Subject: RE: [Isis-wg] Overload Bit

From: "Suresh Boddapati" <[email-masked]>
Date: Fri, 08 Oct 2004 15:08:52 +0000

>
>A -- B -- C
>
>[1] A, B and C are 3 L1/L2 ISIS routers. B has its overload bit set.
>This as per what i have read "ensures that no other router uses this
>local router for transit traffic".
>
>Now, here goes my problem(s):
>
>ISIS adjacencies are up between these 3 routers. A is aware of C's
>loopback and C is aware of A's loopback IP address. That could be say,
>because we have added loopback interfaces to our areas.
>
>Now if I try to ping A from C, should my ping go?
>
Assuming B set the overload bit, no, your ping should not succeed because A 
should ignore B's nbrs in its SPF calculation.

>IMO it shouldnt. Only the traffic destined to B's directly connected
>interfaces should be allowed. Also does this technically mean that
>when A receives an LSP from B with this bit set, it should not install
>it?
No. A should install the LSP from B, precisely to be able to reach B's 
subnets (see below).

>
>If that is the case then why would B advertise its LSP in the first
>place? When it knows that A and C are not going to use that?
>
Because you still want to be able to reach B's directly connected subnets.  
B is saying "Dont use me as a transit, because I may not have the complete 
topology information. But you can use me to reach any of the networks that I 
have advertised". Implementations also allow a node to declare itself 
overloaded for a certain amount of time to avoid transient blackholes ( 
check out the relevant draft on this).

Suresh

>Thanks,
>Abhishek
>
>--
>Class of 2004
>Institute of Technology, BHU
>Varanasi, India
>
>_______________________________________________
>Isis-wg mailing list
>[email-masked]
>https://www1.ietf.org/mailman/listinfo/isis-wg

_______________________________________________
Isis-wg mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/isis-wg

---

From: Henk Smit <[email-masked]>
Date: Sat, 9 Oct 2004 01:08:48 +0200 (CEST)

> should ignore B's nbrs in its SPF calculation.

  It depends.
 If A tries to ping the IP address of the loopback interface of C, it should
 not succeed.
 If A tries to ping the IP address of the p2p interface of C that is
 connected to B, then the ping *will* succeed.

> >interfaces should be allowed.

  Agreed. But you should realize that the IP address of C's p2p interface
 is in fact in an IP range that is advertised by B, because B has an IP
 address in that same range configured on its p2p interface to C.

  Suresh already answered the other questions.
 Hope this helps.

    Henk.