draft-ietf-ospf-shortcut-abr-00.txt   draft-ietf-ospf-shortcut-abr-01.txt 
Network Working Group A. D. Zinin Network Working Group A. D. Zinin
Internet Draft AMT Group Internet Draft AMT Group
Expiration Date: January 2000 July 1999 Expiration Date: April 2000 October 1999
File name: draft-ietf-ospf-shortcut-abr-00.txt File name: draft-ietf-ospf-shortcut-abr-01.txt
OSPF Shortcut ABR OSPF Shortcut ABR
Enhanced OSPF ABR Behavior Enhanced OSPF ABR Behavior
draft-ietf-ospf-shortcut-abr-00.txt draft-ietf-ospf-shortcut-abr-01.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet Drafts are working documents of the Internet Engineering Internet Drafts are working documents of the Internet Engineering
Task Force (IETF), its Areas, and its Working Groups. Note that other Task Force (IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as Internet Drafts. groups may also distribute working documents as Internet Drafts.
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administrator to avoid it or, if virtual links are still used, to administrator to avoid it or, if virtual links are still used, to
decrease the number of configured virtual links. decrease the number of configured virtual links.
This memo also describes possible situations where the proposed This memo also describes possible situations where the proposed
implementation can be used. implementation can be used.
Acknowledgements Acknowledgements
The author would like to acknowledge Christian Hopps and Peter Psenak The author would like to acknowledge Christian Hopps and Peter Psenak
for their help in finding weak points in early versions of this for their help in finding weak points in early versions of this
document and Thomas M. Thomas for reviewing the preceding version of document, Thomas M. Thomas for reviewing the preceding version of it,
it. and James Huang for pointing out potential problems described in
section 7.
Special thanks go to John Moy who contributed a lot to this document Special thanks go to John Moy who contributed a lot to this document
and provided a simpler algorithm representation, used herein. and provided a simpler algorithm representation, used herein.
Table of Contents Table of Contents
1 Overview ..................................................... 3 1 Overview ..................................................... 3
1.1 Introduction ............................................... 3 1.1 Introduction ............................................... 3
1.2 Motivation ................................................. 3 1.2 Motivation ................................................. 3
2 Description of Shortcut ABR behavior ......................... 5 2 Description of Shortcut ABR behavior ......................... 5
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3.1 Changes to Router-LSA origination .......................... 5 3.1 Changes to Router-LSA origination .......................... 5
3.2 Changes to routing table calculation ....................... 6 3.2 Changes to routing table calculation ....................... 6
3.3 Changes to Summary-LSA origination ......................... 9 3.3 Changes to Summary-LSA origination ......................... 9
4 Implementation Details ....................................... 10 4 Implementation Details ....................................... 10
5 Compatibility ................................................ 10 5 Compatibility ................................................ 10
6 Deployment Considerations .................................... 11 6 Deployment Considerations .................................... 11
6.1 Necessity of virtual links ................................. 11 6.1 Necessity of virtual links ................................. 11
6.2 Change of traffic patterns ................................. 11 6.2 Change of traffic patterns ................................. 11
6.3 Optimized inter-area routing ............................... 11 6.3 Optimized inter-area routing ............................... 11
6.4 Gradual deployment of Shortcut ABRs ........................ 13 6.4 Gradual deployment of Shortcut ABRs ........................ 13
7 Security Considerations ...................................... 13 7 Routing loops in transition periods .......................... 14
8 Appendixes ................................................... 14 8 Security Considerations ...................................... 16
A.1 Router-LSA ................................................. 14 9 Appendixes ................................................... 16
9 References ................................................... 15 A.1 Router-LSA ................................................. 16
10 Author's Address ............................................ 16 10 References .................................................. 17
11 Author's Address ............................................ 18
1 Overview 1 Overview
1.1 Introduction 1.1 Introduction
An OSPF routing domain can be split into several subdomains, called An OSPF routing domain can be split into several subdomains, called
areas, which limit the scope of LSA flooding. A router having attach- areas, which limit the scope of LSA flooding. A router having attach-
ments to multiple areas is called an "area border router" (ABR). The ments to multiple areas is called an "area border router" (ABR). The
primary function of an ABR is to provide its attached areas with primary function of an ABR is to provide its attached areas with
Type-3 and Type-4 LSAs (which are used for describing routes and Type-3 and Type-4 LSAs (which are used for describing routes and
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set ShortcutConfigured flag for a given area. set ShortcutConfigured flag for a given area.
Note that the nature of the changes to OSPF presented in this docu- Note that the nature of the changes to OSPF presented in this docu-
ment is so that standard ABR behavior is not altered until at least ment is so that standard ABR behavior is not altered until at least
one area is configured as Shortcut. one area is configured as Shortcut.
5 Compatibility 5 Compatibility
ABRs following the approach described in this document are required ABRs following the approach described in this document are required
to announce their Shortcut capability for a given area in Router- to announce their Shortcut capability for a given area in Router-
LSAs. Since no loops are formed when all ABRs in a given area are LSAs. Since Shortcut ABRs do not consider Summary-LSAs from an area
Shortcut and Shortcut ABRs do not consider Summary-LSAs from an area
when a Shortcut-incompatible ABR in such an area is seen, the when a Shortcut-incompatible ABR in such an area is seen, the
approach described in this document is compatible with standard OSPF approach described in this document is compatible with standard OSPF
described in [Ref1]. described in [Ref1].
6 Deployment Considerations 6 Deployment Considerations
This section discusses the deployment details of Shortcut ABR. This section discusses the deployment details of Shortcut ABR.
6.1 Necessity of virtual links 6.1 Necessity of virtual links
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all ABRs that have links in it must have this area configured as all ABRs that have links in it must have this area configured as
Shortcut. If a single ABR in an area does not announce the S-bit in Shortcut. If a single ABR in an area does not announce the S-bit in
its Router-LSA for this area, no other Shortcut ABRs connected to its Router-LSA for this area, no other Shortcut ABRs connected to
this area will direct inter-area traffic through it (except for the this area will direct inter-area traffic through it (except for the
cases when standard OSPF behavior leads to it). cases when standard OSPF behavior leads to it).
The implementers should note that support of a configurable option The implementers should note that support of a configurable option
described in section 4 is very important for traffic control and suc- described in section 4 is very important for traffic control and suc-
cessful deployment. cessful deployment.
7 Security Considerations 7 Routing loops in transition periods
As it was noted before standard OSPF ABR behavior uses DV approach to
distribute routing information among the areas. While the basic tech-
nique used in OSPF for this purpose provides loop-free environment,
existence of circular virtual link topology may lead to temporary
routing loops basically because of the section 16.3 that can update
backbone routes with non-backbone inter-area ones. The routing loops
formed in such situations are similar to those experienced in DV
routing protocols when the originator of a route loses its connec-
tivity to the network, sends messages withdrawing the route to all
neighbors, but not all messages manage to get through and the origi-
nator receives a false update from an upstream neighbor. An example
of such a temporary loop is illustrated in Figure 3.
.............. ................
. . . .
. Area 1 +----+ Area 2 .
. ______| |________ .
. / 1| R2 |1 \ .
. / +----+ \ . 10
. / . 10| . ***************
. 1/ . | . * \1 . *--+
+----+..... __|_ *......+----+....|R4|.....
| |10 / \ * 10| | +--+ .
| R1 |------* *--------| R3 | |100 .
+----+ \____/ +----+ 1 | .
. 1| . Backbone . |1.. \ | .
. | . . / . . \ | .
. | ................... / . . ----------- .
. | / . . Network N .
. \_______________________/ . . .
. . . Area 3 .
. Area 4 . ............
...............................
Figure 3. Sample topology with temporary routing loop
In this example routers R1, R2, R3, and R4 are ABRs. All of them are
connected to the backbone ring that constitutes the backbone area.
Note that the link from R4 to the backbone ring (marked with aster-
isks) does not belong to area 2, but to the backbone area. The cost
of backbone intra-area route between any given two ABRs is 10, since
they are all connected via a broadcast segment. The cost of non-
backbone intra-area path from R1 to R2, from R2 to R3, and from R3 to
R1 is 1. In this example, it doesn't matter what the cost between R3
and R4 is. We are interested in network N residing in area 3. Both
ABRs (R3 and R4) can reach this network. Suppose R3 can reach it via
a path with cost 1, while R4 via a path with cost 100. Both routers
announce summary-LSAs for network N into the backbone area. If all
ABRs follow the standard ABR behavior, and there are no virtual links
in the domain, R1 and R2 will install inter-area routes to network N
through the backbone area. If virtual links are established between
R1 and R2, R2 and R3, and R3 and R1, then routers R1 and R2 will
choose more optimal paths through areas 4 and 2 correspondingly,
according to the algorithm described in 16.3 of [Ref1]. Also, R1 and
R2 will announce summary-LSAs with cost 2 into area 1, since inter-
area routes to network N in their routing tables are still backbone-
associated. The same happens when R1, R2, and R3 are Shortcut-
capable ABRs and agree to use areas 2 and 4 for shortcutting.
Now assume R3 loses its connectivity to area 3. After the routing
table is recalculated, R3 has an inter-area route to network N
through the backbone area via R4 with cost 110. R3 withdraws its
summary-LSA covering network N advertised into the backbone by flood-
ing corresponding MaxAge LSA (premature aging, as described in 14.1
of [Ref1]) and updates areas 2 and 4 with new version of summary-LSA,
containing the cost of 110. Now assume that R2 successfully receives
and installs the new LSA, while R1 does not (due to packet drops or
other potential problems). R2 recalculates the routing table and
installs the inter-area route via R1, because R1 did not withdraw its
summary-LSA with cost 2. After the new route is installed into R2's
routing table, R2 originates a summary-LSA for network N with cost 3
into area 2. R3 uses this LSA to calculate the route to N via R2 and
announces a new summary-LSA with cost 4 into area 4. This LSA is used
by R1. A loop is formed. Note that R1, R2, and R3 will not use the
backbone path, because it will always be updated by a non-backbone
path with smaller metric. Described looping stops when the cost of
non-backbone inter-area path to network N becomes greater than the
cost of backbone-associated path, that is greater than 110.
The loop described above is not Shortcut ABR-specific, i.e., it can
be seen even with standard ABR behavior when virtual links create a
circle. However, this document encourages administrators to configure
areas as shortcut and thus potentially increases the probability of
such a temporary loop. The author would like to emphasize that 1)
such routing loops are hardly to be seen in real networks with proper
domain design, and 2) even if such a loop forms, it is temporary, the
network finally converges and proper routes are installed. There
exist several methods to minimize the probability of described rout-
ing loop formation and to provide faster convergence in case a loop
has formed, but specification of these methods is outside the scope
of this document and will be delayed until the problem becomes
apparent.
8 Security Considerations
Shortcut ABR behavior specified in this document does not raise any Shortcut ABR behavior specified in this document does not raise any
security issues that are not already covered in [Ref1]. security issues that are not already covered in [Ref1].
8 Appendixes 9 Appendixes
A.1 Router-LSA A.1 Router-LSA
An OSPF router originates a router-LSA into each of its attached An OSPF router originates a router-LSA into each of its attached
areas. The router-LSA describes the state and cost of the router's areas. The router-LSA describes the state and cost of the router's
interfaces to the area. The contents of the router-LSA are described interfaces to the area. The contents of the router-LSA are described
in detail in Section A.4.2 of [Ref1]. One more flag has been added in detail in Section A.4.2 of [Ref1]. One more flag has been added
to the router-LSA, called bit S below. This flag indicates whether to the router-LSA, called bit S below. This flag indicates whether
the area has been configured as Shortcut on the ABR. Note that all the area has been configured as Shortcut on the ABR. Note that all
ABRs in an area must announce the S-bit this area to be used in ABRs in an area must announce the S-bit this area to be used in
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multicast receiver. These routers receive all multicast datagrams, multicast receiver. These routers receive all multicast datagrams,
regardless of destination. Inter-area multicast forwarders and regardless of destination. Inter-area multicast forwarders and
inter-AS multicast forwarders are sometimes wild-card multicast inter-AS multicast forwarders are sometimes wild-card multicast
receivers (see [Ref3] for more details). receivers (see [Ref3] for more details).
o bit S. When set, the router is a Shortcut-capable ABR and intends o bit S. When set, the router is a Shortcut-capable ABR and intends
to use the area for shortcutting provided that all other ABRs in to use the area for shortcutting provided that all other ABRs in
this area agree on that (also announce the S-bit into this area). this area agree on that (also announce the S-bit into this area).
See sections 2 and 3 for more details. See sections 2 and 3 for more details.
9 References 10 References
[Ref1] J. Moy. OSPF version 2. Technical Report RFC 2328, Internet [Ref1] J. Moy. OSPF version 2. Technical Report RFC 2328, Internet
Engineering Task Force, 1998. ftp://ftp.isi.edu/in- Engineering Task Force, 1998. ftp://ftp.isi.edu/in-
notes/rfc2328.txt. notes/rfc2328.txt.
[Ref2] Zinin, Lindem, Yeung. Alternative OSPF ABR Implementations. [Ref2] Zinin, Lindem, Yeung. Alternative OSPF ABR Implementations.
Work in progress, Internet Engineering Task Force. draft- Work in progress, Internet Engineering Task Force. draft-
ietf-ospf-abr-alt-00.txt ietf-ospf-abr-alt-00.txt
[Ref3] J. Moy. Multicast Extensions to OSPF. Internet Engineering [Ref3] J. Moy. Multicast Extensions to OSPF. Internet Engineering
Task Force, 1998. http://www.ietf.org/internet-drafts/draft- Task Force, 1998. http://www.ietf.org/internet-drafts/draft-
ietf-mospf-mospf-01.txt. ietf-mospf-mospf-01.txt.
10 Author's Address 11 Author's Address
Alex D. Zinin Alex D. Zinin
AMT Group AMT Group
8 Maly Znamensky per., bld. 1 8 Maly Znamensky per., bld. 1
Office 3b Office 3b
121019 Moscow, Russia 121019 Moscow, Russia
E-mail: zinin@amt.ru E-mail: zinin@amt.ru
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