Subject: [Isis-wg] Clarification

From: [email-masked]
Date: 30 Dec 2000 22:53:47 -0800

Hi All,

Considering the functionality of a default route,
do we at any time need to advertise the default route in the L1 / L2 LSPs. If so what are the conditions under which we advertise these and when we receive, how do we process the default route.( in case we already have a nearest L2 IS).

Thanx and Regards
Navaneeth.

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From: "Amir H." <[email-masked]>
Date: Sun, 31 Dec 2000 11:45:59 -0000

Navaneeth,

There is a difference between a default route injected into the IS-IS
domain, and the route to the nearest L2 IS.
A router advertising a default route is attracting traffic which fits into
that prefix (which actually is all traffic). So, if there is no more
specific prefix for a given IP address, this route will be used. In this
context, this is no different than any another route. So, you'd process it
just like you'd process other IP External Reachability entries. This is
relevant in both level-1 and level-2 routers.
The nearest L2 IS is only relevant in level-1 (and level-2 un-attached
routers), and is used when a destination is not within an area, according to
RFC1195. However, RFC1195 does not allow external prefixes to be injected
into a level-1 area, whereas the current practice and recent drafts do allow
it. I'd say that this changes a bit the definition in the RFC1195, to
destinations not _advertised_ within an area. So, if a router advertises a
default route into the area, it should be used, just like other external
prefixes not in the area, and not the nearest L2 IS (even if it's closer in
metric). The same is for a level-2 unattached router.
Note, however, that injection of default routes into the IS-IS domain should
be done VERY carefully.

    Hope this helps,
        Amir.

--
Amir Hermelin   mailto:[email-masked]

----- Original Message -----
From: <[email-masked]>
To: <[email-masked]>
Sent: Sunday, December 31, 2000 6:53 AM
Subject: [Isis-wg] Clarification

> do we at any time need to advertise the default route in the L1 / L2 LSPs.
If so what are the conditions under which we advertise these and when we
receive, how do we process the default route.( in case we already have a
nearest L2 IS).