Label swapping between ASBR's

Unanswered Question
Oct 19th, 2007
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Hi Team,

I need a clarification how the lables are swapped between ASBR's for two different AS's in case of Inter-AS L3 MPLS VPN OptionB.

Because when i see a route from PE-ISP-A for a prefix it has two label stack. One has come from the remote end PE (PE-ISP-B) inner label or VPN label, and the other label is called the IGP label which will be used to reach the ASBR-ISP-A.

So my query is when the packet has come to ASBR-ISP-A, Will the packet go as labels or without any lables to ASBR-ISP-B.

If packet has gone to ASBR-ISP-B without any labels then how ASBR-ISP-B knows the packet has to reach the PE-ISP-B because we dont populate the VRF routing table in ASBR's in case of Option-B.

Kindly clarify.

Thanks in advance.

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swaroop.potdar Fri, 10/19/2007 - 05:59
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Arun, the packet has to reach the ASBR-B with a top IGP label. Since you see a 2 label stack, in your configuration you must have done a mutual redistribution of your loopbacks into BGP and remote AS loopbacks from BGP to your IGP. This will results in a end-to-end single LSP without a stack.

Please refer to this previous conversation for more details.



ilya.varlashkin Sat, 10/20/2007 - 12:12
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If ASBR's are from different AS'es or from two member AS'es of the same confederation but doing next-hop-self, then they advertise prefixes with implicit-null label which causes PHP. So when packet with two labels in stack comes to ASBR, lookup for the top label (denoting this ASBR) will return "pop the label". For the second level label ASBR will have to lookup label in the VPNv4 address family and swap the label. VPNv4 traffic is send with single label between ASBR's. But since each ASBR has prefixes for all VRF's (i.e. they're just like PE from this prospective), they perform normal route lookup and find appropriate label.


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