Information Technology Reference
In-Depth Information
The RD is not used to represent VPNs or to control route redistribution. It is only used to
make a VPNv4 address unique in an MPLS backbone even when IPv4 addresses are not.
RD assignments and best practices of RD design in an MPLS VPN are discussed in detail
in the later section “Design Guidelines for RDs.”
NOTE
Example 10-1 shows how a VPNv4 address is received by a multiprotocol BGP speaker. In
the example, the VPNv4 prefix 65000:1:10.0.0.1/32 is received from 192.168.1.1, where
65000:1 is the RD and 10.0.0.1/32 is the IPv4 prefix.
Example 10-1
Output of
debug ip bgp update
...
23:48:33: BGP(1): 192.168.1.1 rcvd 65000:1:10.0.0.1/32
...
Extended Community Attribute
To control VPN route distribution, a new BGP attribute, extended community, is defined.
Comparing it to the standard BGP community attribute (attribute type 8), the extended
community attribute (attribute type 16) is extended from the standard 32 bits to 64 bits, and
it includes a Type field.
Currently, two extended communities are relevant to VPNv4:
•
Route Target, Type 0x02
•
Route Origin, Type 0x03
Route Target Extended Community
A Route Target (RT) extended community is used to identify a set of sites or VPNs. Asso-
ciating a certain RT with a VPNv4 route allows the route to be placed in VPNs/sites that
forward such traffic. As a result, RTs are used to control VPNv4 route redistribution.
A common way of assigning RTs to a route is to split an RT into two components: a 16-bit
AS number and a 16-bit assigned number. An example of an RT is 65000:100, where 65000
identifies an AS number and 100 is a locally assigned number that represents one VPN
or site. Examples of RT assignment in an MPLS VPN are presented in the later section
“Deployment Considerations.”
