Routing

Hi guys,

I am new to MPLS and i am wondering if someone can help me understand label assignment in MPLS.

I understand that label can be assigned manually (Static LSP) and automatically (LDP signalling protocol). During manual LSP setup we define label value manually on each LSR.

Ingress LSR - Push (Label value)
Transit - Incoming Label & outgoing label (Pop/Swap label value)

Question
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Does LSR assign every LSP a different label or same label? I think it should be different Label....but if it uses different label for every LSP then how does a transit router know which LSR its coming from? or Transit router doesnt care where does it come from since LSPs are uni-directional??

I hope this question make sense 🙂

Thanks

Hello,

There are at least 3 types of point-to-point LSPs:

(1) Dynamic, FECs signaled from downstream with LDP or labeled-unicast BGP.
---> In this case, transit LSR label assignment can differ e.g. based on whether you have deaggregate configured under LDP or not.

(2) Dynamic, signaled with RSVP from Ingress PE, hierarchy [edit protocols mpls label-switched-path]
---> LSR assigns a different label for each LSP.

(3) Static, no protocol, configured with [edit protocols static-label-switched-path]
---> The administrator should configured different labels for each LSR.

HTH
Ato

(3) Static, no protocol, configured with [edit protocols static-label-switched-path]

---> The administrator should configured different labels for each LSR.
I meant for each LSP

Great thanks.

 

Now this pops another question if thats ok?

 

 

Since transit routers make the forwarding decision based on incoming label. How do LSRs swap the incoming label with outgoing considering they dont do destination address lookup and every LSP has different label?

 

 

The transit LSR has a forwarding table with:

Incoming label, then action

Where action can be: pop,swap,swap+push, etc...

In dynamic LSPs, the action is determined by the announcement done by the next LSR in the path.

Yup i understand they have the information in mpls.0 table.

 

But how do they get this information i.e. swap 100065 with 100069? How do LSRs get this information.

 

Imagine: Ingress PE - LSR1 - LSR2 - Egress PE

LSR2 announces label X for a LSP/FEC, LSR1 announces label Y for the same LSP/FEC

So LSR1 installs a Y -> swap X entry

Sorry not been difficult but just to double check how will two different LSRs anounce same FEC? 

 

Does it mean LSP have route foe FEC/prefixes via IGP/bgp?

 

I recommend you download this PDF:

http://www.juniper.net/us/en/community/junos/training-certification/day-one/networking-technologies-series/this-week-deploying-mpls/

Will do...thanks