From: Tim (ccie2be@nyc.rr.com)
Date: Mon Aug 21 2006 - 09:51:18 ART
Hey Chris,
Thanks for getting back to me.
Based on what you said, that would seem to imply that a LSR could use the
same label for every route in its route table.
So, for example, if a LSR had only 2 interfaces - 1 going to an upstream
edge LSR and another going to a downstream edge LSR - and 100 routes in it's
route table, then it could advertise the just ONE label for all 100 routes
to its upstream neighbor and another label (or maybe even the same label) to
its downstream neighbor.
Is that right?
------------------
Also, from the previous post, it was noted that a LSR will have 3 labels for
each route in its route table:
1 label which it learned from the downstream router (edge LSR)
1 label which it learned from the upstream router (edge LSR)
and 1 label which it generated itself
Why does it need 3 labels for the same route and what does it do with them?
It's clear that it will use the label from its downstream neighbor to label
packets going downstream and the label it generated itself will be used by
its upstream edge LSR neighbor to send packets its way.
But, what will this router do with the label it learned from its upstream
neighbor for this route. It seems that that label isn't ever used.
Thanks, Tim
_____
From: Chris Broadway [mailto:midatlanticnet@gmail.com]
Sent: Monday, August 21, 2006 7:06 AM
To: Tim
Subject: Re: MPLS newbie w/ a couple basic Q's
Here is a quick email that might help you understand MPLS. Each router will
have a label for each path to their neighbor, not the network destinations.
So rtr2 will have a label to get to rtr1 and one to get to rtr3. for
example, traffic that comes from rtr1 going to NET D will have a label
assigned to it (30 for this purpose). The packet with a label of 30 will
get to rtr2 and see it needs to SWAP label 30 for label 35 (for this purpose
only) which is the path to rtr 3. Rtr3 then POPs the label 35 off and the
packet is once again Native IP. The packet is sent to Net D. The key is
how does the rtr know what label to put on. That information is kept in the
MPLS bindings (mapping) database, which binds labels to IPs (L2 to L3).
This example does not discuss PHP.
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