Subject: [Isis-wg] isisSysInstance

From: "Mammen, Biju" <[email-masked]>
Date: Wed, 10 Oct 2001 19:24:21 -0400

Hello all,
This question seems to have popped up time and again but have gone 
unanswered on all occasions. Some guidance is really appreciated: 

a) What would be the implication of defining multiple instances of 
ISIS (as per draft-ietf-isis-wg-mib-05.txt)?

b) Would all these instances work on the same Unified routing table or
different ones?

c) Would each of this instance have its own LSDatabases?

d) Is this the same as CISCO's view of multi area support in a single
router?
( A single Level 2 and 28 Level 1s)

Thanks and regards
Biju

---

From: Naiming Shen <[email-masked]>
Date: Thu, 11 Oct 2001 02:03:17 -0700

]
 ]Hello all,
 ]This question seems to have popped up time and again but have gone 
 ]unanswered on all occasions. Some guidance is really appreciated: 

   Never seen those questions before.

 ]
 ]a) What would be the implication of defining multiple instances of 
 ]ISIS (as per draft-ietf-isis-wg-mib-05.txt)?

   More work.

 ]
 ]b) Would all these instances work on the same Unified routing table or
 ]different ones?

   It depends.

   One can have a single instance using multiple routing/forwarding
tables, such as in the case of M-ISIS(multi-topology isis).

   One can also have multiple instances using a single routing/forwarding
table. A good example will be this: an ISP has an old backbone, and is
building a new/next-generation backbone. Those two backbones use two
seperate IGPs(ISIS in this case) at least before the complete migration.
If some of the routers are shared among those two backbones, they may
want to put two ISIS instances on those routers and they can
share(or not to share) the same routing table, even though two ISISes
don't talk to each other. Since the backbone ISIS usually handles router's
loopback addresses and internal networks, they may not have overlapping
address space, so there is no reason not to share the same routing
table. Even in the case they have overlapping addresses, they can
always set one instance to be prefered in distance and hopefully
they go to the same destinations. They can even redistribute routes
between the two instances, so they can run two seperate IGPs but
only one instance of BGP. Just to make sure the redistributed routes
are tagged properly or using some address mask scheme so that the
routes will not be redistributed back to it's originating instance.

   In the case of VPN application, they have overlapping address space
and also destinating to different places, then multiple instnaces need
to have multiple routing tables.

   So in general, we can say N instances will result in M routing tables,
where M and N > 0, depends on the network requirement and implementations.

 ]
 ]c) Would each of this instance have its own LSDatabases?

   this is always true.

 ]
 ]d) Is this the same as CISCO's view of multi area support in a single
 ]router?
 ]( A single Level 2 and 28 Level 1s)

   no comment on specific implementation.

thanks.

 ]
 ]Thanks and regards
 ]Biju
 ]

- Naiming
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