Subject: [Isis-wg] Zero Metrics in TLV 128?

From: Abhishek Verma <[email-masked]>
Date: Thu, 14 Oct 2004 22:34:47 +0530

Hi All,

I notice this.

R1 (x.1/24) ----------- R2 (x.2/24)
                              |__ 192.168.1/24

R2 is also connected to another interface 192.168.1/24 and this
interface has been added to ISIS. R1 and R2 are L1L2 routers. R1 in
its L2 LSP advertises 192.168.1/24 network in its TLV 128 (IP Internal
Reachability) with Zero metric. It does not include this interface in
its L1 LSP though.

Thus would an ISIS router advertise the IP reachability information
that it has learnt from other ISIS routers with metric Zero to other ISes? 

Is this normal?

When i connect R3 to R2 on another LAN, I see it advertising the
network x/24 (the one over which R1 and R2 are connected) with metric
20 back to R2 in its L2 LSP. This is again, included in TLV 128.

I thought, since it learnt this information from other ISes it should
have advertised this with metric Zero.

I think i got lost somewhere!

Any clues, anybody?

Regards,
Abhishek.

--
Class of 2004
Institute of Technology, BHU
Varanasi, India

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From: Marc Binderberger <[email-masked]>
Date: Thu, 14 Oct 2004 22:33:36 +0200

Hello Abhishek,

maybe I can answer parts of your question. Based on my experience with 
Cisco equipment (haven't checked if all details are a MUST per specs of 
if Juniper and other may behave differently. Comments about this are 
welcome :-)

R2 put the 192.168.1/24 into the L1 database by default (unless you 
configure "isis circuit-type level-2-only" in the interface context). 
It also redistributes all L1 information into L2 (modulo up/down bit - 
another story). So R2 is advertising the 182.168.1/24 in L1 and L2 LSP.

Other L1L2 routers being connected to R2 via L1L2 links will again 
redistribute the L1 information into L2. This L2 information is newly 
generated on R1 and R3 and thus both routers advertise it. The metric 
should be the calculated metric on R1 and R3, i.e. 10 for standard link 
distance R1--R2 / R3--R2 and 10 again for the default interface 
distance of the 192.168.1/24 interface.

So I'm not surprised by your finding of R3. But I can't explain the 
zero metrics on R1 (?).

Regards, Marc

--
Marc Binderberger    <[email-masked]>