Information Technology Reference
In-Depth Information
It is important to note that an RD has no association with an RT, even though they might
have the same format and sometimes the same value. Additionally, a VPNv4 prefix
might have only one RD but can have multiple RTs attached to it.
NOTE
Example 10-2 shows RT as an attribute of a BGP VPNv4 prefix. The VPNv4 prefix
10:1:10.1.2.0/24 is received from 192.168.110.3, with an RT of 65000:100. The RD of
the VPNv4 prefix is 10:1.
Example 10-2
Output of
debug ip bgp update
...
23:48:51: BGP(1): 192.168.110.3 rcvd UPDATE w/ attr: nexthop 192.168.110.3,
origin ?, path 10, extended community RT:65000:100
23:48:51: BGP(1): 192.168.110.3 rcvd 10:1:10.1.2.0/24
...
Route Origin Extended Community
Route Origin identifies the routers that inject the routes into BGP. This extended commu-
nity is called Site of Origin (SOO) in Cisco IOS software. In an MPLS VPN that has mul-
tihomed connections, SOO is used to identify a customer site to prevent traffic from leaving
the site from one point and being sent back to the same site from another point. The use of
BGP in a Layer 3 MPLS VPN is discussed in the section “Building MPLS VPN Architec-
tures.” SOO is typically configured as a 16-bit AS number and a 16-bit assigned number.
An example of SOO is 65000:100. The use of SOO in preventing routing information loops
is discussed in the section “AS Override.”
Multiprotocol Reachability Attributes
To advertise multiprotocol reachability information or Network Layer Reachability
Information (NLRI) in BGP updates, two BGP attributes are created (refer to RFC 2858,
Multiprotocol Extensions for BGP-4
):
•
Multiprotocol Reachable NLRI (MP_REACH_NLRI), Type 14
•
Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI), Type 15
MP_REACH_NLRI is used to advertise feasible routes. MP_UNREACH_NLRI is used to
advertise, or rather withdraw, unfeasible routes. Each attribute has fields identifying AFI,
SAFI, and NLRI. For VPNv4, AFI/SAFI are 1/128.
