MPLS Label

Unanswered Question
Dec 19th, 2007

Normally LSR router will advertise the label received from downstream router to all router multicast address called 224.0.0.2 so will the receiving router will send the same label to the router from which it already received? if yes then will it create loop? let say we have router a-b-c-d router d advertise network x with label 3 to router c now router c will put that label 3 in its tabel and will generate one more local label 33 for network x and advertise to all router multicast address so here router d will receive the new label 33 for network x so this situation looks confusing to me!

I have this problem too.
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kevin.shi Thu, 12/20/2007 - 04:29

I don't think the lable is advertised globally.

the lable only has local meaning, is propegated one hop, upstream, isn't it?

arun kumar Thu, 12/20/2007 - 04:33

Hi Dan,

There are three functions a LSR will do with the Labels.

Label Generation (Independent & Ordered)

Label Distribution (Downstream On-Demand & Unsolicited)

Label Retention (Liberal & Conservative)

Cisco IP routers will operate on Independent, Downstream Unsolicited and Liberal Retention Mode: LSR will retain the label though it is not the next hop/egress PE. This information is stored in LIB.

and ATM switches and routers work on Ordered, Downstream On Demand and Conservative Retention Mode: LSR will retain the latbels only if it is in the forwarding next hop or the egress PE.

There are two things to consider. First is Control Plane Operation - Generation, Distribution and Retention of Labels stored LIB.

Second is Data Plane operation - How the traffic is forwarded through the LSR's based on the label stored in LFIB.

The above 3 methods are for storing the label values in local LIB. But forwarding path will be based only on LFIB (LIB+FIB from CEF).

Lables from LIB are copied to LFIB only when the router advertised the label is the next hop for the prefix . So in this case though Router D receives the label from Router C for the network "x" and stores it in LIB, D will not install label in its LFIB because Router C is not the next hop for the Network X from Router's D perspective.

You can check in Router D the LIB and LFIB for network x... You can view the label 33 and 3 in LIB but no in LFIB...

LIB: sh mpls ldp bindings x.x.x.x s.s.s.s (control plane)

LFIB: sh mpls forwarding-table x.x.x.x s.s.s.s ( data plane)

Hope this helps....

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