MOSPF can't determine the receiving interface exactly - In John Moy's book OSPF Anatomy of a Routing Protocol

Ian Potts <ijpotts_lists@YAHOO.CO.UK> Thu, 05 June 2003 12:51 UTC

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From: Ian Potts <ijpotts_lists@YAHOO.CO.UK>
Subject: MOSPF can't determine the receiving interface exactly - In John Moy's book OSPF Anatomy of a Routing Protocol
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Hello,

In John Moy's excellent book, Anatomy of an Internet
Routing Protocol, on page 195 (section 10.3.1 The
Multicast Forwarding Cache), it states that a router
does the RPF check in MOSPF on the router's ID, and
not on the incoming interface, since MOSPF can't
determine the receiving interface exactly, since there
is insufficient information in the OSPF lsdb to match
the sending and receiving halves.

Can someone please explain this to me since I am
confused.  Looking at the router LSAs given below,
can't the router first work out the router-id, then
consult the routing table to find the interface?

Many Thanks
Ian

Router 1

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 144.141.252.254
     (Link Data) Router Interface address:
144.128.254.138
      Number of TOS metrics: 0
       TOS 0 Metrics: 1000

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 144.128.254.136
     (Link Data) Network Mask: 255.255.255.248
      Number of TOS metrics: 0
       TOS 0 Metrics: 1000

Router 2

Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 144.141.89.2
     (Link Data) Router Interface address:
144.128.254.137
      Number of TOS metrics: 0
       TOS 0 Metrics: 1000

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 144.128.254.136
     (Link Data) Network Mask: 255.255.255.248
      Number of TOS metrics: 0
       TOS 0 Metrics: 1000




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