Subject: [OSPF] p2mp interfaces

From: sinbad <[email-masked]>
Date: Thu, 20 Sep 2007 10:37:44 +0530

>From RFC 2328 section 12.4.1.4.

For operational       Point-to-MultiPoint interfaces,      one or

more link descriptions are added to the       router-LSA as

follows:

o   "A single Type 3 link (stub network) is added with

    Link ID set  to the router's own IP interface

    address, Link Data set to the mask 0xffffffff

    (indicating  a host route), and cost  set to 0."

o   For     each fully adjacent neighbor associated      with the

    interface, add an additional Type 1   link (point-to-

    point) with  Link ID    set to the Router ID of   the

    neighboring router,     Link Data set to the IP

    interface address and cost equal to the interface's

    configured output cost.

The text in quotes say that for p2mp interfaces , type-3 link should be
added with cost 0, why is this needed?

Thanks

sinbad 

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---

From: Acee Lindem <[email-masked]>
Date: Thu, 20 Sep 2007 09:25:40 -0400

Hi Sinbad,

On Sep 20, 2007, at 1:07 AM, sinbad wrote:

> The text in quotes say that for p2mp interfaces , type-3 link  
> should be added with cost 0, why is this needed?

So you have OSPF reachability to every local interface in the P2MP  
mesh. Why is this needed? It is not needed to calculate routing  
amongst routers in the mesh. However, if you ever want to  
troubleshoot a problem in the mesh, it is nice to be able to ping  
each local interface.

Hope this helps,
Acee