How LDP(TDP) allocate label?

[annayuzhao]

I am reading the MPLS and VPN architechure. It mentions that the router will assign label to all the enties in the routing table then propagate to all its LDP neighbors. But it also mentions it uses downstream distribution method. I am kind of confused here. Can anybody explain here? Thanks

[Harold Ritter]

Your second sentence describes the independent LSP control mode, in which the router assigns labels to prefixes in the RIB and distribute them without waiting for a label from the upstream router for the same prefixes.

The downstream unsolicited mode is used to distribute label information to a downstream router without receiving an explicit request from that same router.

This two modes are used in conjunction on Cisco routers.

For more information on the different modes defined for MPLS, please refer to RFC 3031.

Hope this helps,

[annayuzhao]

If routers A,B,C connected as (E0)A(S0)-(S0)B(S1)-(S1)C, A has network 10.0.0.0 on LAN and updates it to B and C. All the routers are MPLS enabled.If router A assignes a lable to 10.0.0.0 ,but B and C also assign lable to 10.0.0.0. How the system pick up a lable for 10.0.0.0?Initially I thought it always the router originated the routes will assign lable and distribute them. But now it seems every router will allocate lable even the routes they received. Thanks.

[Harold Ritter]

A label is allocated as soon as A, B and C have 10.0.0.0/8 in their respective RIB. This label is referred to as the local or out label. This is the label that is advertised to all LDP neighbors. The neighbors only use the label received from the upstream router. The upstream router is the next-hop router determined by the routing protocol.

Let me know if this explanation clarifies it for you,

[Harold Ritter]

Correction. The locally allocated label is referred to as local or in label.

[annayuzhao]

So router only allocates its own directlry connected network(not the routing entries it received) and then advertise to the downstream router?Thanks

[Harold Ritter]

The router allocates local labels for every entry in the RIB (connect, static or dynamic) to the exception of BGP learnt routes, which use the label assigned to the interior route to the BGP next hop.

Hope tis helps,