Re: [Isis-wg] Carrying MT info in MT-ISIS

["Les Ginsberg (ginsberg)" <ginsberg at cisco.com>]

Leela -

On a given link a router is configured to support a set of topologies. When it forms an adjacency on that link, only the topologies which are supported by both routers (i.e. the intersection of the set of supported topologies advertised by each router in its IIHs) can be used in MT Intermediate Systems TLV (222).

Consider the following simple example:


A----B----C

A supports topologies 0,10
B supports topologies 0, 10, 20 on all links
C supports topologies 0, 20

B forms an adjacency with A. The intersection of the set of supported topologies is (0,10).
B forms adjacency with C. The intersection of the set of supported topologies is (0,20).

In its LSPs B will advertise:

TLV 22
  Neighbor A
  Neighbor C

TLV 222
   MTID 10 Neighbor A
   MTID 20 Neighbor C


This information will be flooded on all links.

LSPs are not generated differently based on which link they are flooded on. And the set of LSPs generated contain information about reachability to all supported topologies.

   Les

> -----Original Message-----
> From: isis-wg-bounces at ietf.org [mailto:isis-wg-bounces at ietf.org] On
> Behalf Of Leela Janaki P
> Sent: Tuesday, November 03, 2009 11:46 PM
> To: 'isis-wg at ietf.org'
> Subject: [Isis-wg] Carrying MT info in MT-ISIS
> 
> Hello,
> 
> I have a question regarding carrying Multi-topology TLVs in ISIS.
> 
> 
> 
> For point to point interfaces, RFC5120 says that MT IS TLV in an LSP
> MUST contain the reachability of the topology only when the adjacency
> is enabled for that topology. This seems to imply that if an IS
> contains multiple adjacencies , each with different topologies , then
> the LSPs sent out on each of them will be different. I just want to
> confirm if this is the desired behaviour. Alternately is it permitted
> for an IS to send a single LSP containing information of all topologies
> supported by the IS or to send out a separate LSP one for each
> topology.
> 
> 
> 
> Thanks,
> 
> Leela
> 
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