From: Vikram Dadlaney (vdadlaney@gmail.com)
Date: Sun Aug 20 2006 - 14:10:35 ART
Hi Tim,
I shall try and take a shot at this.
A router generates a local label for all the routes in its routing table. It
establishes a TDP/LDP relationship with neighboring routers and than
exchanges labels with those routers. I am going to assume Frame mode over
here only (Partly because I don't understand cell mode myself :-) ).
So in your example RTR-1 would generate a local label for network-D since it
is in its routing table. Than it would exchange that label with its TDP/LDP
neighbors (RTR-2 also in this example even though its considered downstream
with regards to network-d) which have also generated its own label for
network D and so on.
Now say you have traffic destined for network-d generated by a host on
network-A. The packet would come to rtr1 (ingress router with regards to
traffic destined for network-d). RTR-1 would look up the label that it
received for network-D from rtr-2 and impose that label on the packet and
send it to RTR-2.
When the packet reaches RTR-2 it is a labelled packet and hence RTR-2
would look up the label attached to the packet and swap that label with the
label it received from RTR-3 for network-d and send the packet on its way.
I haven't taken into account PHP which would basically mean that instead of
swapping the label on the packet and sending it to rtr3, rtr2 would just pop
the label and send a pure ip packet to RTR-3. this way RTR-3 doesn't have to
do a label lookup.
Now for your question that when would RTR-1 use the label for D. Well
in this example it wouldn't use the label for network-d since when a packet
for destination network-d comes into RTR-1 it is a pure ip packet. But say
you had another rtr connected via Net-A to RTR-1 and than a packet would
have been generated by a network connected to that router than RTR-1 would
have to advertise its local label to that router and label switch the packet
based on its label.
I am not quite sure about your other questions so if you could elaborate a
bit that would be helpful.
Hope this helps. I am myself currently learning MPLS so my
explanation may be wrong but am pretty sure and hope the other folks would
correct me should it be wrong.
Regards,
Vikram
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