When BGP is used to distribute a specific route, it can also be used to distribute an MPLS label which is mapped to that route (Labeled BGP). Label distribution can be in the BGP Update message by using the BGP-4 Multiprotocol SAFI 4. It is used to indicate that the NLRI contains a label. Recall that this is a new Capability and it must be negotiated when peering. Also recall that, the next-hop BGP speaker can assign or change the label. 

If  BGP speakers are exchanging routes via a Route Reflector you should think about if the RRs are changing the next-hop field or not. If the Route Reflector is not in the forwarding path, it does not need to be capable of forwarding MPLS packets.

(In this post, I will introduce the Labeled BGP very simply. The usage scenarios for BGP LSP is related with Seamless MPLS and Inter-AS VPN Option C. If you need answer for the question ” Why Labeled BGP? ” , you need to look Seamless MPLS or Inter-AS VPN Option C.)

We have 2 different case when creating Label Switch Path with Labeled BGP.  One is, the BGP peers are directly connected and the second one is the BGP Peers is NOT directly connected. 

In the case 1, no need additional MPLS options. 

In the case 2, you will need another LSP (can be created by LDP or RSVP-TE) between PE devices.

There must be an LSP PE devices and If there is no such LSP, PE cannot make use of BGP label. 

 The first step is to configure BGP speakers to send this capability information in Open Message.

And the second step will be creating policy to send labels for specific routes. Creating the policy may vary depending on vendor implementation but the Huawei logic is simply, matching an ip prefix and applying a label for that prefix. You can see Wireshark output captured while BGP speaker is sending update related with prefix.

One of the main use cases for BGP Labeled Unicast is Inter-AS VPNs. But it can also be used inside an AS in case of multiple IGP Process to have end to end LSP. In this blog post I introduced how BGP updates are send with a label attached.

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