Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

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and MP_UNREACH_NLRI attributes in BGP updates to exchange information about each
router’s route-target membership.
The PE router subsequently advertises VPN NLRI—the routing information carried in
MP-BGP update messages—only to peers that are members of a route target that is
associated with the VPN route. The VPN routes flow in the opposite direction to the
route-target membership information.
Route-target filtering works across multiple ASs and with asymmetric VPN topologies,
such as a hub-and-spoke. Route-target filtering can reduce the size of the BGP routing
table in PE routers, as well as the amount of VPN NLRI exchange traffic between routes
in the VPN. Route-target filtering also reduces router memory requirements by reducing
the amount of routing information stored and propagated. For example, route reflectors
scale according to the total number of VPN routes present in their network. With
route-target filtering, you can reduce the scaling requirements of the reflectors by
restricting the number of VPN routes they must process to only those VPN routes actually
used by the route reflector clients.
Applications such as BGP/MPLS VPNs, VPLS L2VPNs, and VPWS L2VPNs all use route
targets as part of their route reachability information, and can therefore employ
route-target filtering and potentially accrue the benefits of reduced traffic and smaller
routing tables.
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.
Copyright © 2010, Juniper Networks, Inc. 414
JunosE 11.2.x BGP and MPLS Configuration Guide