Subject: [Isis-wg] Doubt

From: selvarajR <[email-masked]>
Date: Mon, 11 Sep 2000 20:44:56 +0530

Hi,
My doubt is regarding , having more than one adjacency with same system Id 
but different SNPA.
I hope this is possible when there is any backup link present

I assume that,  if any Hello PDU be received by an IS with system Id same 
as the received IS, then it can be discarded. ie Receiving own hello PDU.

          IS - 1                              IS - 2
             |                                       |
             | c1                                  | c2
-----------------------------------------------------------
                     c3 |     | c4
                          |     |
                          |     |
                         IS - 3

For the above broadcast topology  let,
   IS-1, IS-2 & IS-3 be three L1 intermediate  systems
   c1, c2, c3, c4 be the respective IS's circuits. where c4 be the back up 
link for IS-3.
   Let priority of c1 be 10, c2 be 20, c3 be 30 and c4 be 40.

Here, IS -1  and IS -2 will have two  adjacencies for  IS -3 with same s  
ystem Id but, different SNPA.
But, IS -3 won't have any adjacency of it's own. ie c4 is not known to c3 
and vice versa

DOUBTS:
1. After DR election c1 , c2 and c4 will say c4 as the DR. But c3 will say 
 c3 as the DR. So the  LAN will have more than on DR at a time
2. If  the IS won't have two interfaces on the same LAN as in the above 
topology , then why do we need to check for system ID and SNPA. for 
creating new adjacency over a circuit.   I feel checking for System ID 
alone sufficient. So we can discard duplicate IIH.

Pls  correct me , if  I'm wrong.

 Selva.

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