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RT Constrained Route Distribution / restrictions

Hello Guys

recently i've started to study "Configuring BGP: RT Constrained Route Distribution" @

and at the Restriction section have found amazingly laconic statement:

Restrictions for BGP: RT Constrained Route Distribution
BGP: RT Constrained Route Distribution constrains all VPN route advertisements.

any clues how to explore it in details?


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Re: RT Constrained Route Distribution / restrictions

Hello Andyo,

the RT Constrained route redistribution is a scalability oriented feature for the control plane.


Instead of having a PE node to receive all possible VPNv4 routes with any RT value associated to them and then to install only those that have a match in locally defined VRFs with route-target import value(s) this feature does the following:

an additional address family is used with a new SAFI value

you enable it on the PE router using

address-family rtfilter unicast

neighbor <RR-server> activate

neighbor <RR-server> send-community extended


Also on the RR server you need to configure the new AF


address-family rtfilter unicast

5.    neighbor {ip-address | peer-group-name} activate

6.    neighbor {ip-address | peer-group-name} route-reflector-client >>>>

7.    neighbor {ip-address | peer-group-name} send-community extended


This new AF uses 12 bytes long NLRI prefixes with the only purpose to advertise to the RR server the locally configured import RT values.

The RR server can in this way build an outgoing filter based only on the extcommunity RT values and will send only VPNv4 prefixes that match the per PE node filter.

It saves memory usage on the PE node and reduces the number of VPNv4 prefixes advertised by RR server.

The use of memory on RR server increases as it has to mantain this outgoing filter for each neighbor that has activated the new RTfilter address family.


This is useful for big service providers to avoid to require high memory capabilities to all PE nodes just to be able to process all the uncostrained VPNv4 prefixes.


Hope to help



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