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New Member

how an LSR determines the FEC to which an incoming MPLS Frame is to be put?

Hello,

It is understood that when a Edge router receiv IP packets , it is put into an FEC. And more than one destination network may be clasiffied into a single FEC. ( For e.g, if all the outgoing prefixes from a router has to go through a certain next hop, all will be given with the same LABEL). Suppose an LSR -LSR 1- is connected to two Edge routers and receives different LABELed frames from both edge routers, and the LSR 1 has to route the Frames from both the Edge routers to another LSR, LSR 2 - Now, without seeing the destination prefix, how LSR1 will know that Frames coming from both the E

dge routers are to be forwarded to LSR2 ( not to LSR3 which is also connected to LSR1). Can sombody tell /Give links to how this is achieved

                               |--------------------|

Edge router 1 ------- >|                     |                 |-------------------

                               |     LSR 1      |---------------->|LSR2           |

Edge Router 2  ------> |__________..|                  |__________ |    

2 REPLIES
Cisco Employee

how an LSR determines the FEC to which an incoming MPLS Frame is

Hi,

Suppose the FEC is based on the BGP next-hop which LSR-1 has learned through IGP.  Now LSR-1 will have an out-label which it has received from downstream router (LSR-2)  and also an In-lable which it has generated and will be sending to upstream routers (Edge 1 and 2).  Now if it has a per-platform label space (which you will normally see on all routers, exception is for LC-ATM interface which used per-interface label space), it would have a single IN-label which it sends to both upstream edge routers.  So both upstream Edge routers would be sending traffic onto this LSP with same Label.  Based on this incoming label, LSR-1 would switch it to the right path.

Regards,

sudeep

New Member

how an LSR determines the FEC to which an incoming MPLS Frame is

Thanks a lot Mr Sudeep......

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