Subject: [OSPF] question about RFC 2636

From: v punati <[email-masked]>
Date: Mon, 24 Apr 2006 11:46:50 -0700 (PDT)

Hi ,
   
   What is the significance of Vertex-id in Intra_SPF run. RFC-2328 says when the metrics are equal we should filter the route based on Vertex-id.
   
  I am attaching contents from RFC 2328 16.1 paragraph. My question is whether we can have any topologies which can run in this situation? Is this only a transit condition ? or any topology can have this one as persistant condition? Whats the problem if i don't filter the route based on vertex-id.? My question is about the high-lighted portion below.

  Any answers highly appreciated?
   
  kris

  (4) Possibly modify the routing table.  For those routing table
            entries modified, the associated area will be set to Area A,
            the path type will be set to intra-area, and the cost will
            be set to the newly discovered shortest path's calculated
            distance.

Moy                         Standards Track                   [Page 164]
 
RFC 2328                     OSPF Version 2                   April 1998

            If the newly added vertex is an area border router or AS
            boundary router, a routing table entry is added whose
            destination type is "router".  The Options field found in
            the associated router-LSA is copied into the routing table
            entry's Optional capabilities field. Call the newly added
            vertex Router X.  If Router X is the endpoint of one of the
            calculating router's virtual links, and the virtual link
            uses Area A as Transit area:  the virtual link is declared
            up, the IP address of the virtual interface is set to the IP
            address of the outgoing interface calculated above for
            Router X, and the virtual neighbor's IP address is set to
            Router X's interface address (contained in Router X's
            router-LSA) that points back to the root of the shortest-
            path tree; equivalently, this is the interface that points
            back to Router X's parent vertex on the shortest-path tree
            (similar to the calculation in Section 16.1.1).

            If the newly added vertex is a transit network, the routing
            table entry for the network is located.  The entry's
            Destination ID is the IP network number, which can be
            obtained by masking the Vertex ID (Link State ID) with its
            associated subnet mask (found in the body of the associated
            network-LSA).  If the routing table entry already exists
            (i.e., there is already an intra-area route to the
            destination installed in the routing table), multiple
            vertices have mapped to the same IP network.  For example,
            this can occur when a new Designated Router is being
            established.  In this case, the current routing table entry
            should be overwritten if and only if the newly found path is
            just as short and the current routing table entry's Link
            State Origin has a smaller Link State ID than the newly
            added vertex' LSA.

            If there is no routing table entry for the network (the
            usual case), a routing table entry for the IP network should
            be added.  The routing table entry's Link State Origin
            should be set to the newly added vertex' LSA.

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---

From: sujay <[email-masked]>
Date: Wed, 26 Apr 2006 10:32:40 +0530

Punati,
Vertex Id is an identifier for a Router, when specified by a Router LSA
and a Network when specified by a Network LSA.
It is the only way to identify nodes in the spf graph.
 
The text you have mentioned is very much a possible
scenario as it says when the DR is coming up for the first 
time.
 
It can easily happen in a Broadcast network (say ethernet)with varied
metrics across the  physical links 
Regds,
Sujay

-----Original Message-----
From: v punati [mailto:[email-masked]] 
Sent: Tuesday, April 25, 2006 12:17 AM
To: [email-masked]
Subject: [OSPF] question about RFC 2636

Hi ,
 
 What is the significance of Vertex-id in Intra_SPF run. RFC-2328 says
when the metrics are equal we should filter the route based on
Vertex-id.
 
I am attaching contents from RFC 2328 16.1 paragraph. My question is
whether we can have any topologies which can run in this situation? Is
this only a transit condition ? or any topology can have this one as
persistant condition? Whats the problem if i don't filter the route
based on vertex-id.? My question is about the high-lighted portion
below.

kris

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From: Acee Lindem <[email-masked]>
Date: Wed, 26 Apr 2006 10:06:40 -0400

v punati wrote:

>  I am attaching contents from RFC 2328 16.1 paragraph. My question is whether we can have any topologies which can run in this situation? Is this only a transit condition ? or any topology can have this one as persistant condition? 
>
Hi Kris,

If the transit network is physically partitioned, this situation could 
be persistent.
If it is a transient condition, the routing table should converge once 
you've received
updates for all the routers connected to the transit network.  A similar 
conflict
could occur between a transit network and stub due to partitioning or a
misconfiguration.

>Whats the problem if i don't filter the route based on vertex-id.?
>
I guess the motivation is to deterministically route to one of the network
partitions. Since the network is partitioned or this is a transient 
condition, I
really don't see much benefit in handling this case separately.

Hope this helps,
Acee

> My question is about the high-lighted portion below.
>   
>   
>------------------------------------------------------------------------
>