-0910-v2.pdf>.
[I-D.ietf-isis-pcr]
Farkas, J., Bragg, N., Unbehagen, P., Parsons, G.,
Ashwood-Smith, P., and C. Bowers, "IS-IS Path Computation
and Reservation", draft-ietf-isis-pcr-00 (work in
progress), April 2015.
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[I-D.ietf-rtgwg-mrt-frr-algorithm]
Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
algorithm-02 (work in progress), January 2015.
[I-D.ietf-rtgwg-mrt-frr-architecture]
Atlas, A., Kebler, R., Bowers, C., Envedi, G., Csaszar,
A., Tantsura, J., and R. White, "An Architecture for IP/
LDP Fast-Reroute Using Maximally Redundant Trees", draft-
ietf-rtgwg-mrt-frr-architecture-05 (work in progress),
January 2015.
[IEEE8021aq]
IEEE 802.1, "IEEE 802.1aq: IEEE Standard for Local and
metropolitan area networks - Media Access Control (MAC)
Bridges and Virtual Bridged Local Area Networks -
Amendment 20: Shortest Path Bridging", 2012,
<http://standards.ieee.org/getieee802/
download/802.1aq-2012.pdf>.
[IEEE8021Q]
IEEE 802.1, "IEEE 802.1Q-2014: IEEE Standard for Local and
metropolitan area networks - Bridges and Bridged
Networks", 2014, <http://standards.ieee.org/findstds/
standard/802.1Q-2014.html>.
[IEEE8021Qca]
IEEE 802.1, "IEEE 802.1Qca Bridges and Bridged Networks -
Amendment: Path Control and Reservation - Draft 2.1",
(work in progress), June 23, 2015,
<http://www.ieee802.org/1/pages/802.1ca.html>.
[RFC6329] Fedyk, D., Ashwood-Smith, P., Allan, D., Bragg, A., and P.
Unbehagen, "IS-IS Extensions Supporting IEEE 802.1aq
Shortest Path Bridging", RFC 6329, April 2012.
Authors' Addresses
Chris Bowers
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: cbowers@juniper.net
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Janos Farkas
Ericsson
Konyves Kalman krt. 11/B
Budapest 1097
Hungary
Email: janos.farkas@ericsson.com
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