Users Guide

BGP 1427

A VRF may be configured to associate all the routes that belong to the VRF

with a particular Route Target attribute. Dell EMC Networking allows a finer

selection of routes with the use of Export and Import maps. Export and

Import maps provides greater flexibility to the administrator where she can

associate some routes of a VRF with a particular Route Target attribute and

some other routes with a different Route Target attribute.

The Route Target attribute assists in configuring selective route leaking

among VRFs using Multi-protocol BGP. Essentially, route leaking between

VRFs within a single router can be achieved with just the import and export

Route Target statements.

Dell EMC Networking allows configuring Route Target attributes in VRF

mode, using IP Extended community lists in association with

inbound/outbound Route maps.

Behavior of VPNv4 Route Leaking into a VRF with Identical Prefixes & RTs

In this scenario, multiple routes with identical prefix/RTs but different RDs

are advertised to a VPNv4 peer from a single router. The standard decision

process rules handle the cases when receiving identical prefix/RTs with the

same RD or with different RDs from different routers. On the receiving

VPNv4 router, the decision process picks the best route from the identical

prefix/neighbor combination routes as follows:

If the next hops of the prefixes are the same, the first received MP-

NLRI/route is imported into the VRFs with a matching import statement.

In the case of an update, the first received MP-NLRI/route is maintained

and is not replaced, even if the next hop is different.

If the next hops of the prefixes are different, the prefix with the highest

RD value is imported into the VRF.

The decision to use the highest RD is arbitrary; however, the scenario where

routes with identical prefix/RT pairs are configured in different VRFs on a

single router is almost certainly a misconfiguration.

How the VPNv4 NLRI Is Carried in BGP

The BGP Multiprotocol Extensions are used to encode the NLRI. If the

Address Family Identifier (AFI) field is set to 1, and the Subsequent Address

Family Identifier (SAFI) field is set to 128, the NLRI is a VPNv4 address. AFI

1 is used since the network layer protocol associated with the NLRI is still IP.