From: Tim (ccie2be@nyc.rr.com)
Date: Mon Aug 21 2006 - 07:27:19 ART
Vikram,
Thanks so much for taking the time to respond to my MPLS Q's. I really
appreciate that.
Thanks also for confirming that each MPLS router generates a label for every
route in its route table.
That raises another question though given this MPLS network:
net-A rtr-1 net-B rtr-2 net-C rtr-3 net-D
Consider rtr-2:
rtr-2 learns about net-D via the IGP.
And, based on what you told me, rtr-2 will generate a label for net-D since
that route is in its route table. It will also learn of the label rtr-1
generated for net-D as well as the label generated by rtr-3 for net-D.
THAT'S 3 labels !!!
Now, it's obvious that rtr-2 will need to know the label generated by its
downstream neighbor, rtr-3, for net-D since when a labeled packet arrives
from rtr-1, rtr-2 will need know which label to use when swapping labels to
send the packet downstream to rtr-3.
But, what the heck does rtr-2 do with the label for net-D it learned from
its upstream neighbor, rtr-1?
Assuming I understand this correctly, rtr-1 will use the label generated by
rtr-2 to send traffic downstream towards net-D to rtr-2, correct?
Also, is it possible for a router, let's say rtr-2 in this example, to
generate a label with the same value as the label it learned from rtr-3 for
net-D?
IOW, why doesn't rtr-2 just advertise the label it learned from its
downstream neighbor, rtr-3 to its upstream neighbor, rtr-1? If it did that,
then wouldn't traffic flow faster since rtr-2 wouldn't have to actually swap
labels for packets going to net-D?
I know I'm asking lots of Q's but as you can see, my understanding so far is
very limited.
Thanks again for sharing your knowledge with me.
Tim
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Vikram Dadlaney
Sent: Sunday, August 20, 2006 1:11 PM
To: ccielab@groupstudy.com
Subject: Re: MPLS newbie w/ a couple basic Q's
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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