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Understanding Proxy BGP Route Target Filtering for VPNs

BGP route target filtering (also known as route target constrain, or RTC) allows you to distribute VPN routes to only the devices that need them. In VPN networks without BGP route target filtering configured, BGP distributes all VPN routes to all VPN peer devices, which can strain network resources. The route target filtering feature was introduced to reduce the number of devices receiving VPN routes and VPN routing updates, thereby limiting the amount of overhead associated with running a VPN. The Junos OS implementation for BGP route target filtering is based on RFC 4684, Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs).

What if you have a network environment where route target filtering is not widely deployed, or what if some devices do not support route target filtering? For example, you might have a BGP speaker with route target filtering enabled that is peered with a BGP speaker that does not support or have route target filtering configured. In this case, the BGP speaker with route target filtering configured must advertise default route target membership (RT membership) on behalf of its peer. The route target filtering resource savings are unrealized because the device supporting the filtering must now send all VPN routes to the device that does not support the filter. Proxy BGP route target filtering (or Proxy RTC) permits the generation of RT membership for devices that do not support route target filtering. This eases the deployment of route target filtering in networks where it is incompletely deployed or not fully supported.

Proxy BGP route target filtering allows you to distribute proxy RT membership advertisements created from the received BGP VPN routes to other devices in the network that need them. These are known as proxy advertisements because the device creates the RT membership on behalf of its peers without the route target filtering functionality. Proxy BGP route target filtering uses BGP route target extended communities that are exported to a specific BGP speaker to generate the route targets. Generated proxy RTC routes are stored in the bgp.rtarget.0 routing table.

You can also configure a policy to control which VPN routes are used to generate the proxy RTC routes. This can help control which RT membership is generated by the proxying device. In addition, you can configure a policy to reduce the memory overhead associated with proxy RTC. Proxy RTC only uses additional memory on a per-VPN route basis when it is permitted by a policy to be used for generating RT membership.