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Example: Configuring Route Resolution on Route Reflectors

Requirements

• Example: Configuring Interprovider Layer 3 VPN Option A
• Example: Configuring Interprovider Layer 3 VPN Option B
• Example: Configuring and Verifying the Auto Export Feature

Overview

One scenario for route resolution is when you have a label-switched path configured from an RR to a provider edge (PE) router, or when the PE routers only peer with the RR. This can result in routes being hidden. To resolve this issue, you can change the default resolution behavior to use inet.0 for next-hop resolution.

By default, the bgp.l3vpn.0 routing table stores all VPN-IPv4 unicast routes. This table is present on any router that has Layer 3 VPNs configured, including PE routers and RRs.

When a Layer 3 VPN router receives a route from another Layer 3 VPN router, it places the route into its bgp.l3vpn.0 routing table. The route is resolved using the information in the inet.3 routing table. This means that when BGP receives a route destined for table bgp.l3vpn.0, the protocol nexthop (received BGP nexthop) has its forwarding nexthop recursively determined from the inet.3 table. The resulting route is converted into IPv4 format and redistributed to all routing-instance-name.inet.0 routing tables if it matches the VRF import policy.

On an RR with no attached customer edge (CE) routers, the resolution rib bgp.l3vpn.0 resolution-ribs inet.0 configuration causes routes in bgp.l3vpn.0 to use the information in inet.0 instead of inet.3 to resolve routes. You should not use this configuration on a router that is directly attached to a CE router. In other words, do not use resolution rib bgp.l3vpn.0 resolution-ribs inet.0 on a PE router.

If you want both inet.0 and inet.3 to be used, you must configure both, as in set resolution rib bgp.l3vpn.0 resolution-ribs [inet.0 inet.3].

In this example, the policy POLICY-limit-resolve-routes limits the route resolution to only routes learned through IS-IS. If you omit the import policy, all routes in inet.0 are evaluated and potentially used to resolve the protocol next hop. If you do not want to resolve against all entries, you use a policy to filter for a subset of the routes from the tables that are used for route resolution.

One common example is when you resolve against all routes in inet.0, except the default route (0/0).

Although the import statement is used in this configuration, no routes are imported or copied. Rather, the import policy-name configuration limits the set of possible routes that can be considered for route resolution.

The resolution rib bgp.l3vpn.0 resolution-ribs inet.0 configuration is useful when a BGP RR is not in the forwarding path. In other words, there are no ingress LSPs at the RR. Consider the case where RSVP is the label signaling protocol, and RSVP is configured full mesh at the edge routers. The RR needs to be able to reflect the routes. To do so, BGP is expected to perform a route resolvability check. If a Layer 3 VPN route is received but the nexthop is not in the inet.3 table, the route cannot be resolved. Because the router is not in the forwarding path, an effective workaround is to use the information in inet.0. The metric information in inet.0 is useful for choosing the best route, even though it cannot be used for forwarding.

An alternative approach is to make sure that LSPs are provisioned to the RR. This happens automatically if you configure LDP.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Step-by-Step Procedure

1. Configure bgp.l3vpn.0 to use the information in inet.0 instead of inet.3 to resolve routes.
   [edit routing-options resolution rib bgp.l3vpn.0]user@RR# set resolution-ribs inet.0
2. (Optional) Configure the routing policy.
   [edit policy-options policy-statement POLICY-limit-resolve-routes]user@RR# set term isis from protocol isisuser@RR# set term isis then acceptuser@RR# set then reject
3. (Optional) Apply the policy.
   [edit routing-options resolution rib inet.0]user@RR# set import POLICY-limit-resolve-routes
4. If you are done configuring the device, commit the configuration.
   [edit]user@RR# commit

Results

Confirm your configuration by issuing the show policy-options and show routing-options commands.

Verification

• show route
• show route forwarding-table
• show route resolution