Understanding RSVP Automatic Mesh

 

When adding sites to BGP and MPLS VPNs that use RSVP signaling, more configuration is needed to add provider edge (PE) routers than is needed to add customer edge (CE) devices. RSVP automatic mesh helps to reduce this configuration burden.

Service providers often use BGP and MPLS VPNs to efficiently scale the network while delivering revenue-generating services. In these environments, BGP is used to distribute the VPN routing information across the service provider's network, while MPLS is used to forward that VPN traffic from one VPN site to another. BGP and MPLS VPNs are based on a peer model. To add a new CE device (site) to an existing VPN, you need to configure the CE router at the new site and the PE router connected to the CE router. You do not have to modify the configuration of all of the other PE routers participating in the VPN. The other PE routers automatically learn about the routes associated with the new site, a process called automatic discovery (AD).

The requirements are a bit different if you need to add a new PE router to the network. A BGP and MPLS VPN requires that the BGP session be fully meshed and that there also be a full mesh of PE router-to-PE router MPLS label-switched paths (LSPs) between all of the PE routers in the network. When you add a new PE router to the network, all of the existing PE routers must be reconfigured to peer with the new PE router. Much of the configuration effort can be reduced if you configure BGP route reflectors (mitigating the full mesh requirement for BGP) and if you configure (LDP) as the signaling protocol for MPLS.

However, if you need to add a new PE router to a network configured with a full mesh of RSVP-signaled LSPs, you must reconfigure each of the PE routers to have a peer relationship with the new PE router. You can configure RSVP automatic mesh to address this particular operational scenario. When you enable RSVP automatic mesh, RSVP LSPs are dynamically created between a new PE router and the existing PE routers, eliminating the need to reconfigure all of the PE routers manually. For dynamic LSP creation to function properly, BGP must be configured to exchange routes between all of the participating PE routers. If two BGP peers do not exchange routes, it is not possible to configure a dynamic LSP between them. The local router’s inet.0 routing table must include a labeled route to each potential IBGP next-hop (future potential PE routers or LSP destinations).

RSVP includes numerous capabilities that are not available in LDP, including fast reroute, end-point control, and link management. RSVP automatic mesh helps to reduce the operation and maintenance requirements for RSVP, making it possible to deploy RSVP in larger and more complicated networks.

Every PE router can reach every other PE router in the network because this information is distributed by the IGP. A PE router can set up a point-to-point RSVP LSP to any other PE router in the network as long as it knows that such an LSP is required. To build a full mesh of LSPs between the PE routers requires that each PE router know which of the other PE routers make up the full mesh.

Note

In Junos OS, RSVP automatic mesh is configured using the rsvp-te configuration statement at the [edit routing-options dynamic-tunnels dynamic-tunnel-name] hierarchy level. The rsvp-te configuration statement is also available for use in routing instances as a provider-tunnel option. When implemented as a provider-tunnel option, rsvp-te is used to configure the RSVP point-to-multipoint LSPs for multiprotocol BGP multicast VPNs, not to configure RSVP automatic mesh.