Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual page 440

JUNOSe 11.0.x BGP and MPLS Configuration Guide
NOTE: The source address of the transmit interface is not used as the source address
of the packet.
Constraining Route Distribution with Route-Target Filtering
In typical BGP configurations, you can use cooperative route filtering to reduce the
amount of processing required for inbound BGP updates and the amount of BGP
control traffic generated by BGP updates. Cooperative route filtering works by having
the remote peer install a BGP speaker's inbound route filter as its own outbound
route filter. This filtering causes the remote peer to advertise only those routes that
the local peer can accept.
For BGP/MPLS VPNs, route-target filtering is a better approach. Route-target filtering
controls the distribution of BGP routes based on the VPNS (indicated by the
route-target extended communities) to which peer routers belong. PE routers use
the MP_REACH_NLRI 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.
404
Constraining Route Distribution with Route-Target Filtering
If the next-hop interface is in the same VRF and the interface is numbered, the
router uses the source address of the interface.
If the next-hop interface is in the same VRF and the interface is unnumbered,
the router uses either the source address of the interface it is pointing to or the
router ID of the VRF.
If the next-hop interface is in a different VRF, the router uses the source address
of the VRF. If the router does not have a router ID value, the packet is discarded.