Unanswered Question
Sep 14th, 2007

Hi all, can anyone tell me what kind of label mpls puts on to the tag ? is it a name or number, and does the label get changed along the path ? also does the label get applied by a setting on the interface ?

can anyone help


I have this problem too.
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spremkumar Fri, 09/14/2007 - 01:36

Hi Carl

Label is of 32 bits (20 bit for label value/8 bit for TTL Field /3 bit MPLS EXP bits for QOS/1 bit for bottom of stack field)

Labels will be identified with label numbers and each individual valid route will have a label assigned.

Labels will get swapped along the path.

Each router along the path will have its own label generated for the respective routes and also a label it recieves from the neigbor for the same.

once it recieves a packet with certain label it just matches the LFIB(label forwarding informatio base) where u have both incoming label and outgoing label it swaps the in label with out and forwards it.


carl_townshend Fri, 09/14/2007 - 02:32

Thanks for the reply, so do you generate the label yourself, or does the router generate its own label ? also how you say it receives a label from the neighbour, what do you mean by this, why does it need to know that ?



spremkumar Fri, 09/14/2007 - 02:49

Hi Carl

Once you enable MPLS on the router the router will generate the labels for each valid route.

Also there are different methods in exchanging the label based on the kinda backbone topolgy deployed (frame relay/ATM)

Since the forwarding here in MPLS cloud is based on labels you need to have some kinda identifier based on which you forward/switch the packets till the egress SP node.

Untill unless you dont recieve any labels from the neighbor for a particular route you wont have in label (in LFIB)which is required in carrying out the forwarding.

since the forwarding decision here is based on label swap the router (A) just check the in label and swaps with the out label generated on its own and forwards the packet.

once it comes to the next level the same process is repeated and router (B) refers the label given by router (A) which is already installed in the LFIB table and swaps it with its own label and forwards it.


paul.matthews Fri, 09/14/2007 - 02:51

The labels are generated by the routers themselves.

MPLS and label distribution is quite an involved topic.

I will constrain my answers to frame-mode MPLS. I'll also ignore VRF!

Each MPLS router generates a tag for each route in its routing table. It will advertise the label to its peers. Effectively it says something like "for network, use label 46"

so the peers that get that add that to the LFIB.

I'll now do a little network:


All four routers will have LAN1 and LAN2 in their routing tables. They are all also MPLS enabled, so they will each generate a label for each of those routes (and other routes in the table). I'll just talk about traffic going LAN1-LAN2, so I will only talk about LAN2. The same things will happen, but with different numbers for tags for LAN1.

So for LAN2, each router generates a label.

R1 R2 R3 R4

35 22 78 19

R3 will tell R2 and R4 about the label for LAN2. R4 has a better route than R3 so it will not do much with the label for now, bur R2 will add it to the LFIB, so it now has a record that says:

LAN2 - ingress label 22, Egress label 78

R4 will tell R3, so R3 gets an entry:

LAN2 - ingress label 78, Egress label 19

R2 will also tell R1 so R1 knows about the label. R1 as the ingress router does not hae an ingress label.

Now all routers know the releval labels, sowe have our LSP (label switch path).

A packet now enters R1. R1 sees it has an entry to it adds the label 22 and forwards it to R2. R2 sees a label, so looks in the LFIB and sees an entry - in 22, out 78 so it removes the old label and adds a new one and forwards it to R3, R3 sees in 78, out 19 and forwards to R4, R4 is the egress router so the label is popped and the packet routed according to the routing table.

There are two label distribution protocols - LDP and TDP Label and Tag distribution protocol.



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