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Versions: 00 01 02

Network Working Group                                        A. D. Zinin
Internet Draft                                             Cisco Systems
Expiration Date: January 2001                                  July 2000
File name: draft-ietf-ospf-shortcut-abr-02.txt



                           OSPF Shortcut ABR
                       Enhanced OSPF ABR Behavior
                  draft-ietf-ospf-shortcut-abr-02.txt




Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Abstract

   OSPF [Ref1] is a link-state intra-domain routing protocol used for
   routing in IP networks. Though the definition of the ABR in the
   current OSPF specification does not require a router with multiple
   attached areas to have a backbone connection, it is actually
   necessary to provide successful routing to the inter-area and
   external destinations. If this requirement is not met, all traffic,
   destined for the areas not connected to such an ABR or out of the
   OSPF domain, is dropped. The rules of originating and processing
   Summary-LSAs given in the current OSPF standard [Ref1] can also



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   result in suboptimal inter-area routing. Though all these problems
   can be fixed using virtual links, this memo describes an alternative
   implementation of the OSPF ABR behavior, which allows the
   administrator to avoid this or, if virtual links are still used, to
   decrease the number of configured virtual links.

   This memo also describes possible situations where the proposed
   implementation can be used.


Acknowledgements


   The author would like to acknowledge Christian Hopps and Peter Psenak
   for their help in finding weak points in early versions of this
   document, Thomas M. Thomas for reviewing the preceding version of 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
   and provided a simpler algorithm representation, used herein.


Table of Contents

   1 Overview .....................................................    3
   1.1 Introduction ...............................................    3
   1.2 Motivation .................................................    3
   2 Description of Shortcut ABR behavior .........................    5
   3 Proposed changes to OSPF ABR behavior ........................    6
   3.1 Changes to Area Data Structure .............................    6
   3.2 Changes to Router-LSA Origination ..........................    8
   3.3 Changes to Routing Table Calculation .......................    8
   3.4 Changes to Summary-LSA Origination .........................   10
   4 Implementation Details .......................................   12
   5 Compatibility ................................................   12
   6 Deployment Considerations ....................................   12
   6.1 Necessity of Virtual Links .................................   12
   6.2 Change of Traffic Patterns .................................   13
   6.3 Optimized Inter-area Routing ...............................   13
   6.4 Gradual Deployment of Shortcut ABRs ........................   14
   7 Routing Loops in Transition Periods ..........................   15
   8 Security Considerations ......................................   17
   9 Appendixes ...................................................   17
   A.1 Router-LSA .................................................   17
   10 References ..................................................   19
   11 Author's Address ............................................   19




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1 Overview

 1.1 Introduction

   An OSPF routing domain can be split into several subdomains, called
   areas, which limit the scope of LSA flooding. A router having attach-
   ments to multiple areas is called an "area border router" (ABR). The
   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
   ASBRs in other areas) as well as to perform actual inter-area rout-
   ing.

 1.2 Motivation

   In OSPF, flooding of Type-1 and Type-2 LSAs is limited to the area
   borders, so routers in other areas must somehow know how to reach
   destinations and ASBRs residing in different areas. OSPF uses
   Distance-Vector (DV) approach to achieve this goal, i.e., Area Border
   Routers announce networks and ASBRs internal to directly connected
   areas in Type-3 and Type-4 Summary-LSAs.

   If routers using a DV protocol announce only directly attached net-
   works, they must be fully meshed to provide complete routing informa-
   tion to each other. This condition cannot always be met, so routers
   also announce the networks they heard about from their neighbors.
   This is the main reason for loops of routing updates in DV protocols,
   which are solved using methods like split-horizon, limitting the max-
   imum metric value or hop count, and hold-down timers. Application of
   these rules to OSPF inter-area routing would make the code very com-
   plex, but since areas in OSPF need not be fully meshed, ABRs are
   allowed to reannounce inter-area routes. In order to prevent loops of
   summaries in OSPF, ABRs reannounce only those inter-area routes which
   are associated with the backbone area. Summaries from non-backbone
   areas are just not considered by ABRs. Because inter-area routes are
   not reannounced back into the backbone area, the latter functions as
   a loop-free inter-area routing information repository. In order to
   achieve normal routing to inter-area and AS-external destinations,
   all areas in OSPF should be connected to the backbone either physi-
   cally (via an interface) or logically (via a virtual link). This is
   to ensure that all areas are provided with inter-area routes from the
   backbone.

   A basic discussion of the disadvantages of the standard inter-area
   approach are given in [Ref2] and are applicable to this document as
   well.  In addition to that, consider another problem caused by stan-
   dard OSPF ABR behavior (Figure 1).





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                         .  Area 2         .
                         .                 .
                         .      +--+       .
                          ......|R5|.......
                         .      +--+       .
                        .       /  \        .
                       .       /    \        .
                       . BB   /10Mb  \ 2Mb   .
                       .     /        \      .
                        .    |         \    .
                         .  +--+      +--+ .
                          ..|R1|......|R2|..
                         .  +--+      +--+  .
                         .   |  10Mb   |\   .
                         .  ------------ |  .
                         .               |  .
                         .             +--+ .
                         .             |R4| .
                         . Area 1      +--+ .
                          ..................

                   Figure 1. Suboptimal inter-area routing


   In this example router R2 has a 2Mb link to R5. At the same time R1
   has a better link (10 Mbps), but R2 cannot route traffic going to
   area 2 through R1. This is because according to [Ref1] R2 is not
   allowed to consider summary-LSAs from non-backbone areas and, conse-
   quently, does not have routes covering destinations in area 2 via R1.
   The situation looks even more interesting if R4's routing table is
   considered. Since R2 floods summary-LSAs from R1 to R4, router R4
   will have routes to the area 2 via R1 (the best path), expecting
   traffic to go via 10Mbps links. In reality, R2 will not direct
   traffic to R1, but will forward it via 2Mbps link attached to itself.

   The last example shows how the main principle of OSPF---prefer the
   shortest path---is broken due to distance vector approach used for
   inter-area routing. Again, the problem can be fixed using the virtual
   links between R1 and R2 in standard OSPF, but the solution proposed
   in this document appears to be more elegant and involving less admin-
   istrative and traffic overhead. More sophisticated examples of how
   Shortcut ABR approach improves inter-area routing are given in sec-
   tion 6.








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2 Description of Shortcut ABR behavior

   This section describes an alternative implementation of OSPF ABR
   behavior, named "Shortcut ABR". It is an improvement on standard ABR
   behavior, based on relaxation of the restrictions applied to the cal-
   culation of the inter-area routes.

   With the Shortcut ABR approach, ABRs are allowed to consider
   summary-LSAs from all (or a subset of) attached areas by performing a
   modified version of section 16.3 of [Ref1]. This gives Shortcut ABRs
   a chance to install inter-area routes through non-backbone areas (if
   non-backbone paths are really better), i.e. to "shortcut" through
   them.  The routing loop prevention is ensured by restricting the ori-
   gination of summary-LSAs---inter-area routes are readvertised only if
   they are associated with the backbone area (there is a valid LSA for
   the destination learned from the backbone). Origination of summary-
   LSAs for intra-area routes is done as in standard OSPF [Ref1].

   There is a probability of forming constant routing loops if
   Shortcut-capable and standard ABRs are present in an OSPF domain and
   can see each other through the backbone. Also, if transit or shortcut
   areas form a circle, it is possible (though the probability is really
   low) to have temporary routing loops as described in section 7. To
   prevent these types of loops, two new variables (ShortcutConfigured
   and ShortcutCapability) are introduced to the OSPF area data struc-
   ture for non-backbone areas, and a new bit (S-bit) is announced in
   the router-LSAs by the ABRs.


   If the ABR doesn't have the backbone area connected, it considers
   summary-LSAs from all attached areas. This is safe, because no
   inter-area routes are associated with the backbone and get readver-
   tised.  The relaxation of the routing table calculation allows ABRs
   without a backbone connection to route traffic between the attached
   areas, as well as to route traffic destined for the backbone and
   other areas using the routes derived from the summary-LSAs in each
   attached area. This approach also enables router R2 in Figure 1 to
   route inter-area traffic via R1.

   Note that the proposed solution does not obviate the need of virtual
   link configuration in case an ABR has no physical backbone connection
   at all, but at the same time should reannounce inter-area routes
   (intra-area routes are always announced to other areas). However,
   this approach requires only a single backbone link per ABR or no
   backbone link at all (if the ABR does not have to reannounce inter-
   area routes and just needs to find the best routes through attached
   areas itself).




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3 Proposed changes to OSPF ABR behavior

   This section describes the changes made to the base OSPF described in
   [Ref1].

 3.1 Changes to Area Data Structure

   Two new flags are introduced to OSPF area data structure---
   ShortcutConfigured and ShortcutCapability.

   The ShortcutConfigured flag can be assigned three values: Default,
   Enable, and Disable. The flag is set to Default value when an area
   data structure is created. Description of the flag values is given
   below.

          Default

             If area A's ShortcutConfigured flag is set to Default, and
             the ABR has an active backbone connection, area A is not
             used for shortcutting and the ABR does not set the S-bit in
             the router-LSA originated for that area. If the ABR has no
             backbone connection, area A is always used for shortcutting
             and the ABR sets the S-bit in the router-LSA for that area.


          Enable

             If area A's ShortcutConfigured flag is set to Enable, and
             the ABR has an active backbone connection, it sets the S-
             bit in the router-LSA for area A and uses it for shortcut-
             ting, provided that all other ABRs seen through this area
             also report the S-bit.  If the ABR has no backbone connec-
             tion, it unconditionally uses area A for shortcutting and
             sets the S-bit in the router-LSA originated for that area.

          Disable

             If an area's ShortcutConfigured flag is set to Disable, the
             ABR doesn't use this area for shortcutting and doesn't set
             the S-bit in the router-LSA originated for it.


       Treamtment of the ShortcutConfigured flag described above ensures
       that Shortcut ABRs operate correctly and efficiently without
       explicit configuration. For example, when a Shortcut ABR is
       attached to non-backbone areas only, the Default value will allow
       it to shortcut through these areas. When a Shortcut ABR is con-
       nected to the backbone, it doesn't shortcut through non-backbone



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       areas until it is explicitly configured to do so by setting the
       ShortcutConfigured flag for specific (or all) areas to Enable
       value and all other ABRs announce the S-bit (either because they
       are not connected to the backbone, or because they were also con-
       figured to shortcut through that area). Again, this behavior
       ensures that no routing loop is established between a shortcut-
       ting and not shortcutting ABR, as well as that shortcut areas do
       not form a circle.

       In addition to ShortcutConfigured, Shortcut ABRs maintain
       ShortcutCapability flag in Area Data Structure for every non-
       backbone area. These two flags are used to prevent permanent
       routing loops in the networks where Shortcut-incapable ABRs are
       used along with Shortcut ABRs.

       While ShortcutConfigured flag indicates what the administrator
       has configured for a particular area, ShortcutCapability indi-
       cates that the area may actually be used for shortcutting either
       because all other ABRs in the area agree on this using the S-bit
       or because the calculating ABR does not have a backbone connec-
       tion and was not explicitly configured not to do so.

       Note that backbone-connected Shortcut ABRs are allowed to con-
       sider summary-LSAs from a non-backbone area only if that area's
       ShortcutCapability flag is set to TRUE. An area's ShortcutCapa-
       bility flag, in turn, is set to TRUE when the ABR does not have a
       backbone attachment and area's ShortcutConfigured is not set to
       Disables, or when the ABR has a backbone connection, area's
       ShortcutConfigured is set to Enable, and all other ABRs connected
       to the area set their S bits in their router-LSAs.  This means
       that the calculating ABR and all other ABRs connected to that
       area should be allowed to consider that area's summary-LSAs.

       If, during the routing table calculation, a Shortcut ABR notices
       that there is an ABR which does not announce the S-bit in any
       area, the Shortcut ABR will probably need to clear the Shortcut-
       Capability flag for that area (depending on whether it has a
       backbone connection and the value of ShortcutConfigured flag).
       Should the ABR in question find that all ABRs in an area agree on
       the S-bit it may need to set the ShortcutCapability flag for that
       area.

       Note that announcement of S-bit does not depend on the results of
       routing table calculation, but only on the setting of Shortcut-
       Configured and backbone attachment.






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 3.2 Changes to Router-LSA Origination

       The algorithm of Type 1 LSA (router-LSA) origination is changed
       to have the Shortcut ABR announce its Shortcut capability in the
       Router-LSA as described in A.1. A Shortcut ABR should set the S-
       bit in the Router-LSA for Area A only if:

          o    the router does not have a backbone connection and
               ShortcutConfigured flag for this area is NOT set to Dis-
               able value, or

          o    the router has a backbone connection and area A's
               ShortcutConfigured flag is set to Enable.

       As in [Ref1] Shortcut ABRs identify themselves as ABRs by setting
       the bit B in their Router-LSAs when they have more than one
       attached area.

 3.3 Changes to Routing Table Calculation

       In order to maintain correct state of the ShortcutCapability
       flag, steps 1 and 2 in section 16.1 of [Ref1] are changed as fol-
       lows:

          Step 1:

             "Initialize the algorithm's data structures.  Clear the
             list of candidate vertices.  Initialize the shortest-path
             tree to only the root (which is the router doing the calcu-
             lation).  Set Area A's TransitCapability to FALSE.
             ShortcutCapability flag is set as follow.

               o    If the router is not connected to the backbone and
                    Area A's ShortcutConfigured flag is NOT set to Dis-
                    able, or the router is connected to the backbone and
                    Area A's ShortcutConfigured flag is set to Enable,
                    set ShortcutCapability flag to TRUE.

               o    Otherwise, set Area A's ShortcutCapability flag to
                    FALSE."

          Step 2:

             "Call the vertex just added to the tree vertex V.  Examine
             the LSA associated with vertex V.  This is a lookup in the
             Area A's link state database based on the Vertex ID.  If
             this is a router-LSA, and bit V of the router-LSA (see Sec-
             tion A.4.2) is set, set Area A's TransitCapability to TRUE.



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             If this is a router-LSA, and bit B of the router-LSA is set
             (the router is an ABR), and bit S of the router-LSA is not
             set (the ABR is either not Shortcut-capable or has a back-
             bone connection and is not configured to use Area A for
             shortcutting), and the ABR has a backbone connection, set
             Area A's ShortcutCapability to FALSE. In any case, each
             link described by the LSA gives the cost to an adjacent
             vertex.  For each described link, (say it joins vertex V to
             vertex W):"

       Note that the above algorithm, ensures that it is enough to check
       only ShortcutCapability flag while deciding whether summary-LSAs
       of a particular area should be considered or not.

       The algorithm of calculating inter-area routes is changed as fol-
       lows.

       ABRs consider summary-LSAs only from those attached non-backbone
       areas that have ShortcutCapability flag set to TRUE. This is
       achieved by applying section 16.3 of [Ref1] to such areas. The
       following changes to 16.3 are made.

       Paragraph 1 of 16.3 is changed to be as follows:

          "This step is only performed by area border routers attached
          to one or more non-backbone areas that are either capable of
          carrying transit traffic (i.e., "transit areas", or those
          areas whose TransitCapability parameter has been set to TRUE
          in Step 2 of the Dijkstra algorithm (see Section 16.1) or can
          be used for shortcutting (those areas whose ShortcutCapability
          parameter has NOT been set to FALSE during the Dijkstra algo-
          rithm otherwise)."

       Paragraph 4 of 16.3 is changed to be as follows:

          "The calculation proceeds as follows. All summary-LSAs of the
          areas with TransitCapability or ShortcutCapability parameter
          set to TRUE are examined in turn. Each such summary-LSA
          describes a route through a non-backbone area Area A to a Net-
          work 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) or in the case of a Type 4 summary-LSA, to an AS
          boundary router N.  Suppose also that the summary-LSA was ori-
          ginated by an area border router BR."

       Step (3) of the algorithm in 16.3 is changed to be as follows:

          "Look up the routing table entry for N. (If N is an AS



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          boundary router, look up the "router" routing table entry
          associated with the backbone area). If the route type is other
          than backbone intra-area or inter-area (associated with any
          area) then examine the next LSA.

          In other words, this calculation updates backbone intra-area
          routes found in Section 16.1, inter-area routes found in Sec-
          tion 16.2 and installs new inter-area routes if the ABR does
          not have a backbone connection."

       Step (5) of the algorithm in 16.3 is changed to be as follows:

          "If this cost is less than the cost occurring in N's routing
          table entry, overwrite N's list of next hops with those used
          for BR, and set N's routing table cost to IAC. Else, if IAC is
          the same as N's current cost, add BR's list of next hops to
          N's list of next hops. If the area associated with N's routing
          table entry is the backbone, then the area and the type of the
          path (either intra-area or inter-area) should remain
          unchanged. Otherwise (the routing table entry does not exist
          or the associated area is not the backbone), the type of the
          route should be set to inter-area and associated area should
          be set to the area associated with the summary-LSA being pro-
          cessed."

       In order to prevent routing loops, section 16.2 of [Ref1] is
       changed. Step (3) of section 16.2 is changed to instruct the ABRs
       to ignore summary defaults received from stub areas:

          "If it is a Type 3 summary-LSA, and the collection of destina-
          tions described by the summary-LSA equals one of the router's
          configured area address ranges (see Section 3.5), and the par-
          ticular area address range is active, then the summary-LSA
          should be ignored.  "Active" means that there are one or more
          reachable (by intra-area paths) networks contained in the area
          range. The summary-LSA should also be ignored if it is a sum-
          mary default (Destination ID = DefaultDestination, Address
          Mask =  0x00000000) and the area it has been received from is
          a stub area. This is to prevent possible routing loops."

       It is also reemphasized that routers are supposed to install dis-
       card routing entries for active area ranges per 11.1 of [Ref1]

 3.4 Changes to Summary-LSA Origination

       The algorithm of the summary-LSAs origination is changed to
       include an explicit restriction not to originate summary-LSAs for
       inter-area routes if the route to the destination is not



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       associated with the backbone.

       Note that if there are multiple alternative paths to a destina-
       tion, some of which are via the backbone and the rest are via
       non-backbone areas, the area associated with the corresponding
       routing table entry will remain the backbone area, but the set of
       next hops will actually direct traffic along the best path even
       through non-backbone areas.

       If the ABR in question has no backbone connection, it will not
       originate summary-LSA for any inter-area route in any area,
       because the area associated with the routing table entry will
       never be the backbone area.

       The ABR will also not readvertise an inter-area route from non-
       backbone area if its backbone link state database does not con-
       tain a summary-LSA, router-LSA, or network-LSA covering a
       specific destination.

       In order to implement described policy, the paragraph 2 in sec-
       tion 12.4.3 of [Ref1] should be read as follows:

          "... Note that only intra-area routes are advertised into the
          backbone, while both intra-area and inter-area routes are
          advertised into the other areas. Also, summary-LSAs for
          inter-area routes are originated if and only if these routes
          are associated with the backbone area (to prevent loops of
          summary-LSAs)."

       The 6th step of the algorithm given in sections 12.4.3 of [Ref1]
       should be interpreted as shown below:

          "Else, if the destination of this route is an AS boundary
          router, a summary-LSA should be originated if and only if the
          routing table entry describes the preferred path to the AS
          boundary router (see Step 3 of Section 16.4) and it is associ-
          ated with the backbone area.  If so, a Type 4 summary-LSA is
          originated for the destination, with Link State ID equal to
          the AS boundary router's Router ID and metric equal to the
          routing table entry's cost. Note: these LSAs should not be
          generated if Area A has been configured as a stub area."

       The 7th step of the algorithm given in sections 12.4.3 of [Ref1]
       should be interpreted as shown below:

          "Else, the Destination type is network. If this is an inter-
          area route and it is associated with the backbone area, gen-
          erate a Type 3 summary-LSA for the destination, with Link



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          State ID equal to the network's address (if necessary, the
          Link State ID can also have one or more of the network's host
          bits set; see Appendix E for details) and metric equal to the
          routing table cost."

   Described changes in the ABR behavior allow selection of most optimal
   paths to inter-area and external destinations.  Note that backbone
   intra-area routes can be updated with better non-backbone inter-area
   one, thus directing internal backbone traffic along more optimal
   paths through other areas.

4 Implementation Details

   If the current implementation of OSPF uses the standard described in
   [Ref1], then support of the proposed Shortcut ABR behavior strategy
   should be implemented as configurable options, allowing to change the
   ABR behavior and set the ShortcutConfigured flag for a given area.

   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
   one area is used for shortcutting.

5 Compatibility

   ABRs following the approach described in this document are required
   to announce their Shortcut capability for a given area in Router-
   LSAs. Since backbone-attached Shortcut ABRs do not consider Summary-
   LSAs from an area until all ABRs agree on the S-bit, and ABRs not
   attached to the backbone do not readvertise the inter-area routes,
   the approach described in this document is compatible with standard
   OSPF described in [Ref1].

6 Deployment Considerations

   This section discusses the deployment details of Shortcut ABR.

 6.1 Necessity of Virtual Links

   It should be repeated that Shortcut ABR behavior does not obviate the
   need for virtual links in case an ABR has no physical backbone con-
   nection. The difference with standard OSPF is that the administrator
   does not need to configure virtual links through all areas he or she
   wants the inter-area traffic to go through.  Shortcut ABR needs a
   single backbone connection (physical or virtual) to be able to rean-
   nounce optimal inter-area routes to other areas.  Note that it is not
   necessary for a Shortcut ABR itself to have a backbone connection in
   order to find the best inter-area paths, since it considers summary-
   LSAs from all attached areas if the backbone is not configured.



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 6.2 Change of Traffic Patterns

   Use of Shortcut ABR can lead to changes in the paths inter-area
   traffic flows take comparing to those experienced with standard OSPF.
   This happens because the Shortcut ABR approach allows a router to
   find paths better than it is possible with the standard OSPF.  While
   standard OSPF tries to forward all inter-area traffic through the
   backbone area (though it is not guaranteed), the Shortcut ABR finds
   best routes in the domain even across non-backbone areas. With
   Shortcut ABR the backbone area is used as a dedicated place of
   inter-area routing information exchange and inter-area traffic is
   allowed to cross non-backbone area borders if such a path is really
   the best.

 6.3 Optimized Inter-area Routing

   Use of Shortcut ABR improves inter-area routing in OSPF domains by
   allowing ABRs to consider summary-LSAs from all attached area and
   consequently readvertise them into non-backbone areas.  Consider an
   example show in the Figure 2:

                        .......................
                       .       Backbone        .  .........
                      .                        . .         .
                      .      +--+            +--+          .
                      .      |R1|            |R5|--|       .
                      .      +--+            +--+ 1|       .
                      .     8/ 8\          1/  .   |       .
                      .     /    \   --------  .   |       .
                       .  8/     8\  /1    1\  .   | Net N .
                        .+--+     +--+       +--+ 1|       .
                   ......|R2|.....|R3|.......|R4|--|       .
                  .      +--+     +--+       +--+          .
                  .       .|1    1/  \1    1/ . .   Area 3 .
                  .       .--------  -------- .  ..........
                  .       .                   .
                  .       .                   .
                  .Area 1 .    Area 2         .
                   ....... ...................

                  Figure 2. Optimized inter-area routing

   In case all ABRs use standard OSPF approach, routing to the net N
   would be as follows:


       o    R4 and R5 inject summary-LSAs into the backbone




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       o    R4 also injects a summary-LSA into area 2

       o    R3 is limited to consider summary-LSAs from the backbone
            only, so it doesn't see the alternative path through area 2
            and always routes through the backbone (though parallel
            paths are available)


       o    R3 injects summary-LSA for the inter-area routes derived
            from the backbone summary-LSAs and received from R4 and R5
            into Area 2


       o    R2 is not allowed to consider non-backbone summary-LSAs and
            routes via serial links to R1, though more optimal paths do
            exist


       If R2, R3, and R4 use Shortcut ABR approach inter-area routing is
       improved as shown below:


       o    R4 and R5 inject summary-LSAs into the backbone


       o    R4 also injects a summary-LSA into area 2


       o    R3 considers summary-LSAs from both attached areas and
            installs the route through area 2 (it has three routes in
            the routing table---via R5, via R4 through the backbone, and
            via R4 through area 2) and performs traffic sharing between
            the two ethernet links.


       o    R3 injects summary-LSA for the inter-area routes to N (it
            will be the same as in the previous case, actually)


       o    R2 considers summary-LSAs from all attached areas and
            prefers the route through area 2 rather than the backbone.


 6.4 Gradual Deployment of Shortcut ABRs

   Shortcut ABR behavior is designed in such a way that the administra-
   tor can enable shortcutting through non-backbone OSPF areas gradu-
   ally.



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   Since Shortcut ABRs are allowed to consider summaries only of those
   areas that were configured as Shortcut (ShortcutConfigured flag in
   area data structure is set to TRUE) and whose ShortcutCapability flag
   is set to TRUE, it is easy to control which areas will accept addi-
   tional inter-area traffic. For an area to become Shortcut-capable,
   all ABRs that have links in it must agree on their configuration.. 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 this area will
   direct inter-area traffic through it (except for the cases when stan-
   dard OSPF behavior leads to it).

   The implementers should note that support of the configurable option
   described in section 4 is very important for traffic control and suc-
   cessful deployment.

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.

   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 the 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 esta-
   blished 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



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   [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.

                ..............    ................
               .              .  .                .
               .  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


   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 a new version of the
   summary-LSA, containing the cost of 110. Now assume that R2 success-
   fully receives and installs the new LSA, while R1 does not (due to
   packet drops or other potential problems). R2 recalculates the rout-
   ing 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



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   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
   security issues that are not already covered in [Ref1].

9 Appendixes

 A.1 Router-LSA

   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
   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
   to the router-LSA, called bit S below. This flag indicates whether
   the area has been configured as Shortcut on the ABR. See Sections 2
   and 3 for more details.

     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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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     |    rtype      |        0      |            # links            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
     |                          Link ID                              | P
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E
     |                         Link Data                             | R
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     # TOS     |        TOS 0 metric           | #
   + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ L
   # |      TOS      |        0      |            metric             | I
   T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ N
   O |                              ...                              | K
   S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S
   | |      TOS      |        0      |            metric             | |
   + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
     |                              ...                              |

                              The router LSA

                     +---+---+---+---+---+---+---+---+
                     | * | * | S | Nt| W | V | E | B |
                     +---+---+---+---+---+---+---+-+-+

                              The rtype field


The following defines the flags found in the rtype field. Each flag
classifies the router by function:


o    bit B. When set, the router is an area border router (B is for
     border). These routers forward unicast data traffic between OSPF
     areas.


o    bit E. When set, the router is an AS boundary router (E is for
     external). These routers forward unicast data traffic between Auto-
     nomous Systems.


o    bit V. When set, the router is an endpoint of an active virtual
     link (V is for virtual) which uses the described area as its Tran-
     sit area.


o    bit W. Used in MOSPF, when set, the router is a wild-card multicast
     receiver. These routers receive all multicast datagrams, regardless
     of destination.  Inter-area multicast forwarders and inter-AS mul-
     ticast forwarders are sometimes wild-card multicast receivers.



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o    bit Nt. Used in NSSA [Ref3], 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).


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
     this area agree on that (also announce the S-bit into this area).
     See sections 2 and 3 for more details.


10 References

   [Ref1] J. Moy. OSPF version 2. Technical Report RFC 2328, Internet
          Engineering Task Force, 1998. ftp://ftp.isi.edu/in-
          notes/rfc2328.txt.

   [Ref2] Zinin, Lindem, Yeung. Alternative OSPF ABR Implementations.
          Work in progress, Internet Engineering Task Force.
          http://www.ietf.org/internet-drafts/draft-ietf-ospf-abr-alt-
          02.txt

   [Ref3] Coltun, Fuller, Murphy. The OSPF NSSA Option. Work in
          progress, Internet Engineering Task Force, 1999.
          http://www.ietf.org/internet-drafts/draft-ietf-ospf-nssa-
          update-08.txt



11 Author's Address

   Alex Zinin
   Cisco Systems
   150 West Tasman Dr.
   San Jose,CA
   95134
   E-mail: azinin@cisco.com














Zinin                                                          [Page 19]


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