Subject: Re: [Isis-wg] draft-ietf-isis-traffic-01.txt

From: Henk Smit <[email-masked]>
Date: Wed, 1 Dec 1999 16:00:47 +0100 (MET)

> In section 3.0 of the subject draft, regarding the router ID TLV, the
> following statement appears:
> 
>    Implementations MUST NOT inject a /32 prefix for the router ID into
>    their forwarding table, because this can lead to forwarding loops
>    when interacting with systems that do not support this TLV.
> 
> Could someone provide further explanation or, better yet, an illustration
> of the scenario involved?

  See here a simple example:

       22.22.22/24

     R1 ----10----- R2
     |               |
     10              4
     |               |
     R3 -----4----- R4

  Suppose R1 and R2 are connected via a link with prefix 22.22.22/24.
 Suppose R1 has interface address 22.22.22.1 on that link.
 Suppose R1 selects 22.22.22.1 as its MPLS TE router ID.
 Suppose R3 does not inject route 22.22.22.1/32 into the RIB (good).
 Suppose R4 does inject route 22.22.22.1/32 into the RIB (bad).

  This will cause a routing loop for traffic to 22.22.22.1:
      R3 wants to send a packet to 22.22.22.1.
      The matching route will be 22.22.22/24, cost 18, via R3->R4->R2.
      R4 wants to send a packet to 22.22.22.1.
      The matching route will be 22.22.22.1/32, cost 14, via R4->R3->R1.

  Voila, routing loop.
 Hope this helps,

       Henk.

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