Route Distinguishers - Avaya 8800 Planning And Engineering

Route distinguishers

PE routers use BGP to allow distribution of VPN routes to other PE routers. BGP Multiprotocol
Extensions (BGP-MP) allows BGP to forward routes from multiple address families, in this
case, VPN-IPv4 addresses. The BGP-MP address contains a 12-byte VPN-IPv4 address
which in turn contains an 8-byte Route Distinguisher (RD) and a 4-byte IPv4 address. The
Route Distinguisher makes the IPv4 address globally unique. As a result, each VPN can be
distinguished by its own RD, and the same IPv4 address space can be used over multiple
VPNs.
The following figure shows the VPN-IPv4 address.
Figure 101: VPN-IPv4 Address
The RD is configured on each and every VRF created on the PE nodes and must be configured
such that no other VRF on any other PE in the backbone has the same value. RDs are encoded
as part of the Network Layer Reachability Information (NLRI) in the BGP Update messages.
Please note that the RD is simply a number that you configure. It provides a means to identify a
PE node which may contain one or more VRFs. It does not identify the origin of the route nor
does it specify the set of VPNs or VRFs to which the routes are distributed. Its sole purpose
is to provide a mechanism to support distinct routes to a common IPv4 address prefix. By
allowing different RDs to support the same IPv4 addresses, overlapping addresses are
supported.
Planning and Engineering — Network Design
Route distinguishers
November 2010 221