MPLS VPN RT Constrained Route Distribution


Route Target constrained route distribution is a method to prevent other PEs or Route Reflects from sending VPN prefixes for VRFs that are not locally attached.

RT Constrained Route Distribution

  • By default all MPLS VPN BGP peers send to each other all of the VPN prefixes either VPNv4 or VPNv6. If a PE route doesn't have an import policy for a RT attached to the prefix, it will drop it. The problem is if the full internet routing table is advertised out to everyone and each router has to send and drop all of these routes. This takes resources transmitting the prefixes and could be avoided with this feature.
  • RT Constrained Route Distribution is another address-family in BGP that advertised the import RTs as a prefix to PEs and RR. It works in a similar way that BGP Outbound Route Filtering (ORF) works.
  • The RT Constraint (RTC) NLR is a prefix with a format of:
    • 4-byte origin autonomous system
    • 8-byte RT extended community value
    • Example 65000:2:100:1, where 65000 is the ASN, 2 is the ext community type, 100:1 is the RT.
  • Is it 12 bytes long.
  • Router Reflectors should use the default-originate option, which advertises that they should receive all RTC prefixes.

Relevant IOS Commands

address-family rtfilter unicast

Enables the RT Filter address family, make sure to activate the neighbor and send extended community.

R1(config-router)#     address-family rtfilter unicast

show bgp rtfilter unicast all summary

Shows the neighbors for rt-filtering address-family.
Notice that there is a default one received from a Route Refresh and

R1#bgp rtfilter unicast all summary

show bgp rtfilter unicast all

Shows all of the RTC prefixes.

R1#show bgp rtfilter unicast all

Relevant IOS-XR Commands

address-family ipv4 rt-filter

  • Under the BGP process, enables the address-family
RP/0/RSP0/CPU0:R2(config-bgp)# address-family ipv4 rt-filter
  • Under a BGP neighbor, enables the address-family session.
RP/0/RSP0/CPU0:R2(config-bgp-nbr)#  address-family ipv4 rt-filter


Originates a default RTC, usually configured on the Route Reflector. Asks for all RT prefixes.

RP/0/RSP0/CPU0:R2(config-bgp-nbr-af)#  default-originate

show bgp ipv4 rt-filter summary

show bgp ipv4 rt-filter summary

Show bgp ipv4 rt-filter

show bgp ipv4 rt-filter

Additional Resources

Constrained Route Distribution for Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual Private Networks (VPNs)
Configuring BGP: RT Constrained Route Distribution - IOS-XE Configuration Guide.


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