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.

---

From: Naiming Shen <[email-masked]>
Date: Tue, 12 Sep 2000 00:34:14 -0700

]
 ]          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

this actually will be worse, the IS-1 and IS-2 may randomly setup
adjacnecies with IS-3 on either c3 or c4 depends on the timing. since
they will rejects the second adjacency setup due to conflict in systemID
and mac addresses.

 ]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.

i guess the idea was to prevent accident. if you want to bring a new
router into the LAN and misconfigure the systemID which duplicates the
one already in operation, the proper behaviour should be the new router
can not join the ISIS on this LAN instead of knocking off the one already
working. since the chances are the new router probably is configured wrong.
so this check on both systemID and mac address is useful.

 ]
 ]Pls  correct me , if  I'm wrong.
 ]

- Naiming

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