 | Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |
You can use BGP as a label distribution protocol both for IP routes and for VPN routes.
When BGP distributes a particular IP route, it can also distribute an MPLS label that has been mapped to that route, as described in RFC 3107. The MP-BGP extensions (RFC 2858) enable the BGP update message to carry both the route and the label mapping information for the route. The label is encoded into the NLRI field of the attribute, and the SAFI field is set to 4 to indicate that the NLRI contains a label. A BGP speaker can use BGP to send labels to a particular BGP peer only if that peer advertised the capability to process update messages with SAFI 4. BGP speakers advertise this capability only to peers for which the neighbor send-label command has been configured.
When BGP advertises labeled routes, it adds a label-to-next-hop mapping (cross-connect) to the MPLS forwarding table. This mapping consists of the in label that BGP allocates from the platform label space plus the MPLS next hop information related to the labeled route's next hop.
BGP can also distribute labels for VPN routes in BGP/MPLS VPNs. In a BGP/MPLS VPN network, BGP is used to exchange the routes of a particular VPN among the provider edge routers attached to the VPN. To ensure that routes from different VPNS remain distinct even if the VPNs use overlapping address spaces, an MPLS label is assigned to each route within the VPN. BGP distributes both the VPN routes and the associated MPLS label for each route.
The label mapping information for a particular VPN route is included in the same BGP update message that distributes the route. The label is encoded into the NLRI field of the attribute, and the SAFI field has a value of 128 to indicate that the NLRI contains both an RD (route distinguisher) and a label.
For more information on BGP capabilities, see Configuring BGP Routing. For more information on MP-BGP extensions, NLRIs, and BGP/MPLS VPNs, see Configuring BGP-MPLS Applications..
| Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |