Subject: LSP Failure Notification

From: "Dekany, Steven" <[email-masked]>
Date: Fri, 20 Apr 2001 09:30:34 -0400

Greetings;

I am looking for help, with the following:

Assume I have an MPLS Service Provider network, running RSVP-TE as a
signaling protocol. Customer sites are connected to my network via various
IP only links - no MPLS.   Upon a LSP link failure in my core - assume that
I have exhausted all back-up, re-route etc., options and my LSP tunnel is
still down - what happens to my incoming IP traffic at the ingress LER? Is
there an ICMP message generated back to IP Source with "Destination
Unreachable? , or an SNMP Trap, or both? Or something else? 

(Or is this a dumb question, because I have not read the relevant draft...)

Any help or pointers would be greatly appreciated.

Thanks in advance,

Steven

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From: Haoxin Song <[email-masked]>
Date: Fri, 20 Apr 2001 09:58:05 -0400 (EDT)

Assume you have L3 routing table, wouldn't the pure ip routing be used for
the incoming IP traffic in case of LSP Failure? 

Haoxin

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From: Giles Heron <[email-masked]>
Date: Fri, 20 Apr 2001 15:53:59 +0100

> Unreachable? , or an SNMP Trap, or both? Or something else?

It depends...

What happens when a customer packet arrives at your ingress LER (under
normal circumstances) is the following:

1.  The destination IP address is looked up in the forwarding table.
2.  An address is returned from the lookup (this will be the BGP
next-hop associated with the prefix which matched the destination IP
address.)
3.  This address is looked up in the forwarding table (so this is a
recursive lookup.)
4.  An RSVP-TE signalled LSP will be found which matches this address.
5.  The packet is labelled and then forwarded via the LSP.

In the case you have outlined steps 1 to 3 will proceed as before. 
However step 4 will return an IP next hop on a connected interface,
rather than returning an LSP.  There should always be an IP route
matching the BGP next-hop (unless the lack of this route is the reason
why RSVP-TE is failing, but then you probably wouldn't have a working
iBGP session - but I digress!)

So the packet will be forwarded using normal IP forwarding.

If the core of your network is also running full BGP routes this will
work fine (and in fact the traffic may be switched into an LSP by a
subsequent router.)

However if the core is not running BGP (as I would expect to be the case
with ATM LSRs) then the packet will be discarded at the next hop.  But
of course the next hop won't know how to route an ICMP packet back to
the source (since the address of the source is also in BGP) so I'd
expect this ICMP to be discarded at source.

This is probably why running LDP is generally seen as a precursor to
removing BGP from the core of the network?

Someone (or many someones) will no doubt correct me if I'm wrong.

HTH,

Giles

-- 
============================================================
Giles Heron      Principal Network Architect      Gone2 Inc.
ph: +44 7880 506185         "if you build it they will yawn"
============================================================

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From: Bora Akyol <[email-masked]>
Date: Fri, 20 Apr 2001 07:00:13 -0700 (PDT)

The tunnels are established using IGP reachability information. When the
tunnel goes away, IGP route will become the next best route and traffic
will be traveling via IP.

This may take a while, but until the head end realizes that the tunnel is
down, there really is not much you can do. The packets will fall on the
ground.

I believe G-MPLS defines a failure notification mechanism.

Bora

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From: Rajiv Papneja <[email-masked]>
Date: Fri, 20 Apr 2001 12:08:07 -0400 (EDT)

Hi Bora,

 See the inlines:)

> down, there really is not much you can do. The packets will fall on the

 I think, probably using local repair can reduce the loss of packets. 

 -Rajiv
> > Any help or pointers would be greatly appreciated.

********************************
Rajiv Papneja (GRA)
Advanced Internet Laboratory
George Mason University,
G10, Johnson Center,
Fairfax, VA 22030-4444
Tel: 703.993.4703
email: [email-masked]
********************************