Introduction
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 10.1.0.2 and 10.1.0.1.
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
default-originate
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.