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

Yasuhiro Ohara <yasu@SFC.WIDE.AD.JP> Tue, 10 June 2003 23:29 UTC

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Date: Wed, 11 Jun 2003 08:29:06 +0900
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From: Yasuhiro Ohara <yasu@SFC.WIDE.AD.JP>
Subject: Re: MOSPF can't determine the receiving interface exactly - In John Moy's book OSPF Anatomy of a Routing Protocol
Comments: To: ijpotts_lists@YAHOO.CO.UK
To: OSPF@PEACH.EASE.LSOFT.COM
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Hi, I try to explain.

Even in your example, actually you don't determine the receiving half
of the point-to-point link. You can only know the receiving interface
of the other router by *guessing*, and you can guess only because
they don't have any other links to the same router.

Let's suppose your example have two point-to-point links between
Router 1 and Router 2 as it will be easier to understand.
(The book also mentions that configuration, "when multiple
point-to-point links connect the router to the neighbor"
in the same section)

The Router-LSAs in the database will be like below.

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: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 144.141.252.254
     (Link Data) Router Interface address: 10.1.1.1 (or whatever)
      Number of TOS metrics: 0
       TOS 0 Metrics: 10000 (or whatever)

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: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 144.141.89.2
     (Link Data) Router Interface address: 192.168.0.1 (or whatever)
      Number of TOS metrics: 0
       TOS 0 Metrics: 10000 (or whatever)

Then, can you illustrate the network configuration, specifically
which interface of Router 1 is connected to the interface which is
described as Link-Description #1 in the Router 2's Router-LSA ?
Why ?

Note that point-to-point link can be configured not to share the
IP-subnet. So Router 1's interface having address 144.128.254.138
may not necessarily be connected to the Router 2's interface having
address 144.128.254.137.

For the multicast forwarding of direction from Router 1 to Router 2,
both router can calculate that Router 1 should send the datagrams
on the interface #1 (having address 144.128.254.138), because it has
lower cost. But neither router can determine which interface the
Router 2 will receive that datagram on.

This problem is resolved in OSPFv3 as each single Link Description in
Router-LSA describes both sending and receiving halves (by describing
both side of IfIndex). OSPFv2's Link Description only describes sending
half and the *Router-ID* of receiving router. It doesn't describe
receiving half (interface) of receiving router.

Hope this helps.

regards,
yasu

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
Date: Thu, 5 Jun 2003 13:41:50 +0100

> 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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