Subject: [Isis-wg] Setting the over load bit in an MPLS network

From: Amir Birjandi <[email-masked]>
Date: Mon, 27 Aug 2001 17:12:48 -0400

Hi,

After reading  IS-IS Transient Black hole Avoidance
<draft-ietf-isis-transient-01.txt> draft i have this basic question.

Imagine,

we have a multihop Label switch path, which is carrying the traffic. 

routerA---routerB----routerC----routerD

the label switch path---> from A to D.

IGP== ISIS

there are also a number of disjoint paths between A and D.

If we set the overload bit on C or B,  how would router A know it should
stop sending traffic over that Label switch path.

Any comments is highly appreciated

Amir 
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---

From: stefano previdi <[email-masked]>
Date: Tue, 28 Aug 2001 09:39:32 +0200

>stop sending traffic over that Label switch path.

it will know it through spf.

labels are bound to routes. When your neighbors advertise you label
bindings, you select the label for a given route if such label has
been bound by the neighbor who is also your next-hop for that given
route.

Setting the ovl bit will change the topology, therefore the routing
table, and therefore your next-hops. So, you will use other labels
from other neighbors (the new next-hops).

s.

---

From: Naiming Shen <[email-masked]>
Date: Tue, 28 Aug 2001 12:10:21 -0700

]we have a multihop Label switch path, which is carrying the traffic. 
 ]
 ]routerA---routerB----routerC----routerD
 ]
 ]the label switch path---> from A to D.
 ]
 ]IGP== ISIS
 ]
 ]there are also a number of disjoint paths between A and D.
 ]
 ]If we set the overload bit on C or B,  how would router A know it should
 ]stop sending traffic over that Label switch path.
 ]

I would think you may not want to stop sending traffic down this Label
switched path.

The overload bit here is used as a hint to say C or B's bgp has not yet
converged, it is possible that the label switched path calculation does
not pay attention to ISIS overload bit and still sets up the label switched
path through B and C to D before BGP on B or C can converge. In this case,
routers B and C knows how to switch the mpls packets regardless of BGP
convergence.(in some cases, the forwarding and rsvp module are always up,
so the label switched path is never down)

But whether routerA can send traffic down this A->B->C->D lsp depends
on implementation, configuration and topology of the network. In some
implementation and configuration, as long as the tunnel destination is
the bgp nexthop, it can always switch; Even if the IGP routes are
tightly coupled with the label switched paths, as long as the IGP has
some alternative paths for routerA to reach routerD, it may still forward
packets into this label switched path.
If the IGP was ospf, then the "overload bit" equivalent should not prevent
routerA sending traffic down this label switched path regardless of
the topology of network.

- Naiming