NH ???

Unanswered Question
Sep 25th, 2007

When using the 10-B option of the RFC:4364 from an Inter-AS scenario.

1>We have AS1 having[CE1-PE1-ASBR1] & AS2 having[ASBR2-PE2-CE2].

2>Now with all the devices up and configs for option 10-B loaded having >>DEFAULT<< settings for the control-plane protocols.

What shud be the NH @ PE2 for the networks learned via CE1 ???

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Overall Rating: 4 (3 ratings)
swaroop.potdar Tue, 09/25/2007 - 07:25

It will be ASBR1 IP.(the interface IP connecting ASBR1 and ASBR2-EBGP link). This will change to ASBR2's loopback IP used for IBGP if you have made if next-hop self on ASBR2 for routes learnt from ASBR1.



dara111983 Tue, 09/25/2007 - 12:32

Thanks, Swaroop.

Saying more can break the NDA.

But Cisco should use the word default or not/else we assume it that way ;)

swaroop.potdar Tue, 09/25/2007 - 12:55

There shouldnt be any concern of NDA in this, its quite open and reproducible. I would be glad to clear any more doubts you have to the best of my understanding.



dara111983 Tue, 09/25/2007 - 12:58

I was just willing to clear this thing up!!!

And undoubtedly, you made me more confident that the answer I thought and learned was correct!

I am Thank full to you :)



swaroop.potdar Tue, 09/25/2007 - 13:12

Dara, there is one more clarification, in your topology in absence of the P router, you will see a 3 label stack without using the next-hop-self. And if you use the next-hop-self you will see only 2 label stack.

If you add a P device then without the next-hop-self you will have a 4 label stack, and with it 3 label stack.



dara111983 Tue, 09/25/2007 - 15:01

If I am Correct!!!

This third label in discussion is the middle label.

When using NH-self @ ASBR2 the labelling @ PE2:

a]label 1 will be VPN label for the destination prefix FEC.

b]label 2 will be the label generated via LDP by P-router for FEC of PE1 IP.

iff the prefix is known via LDP

the third middle label is bcos of the BGPV4+label @ ASBR2 distributing the label's for the prefixes in AS1.if we redistribute these prefixes in IGP than there shud be no third/middle label.

am I right on this???



swaroop.potdar Tue, 09/25/2007 - 16:27

Yes you got it right!.

Also have a look at when and why you get a 4 label stack ..that should affirm things more.



dara111983 Wed, 09/26/2007 - 01:26

Hi Swaroop,

Can you provide in a scenario for that!

My guess here would be,

1>More than one technology of MPLS integerated would require 4 labels to achieve it's purpose.

2>CSC + some features in carrier backbone's.


a] MPLSVPN/ATOM/6PE/6VPE + TE = 2labels

b] MPLSVPN/ATOM/6PE/6VPE + FRR = 3labels

So I need a scenario and the purposes which we can accomplish when we using control-plane to get 4 labels!!!

swaroop.potdar Wed, 09/26/2007 - 10:50

Dara, we dont accomplish anything by using a 2 or 3 or 4 lable stack. Its more for the dependency that we have to use the label stack. When a certain FEC at the remote end is conveyed to the local end through multiple hops which use different methods to assign labels (apart form LDP) this gives use the label stack. If we can override the dependencies and have a transparent LSP (redistrbuting other AS learnt routes from BGP to IGP and vice versa) then we can achieve the same using 2 label stack.

So to conclude its more for the dependency that an edge LSR has to assign multiple labels for a packet to be forwarded.

Now to simulate the 4 label stack, in your original topology you can add one more router as a P router between any PE and ASBR and dont set the next-hop self on the ASBR which has a P router in its AS, this will show a 4 label stack. If you see the CEF output it will be evident when and why its its a label and where all its utilized the 4 labels.



dara111983 Wed, 09/26/2007 - 13:46

Thanks Swaroop,

Good explanation,

Thanks a lot and now I can get your logic!!!


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