From: Tim (ccie2be@nyc.rr.com)
Date: Mon Aug 21 2006 - 11:29:46 ART
Hey Chris,
Thanks for bearing with me as I try to absorb what's going on here.
I need to think about the first part of what you wrote some more but let me
confirm I understand the 2nd part regarding the 3 labels for net-D on rtr-2.
To stay in sync, I'll repeat the network diagram here:
net-A rtr-1 net-B rtr-2 net-C rtr-3 net-D
rtr-1 generates a label, let's say its = 10, for net-D and advertises it to
rtr-2.
rtr-2 generates a label, = 20, for net-D and advertises it to rtr-1 and
rtr-3.
rtr-3 generates a label, = 30, for net-D and advertises it to rtr-2.
So, now rtr-2 knows of 3 labels for net-D.
When a packet for net-D arrives on rtr-1, rtr-1 will use the label 20 it
learned from rtr-2 to send the packet to rtr-2, correct?
And, once the labeled packet gets to rtr-2, rtr-2 will then use the label it
learned from rtr-3 (30) to send the labeled packet to rtr-3, correct?
So, in this example, label 10 will never be used, right?
However, if rtr-2 for some reason ever did need to send traffic for net-D to
rtr-1 ( assuming a different topology), it would then use label=10 to send
traffic to rtr-1.
Am I close to being accurate in this description?
Thanks again, Tim
_____
From: Chris Broadway [mailto:midatlanticnet@gmail.com]
Sent: Monday, August 21, 2006 9:52 AM
To: Tim
Subject: Re: MPLS newbie w/ a couple basic Q's
To answer the first portion, you have to look at the whole picture and the
current method of MPLS deployment. You would have your LSRs running MPLS in
the MPLS domain and either OSPF or ISIS be the IGP for this domain. This is
on your diagram as rtr1,rtr2, and rtr3. Network "D" is also important and
contributes to the operation and how things get labeled. Consider rtr3 the
"edge" of the MPLS domain (LER). It would have a BGP relationship to the
gateway router for Network "D". This would make rtr 3 aware of the BGP
table from Network D. YOU DO NOT WANT THE CORE LSRs TO HAVE THIS KNOWLEDGE.
Since your diagram is only three routers, rtr2 would be the "CORE" LSR,
simply because it is not a LER. All LERs would have an IBGP peering to each
other. This would make all LERs aware of Network D. This means rtr1 would
now know how to get to Network D, is through rtr3. But how does he get to
rtr3? This is where the IGP is used. The IGP is used only as the transport
for the BGP. Even if there were 100 routes in network D, rtr2 would only
see the IGP to rtr1 and 3 in its routing table. The BGP table of the LERs
is what would show all the routes. With all this said, I have not discussed
layer2 or 3 VPN and VRFs used with MPLS. But it would generally still
follow this principle.
The second portion:
the router knows of the label coming from rtr1 and rtr3. Also the router
knows of the label it gives for traffic going to rtr1 and 3. The operation
from rtr2 would sound something like this:
"I see signaling coming in on label 30...I know anything from label 30 is
from rtr1. The signaling is trying to get to rtr3...I know I have to swap
label 30 with 35 for signaling going to rtr3"
Then the opposite happens for the reverse traffic. The important thing here
is that all that is set up before the actual traffic flow. The MPLS
signaling (LDP or RSVP) is what determines the label paths and sets up the
binding database. If the signaling does not happen, MPLS cannot work.
-Broadway
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