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There is a tight coupling between IP and MPLS forwarding. MPLS forwarding equivalence classes (FECs) are often an IP prefix – that is to say that traffic matching a given IP prefix is routed into a MPLS label switch path (LSP). It is thus necessary to be able to associated a given prefix/route with an [out-going] MPLS label that will be imposed when the packet is forwarded. This is configured as:

ip route add via GigE0/0/0 out-labels 33

packets matching will be forwarded out GigE0/0/0 and have MPLS label 33 imposed. More than one out-going label can be specified. Out-going MPLS labels can be applied to recursive and non-recursive routes, e.g;

ip route add via out-labels 34

packets matching will thus have two MPLS labels imposed; 34 and 33. This is the realisation of, e,g, an MPLS BGP VPNv4. To associate/allocate a local-label for a prefix, and thus have packets to that local-label forwarded equivalently to the prefix do;

mpls local-label 99

In the API this action is called a ‘bind’. The router receiving the MPLS encapsulated packets needs to be programmed with actions associated which each label value – this is the role of the MPLS FIB. The MPLS FIB Is a table, whose key is the MPLS label value and end-of-stack (EOS) bit, which stores the action to perform on packets with matching encapsulation. Currently supported actions are:

1)	Pop the label and perform an IPv[46] lookup in a specified table
2)	Pop the label and forward via a specified next-hop (this is penultimate-hop-pop, PHP)
3)	Swap the label and forward via a specified next-hop.

These can be programmed respectively by:

1)	mpls local-label 33 ip4-lookup-in-table X
2)	mpls local-label 33 via GigE0/0/0
3)	mpls local-label 33 via GigE0/0/0 out-labels 66

the latter is an example of an MPLS cross connect. Any description of a next-hop, recursive, non-recursive, labelled, non-labelled, etc, that is valid for an IP prefix, is also valid for an MPLS local-label.


To configure an MPLS VPN for a PE the follow example can be used.

Step 1; Configure routes to the iBGP peers - note these route MUST have out-going labels;

ip route add via Eth0 out-labels 33
ip route add via Eth0 out-labels 34

Step 2; Configure the customer 'VRF'

ip table add 2

Step 3; add a route via the iBGP peer[s] with the MPLS label advertised by that peer

ip route add table 2 via next-hop-table 0 out-label 122
ip route add table 2 via next-hop-table 0 out-label 121

Step 4; add a route via the eBGP peer

ip route add table 2 via next-hop-table 2

Step 5; depending on the label allocation scheme used, add routes to the MPLS FIB to accept incoming labelled packets

1 per-prefix label scheme - this command 'binds' the label to the same forwarding as the IP route

mpls local-label 99 

2 per-CE label scheme - this pops the incoming label. Append config for 'out-labels' if so desired.

mpls local-label 99 via next-hop-table 2

3 per-VRF label scheme

mpls local-label 99 via ip4-lookup-in-table 2