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New Member

MPLS Doubt

A packet's LSP can extend only until it encounters a router

whose forwarding tables have a longer best match address prefix for

the packet's destination address. At that point, the LSP must end

and the best match algorithm must be performed again."

How is possible for an transient LSR to know abt the destination prefix.It has just to do Label SWAP/POP/PUSH?

5 REPLIES
Cisco Employee

Re: MPLS Doubt

When a packet enters an LSP, it travels along the LSP till the end regardless whether any of the transit routers have a more specific entry in their RIB to get to destination. This is because the ip destination address is not looked up on a hop by hop basis once the packet has entered the LSP.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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Re: MPLS Doubt

The quote you have used is more mapped to any kind of summarization towards the core.

That is till the first point where you have originated the summary you will have LSP1 which will carry all the specific IP prefixes falling in the summary range using the LSP and when it reaches the the summarization originator router, it will have to do a best path lookup and do a second imposition of the Label towards the longest matching destination uisng an LSP2.

So in such a scenario there will be a intermediate L3 lookup, and when such a intermediate router

receives a frame via an LSP it will be already popped without a label, as it the orginator of the summary, and then the same router needs to do imposition for the more specific prefix towards the egress.

The intermediate routers only should be used for SWAP/POP/PUSP ideally.

This is since because, if the LSP wasnt for a normal IP packet, and infact a VPN packet,

then there cannot be any kind of further L3 lookup done and it will get dropped.

As intermediate routers do not have VPN information.

And on the contrary, if you have a summary and a specific available across all the P/PE devices in your network, then this will never happen, as the routers will put the incoming frames onto the LSP which is created end to end using the specific prefix information.

HTH-Cheers,

Swaroop

New Member

Re: MPLS Doubt

Thanx Hritter and Swaroop for your valuable posts. Hritter, this is wat my understanding regd LSP.But RFC language is bit confusing, RFC says LSP breaks when summariztion happens, Section 4.1.3 of RFC 3031.

Swaroop, is FEC the same concept like aggregation / summarization.....if yes even then after label binding, FEC are never taken in consideration and only swapping/push/pop has to happen.

So the question remains the same how come transient LSR will come to know about IP destination ?

Re: MPLS Doubt

Yes an aggregate/summary is also a FEC.

Now what this means is, this is an eligible entry for which LDP(or other protocol) can generate a Label binding.

So this means that using this FEC label bindings are exchanged with all the LSP peers.

Once the label bindings are in place, then you install they in the Data Plane to be used for forwarding.

If any transient LSR receives a packet without a Label, (if there is summary done by this router then he will recieve a packet without MPLS Label, as it would be popped by the PH.) then he needs to do a second lookup, and do a right imposition of the label to be used next.

This is how a LSP breaks and how and when a transient LSR has to do more than PUSH/SWAP/POP.

HTH-Cheers,

Swaroop

New Member

Re: MPLS Doubt

Thanx Swaroop

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