Subject: [Isis-wg] RE: ISIS over NBMA

From: Jeff Parker <[email-masked]>
Date: Tue, 24 Jul 2001 09:43:17 -0400

I will try the third question

> 3) In RFC-2973, ISIS Mesh Groups (page#4), 
> How the numbered mesh groups are connected by "transit circuits" 

> Let's consider a full-mesh topology with three routers A, B, C is divided
in to two mesh groups 1 & 2 (full-mesh)
 
> mesh group 1: A and B
> mesh group 2: B and C

> here B is the transit router whose both interfaces are 'meshset'
> and if the interface link between A and C is 'meshblocked'
> then where comes the transit interface ?
 
> any response in this regard would be of great helpful 
 
> thanks in advance,
> lakshminarayan r

You assume a full mesh of three routers.  
The routers themselves are not in mesh groups: the
circuits are. 

It is easy to configure mesh groups on the network below so 
it is not connected.  One way to configure it so that it
is connected would be to meshblock circuits v and x,
and to put the circuits w and y in different mesh
groups.  This configuration is brittle: breaking
either link AB or BC will disconnect the network,
as link AC does not carry LSPs.  

        A
       u  v
      /    \
     /      \
    w        x
   B y ----- z C

There are more examples in the RFC to check out:
mesh groups become more interesting as N gets
much bigger than 3.  In the example above, if
there is any router that uses mesh groups to
reduce flooding, then we can break a single link
and break flooding.  

If there are 4 routers in a full mesh, it is 
possible to reduce flooding without risking 
disconnection through a single link failure.  

- jeff parker
- axiowave networks
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