Subject: Re: Re: DVMRP Forwarding

From: Bill Fenner <[email-masked]>
Date: Tue, 20 Jun 2000 08:38:05 -0700

You can only combine forwarding entries as far as the granularity of
DVMRP routes (i.e. it's OK to forward on subnets), but if S1 and S2
are described by different routes in the DVMRP routing table, they may
have different broadcast trees so you cannot forward (S2,G) using the
information for (S1,G).

      S1            S2
       |             |
      R1            R2
       |           / |
       |        /    |
       |     /       |
       |  /          |
      R3             |
       |             |
       |             |
       |             |
      R4------------R5
                     |
                     M

S1 and S2 are sources.  R1-R5 are routers.  M is a member.  All links
have metric 1 except the R2/R5 link which has metric 2.  The broadcast
tree for S2 contains R2, R3, R4 and R5 (but not the R4-R5 link); the
broadcast tree for S1 conains R1, R3, R4, R5.  When S2 starts sending
to G, R4 sends a prune for (S1,G) to R3.  If S1 later starts sending,
its traffic must transit the R3->R4 link to get to M, so cannot use the
S2 prune information that's already there.

  Bill