Subject: [Isis-wg] Doubt on L2 router

From: "Swati Rastogi" <[email-masked]>
Date: Mon, 6 Aug 2001 20:32:38 +0530

Hi All,
In OSPF if the designated router fails then the backup designated router
takes over and all of this is done automatically. But in ISIS we have a
router configured as a L2 router drilled in the profiles. What would happen
if this router goes down for some reason ? Since this is the only link via
which the routers inside this area can communicate to the outside world.
Does it imply that the routers are now blackholed !

This would imply that the protocol has a single point of failure ..

Regards,
Swati Rastogi

_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com

_______________________________________________
Isis-wg mailing list  -  [email-masked]
http://external.juniper.net/mailman/listinfo/isis-wg

---

From: "Manav Bhatia" <[email-masked]>
Date: Mon, 6 Aug 2001 20:50:17 +0530

If a single level 2 router looses connectivity to the level 2 backbone then
this router will indicate in its L1 routing packets that it is not
"attached", thereby allowing level 1 routers in the area to route traffic
for outside of the area to a different L2 router. Level 1 routers therfore
route traffic to destinations outside of their area only to L2 routers which
indicate in their level 1 LSPs that they are "attached"

Coming back to your question .. yes the networks get blackholed !

There is no provision for use of L1 routers to repair a level 2 partition !

Hope this helps
Manav

---

From: Jeff Learman <[email-masked]>
Date: Mon, 06 Aug 2001 11:45:21 -0400

>Does it imply that the routers are now blackholed !

Are you confusing "designated router" with "L2 router"?
In IS-IS, they're separate -- there is one L1 designated router
on a LAN, and the election is automatic.  On a LAN with
more than one L2 router, there is also one L2 DR, and
it is elected in the same way.

If your L1 area has only one router connecting it to other
areas, then you have a single point of failure that has
nothing to do with IS-IS.  If your area has multiple
connections to other areas, then you don't.  (There is
also a practical requirement that your L2 routers are
connected within your area, either directly or by L2
routers, if your area is used for transit routes between
other areas.)

---

From: "Ken Larmer" <[email-masked]>
Date: Mon, 6 Aug 2001 13:58:26 -0400

> -----Original Message-----
> From: [email-masked]
> [mailto:[email-masked]]On Behalf Of Jeff Learman
> Sent: Monday, August 06, 2001 11:45 AM
> To: Swati Rastogi
> Cc: isis-wg
> Subject: Re: [Isis-wg] Doubt on L2 router
>
>

Of course, as with most things in life, there are exceptions. If you have an
IS, which has multiple L1 IS instances configured, in order to get between
L1 instances, some vendors set the Attached bit in the L1 LSP (LSP Number =
0)  which they generate. While this allows for connectivity between L1 areas
(each L1 IS instance is a separate L1 area "Sub-Domain"), it does not take
into account some of the basic rules Jeff mentioned for setting the attached
bit. That is, the attached bit will be set, regardless of whether or not
your L2 Instance is connected to another L2 area. As a consequence, you may
be able to get between L1 instances, but you may not be able to get outside
of the physical router you are connected to if the L2 backbone is not
contiguous. Or, you may take a less than desirable path to your destination
because the "nearest L2 router" may contain a less desirable route. This is
why it may make more sense in some situations to advertise an IP default
route instead of relying on the "Attached Bit".

Cheers,
Ken Larmer