On 25/06/07, paul at clubi.ie <paul at clubi.ie> wrote:
> On Mon, 25 Jun 2007, Thomas Bridge wrote:
> > Not to mention the fact that in an MPLS core - the ICMP TTL expired
> > packet often has to continue down the LSP to reach the remote end
> > point before being sent back over the return LSP, mean that the
> > "latency" value for all the MPLS hops tends to be pretty identical.
> Shudder.
not shudder - it's part of how it works. And why you have to be
careful parsing the output of traceroutes over an MPLS network.
> > Bonus geek points for anyone who can tell me why this doesn't apply to
> > the second last router in the LSP.
> Cause MPLS has some kind of hack whereby the second last can pop the
> label on behalf of the last.
It doesn't POP "on behalf of" - it POPs the label to forward the
encapusalted MPLS packet / IP packet / whatever it is so that the
final device doesn't need to do two lookups.
You are in fact correct - depending on how the second last hop is
configured you either loose the packets altogether or they are
returned directly as an IP packet.
> Does that always apply though (if it
> does not, as I suspect you meant "need not apply").
In theory, no. In practice, everyone uses PHP.
> MPLS.. shudder.
Another example of why Paul is right and the rest of the world is wrong? :)
Thomas
--
Thomas Bridge
CCIE #14108
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