Exchanging Route-Target Membership Information
BGP peers exchange route-target membership information in the following sequence:
1. When the BGP peers negotiate the BGP multiprotocol extensions capability during
the establishment of a BGP session, they indicate support for the route-target
address family by including the (AFI, SAFI) value pair for the route-target
membership NLRI (RT-MEM-NLRI) attribute. This pair has an AFI value of 1 and
a SAFI value of 132.
2. If the capability is successfully negotiated, BGP speaker Router A expresses its
interest in a VPN route target by advertising to its peers the RT-MEM-NLRI
attribute that contains the particular route target. This attribute is represented
as a prefix in the following format:
AS number:route-target extended community/prefix length
■ AS number—Number of the originating AS
■ route-target extended community—Two-part number identifying the route
target extended community. Consists of number1:number2, where:
■ number1—Autonomous system (AS) number or an IP address
■ number2—Unique integer; 32 bits if number1 is an AS number; 16 bits
if number1 is an IP address
■ prefix length—Length of the prefix. A prefix less than 32 or greater than 96
is invalid. However, the prefix for the Default-RT-MEM-NLRI attribute is an
exception to this rule. For the Default-RT-MEM-NLRI attribute, 0 is a valid
prefix length.
For example, 100:100:53/36 is a valid RT--MEM-NLRI.
3. Remote peers of Router A use the route-target membership advertised by Router
A to filter their VPN routes that are outbound to Router A. A peer advertises a
VPN route to Router A only when one of the following conditions is true
■ Router A advertised a default route-target membership.
■ Router A advertised membership in any of the route targets associated with
the VPN route.
4. Router A then receives and processes the RT-MEM-NLRI attributes sent by its
peers to determine which VPN routes it advertises to the peers.
BGP speakers advertise and withdraw the RT-MEM-NLRI attribute in MP-BGP update
messages. BGP speakers ignore RT-MEM-NLRI attributes received from peers that
have not successfully negotiated this capability with the speaker.
If dynamic negotiation for the route-refresh capability is enabled, BGP negotiates the
route-refresh capability for the RT-MEM-AFI-SAFI address family when a peer is
activated in that family. As a consequence, you can use the clear ip bgp soft
command to refresh the RT-MEM-NLRI routes in the BGP speaker’s Adj-RIBs-Out
table.
Constraining Route Distribution with Route-Target Filtering ■ 411
Chapter 5: Configuring BGP-MPLS Applications
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