In this example the two customer sites use different AS numbers, which simplifies
configuration. Alternatively, the same AS numbers can be used.
Customer site 1 has two networks that need to be reachable from customer
site 3—10.3.0.0/16 and 10.12.0.0/16—and uses BGP to announce these prefixes to
PE 1. CE 1 uses a standard BGP update message as shown in Figure 72 on page 388
to carry this and additional information. CE 1 is withdrawing prefix 10.1.0.0/16. CE
1 specifies its own address as the next hop; 10.4.1.1 is from the private address
space of VPN A.
PE 1 passes the advertisement along the backbone through an IBGP session, but uses
MP-BGP rather than standard BGP-4. Consequently, PE 1 uses an extended BGP
update message, which is different in format from the standard message, as shown
in Figure 72 on page 388.
The extended update uses different attributes for some of the advertised information.
For example it carries the advertised prefixes in the MP-Reach-NLRI attribute instead
of the NLRI attribute. Similarly, it uses the MP-Unreach-NLRI attribute for withdrawn
routes rather than the withdrawn-routes attribute.
PE 1 advertises the customer site addresses by prepending information to the
addresses as advertised by CE 1, thus creating labeled VPN-IPv4 prefixes. The
prepended information consists of a route distinguisher and an MPLS label.
Because the CE router uses IPv4 addresses from the VPN’s private address space,
these addresses can be duplicated in other VPNs to which PE 1 is attached. PE 1
associates a route distinguisher with each IPv4 address to create a globally unique
address. In this example, the RD consists of the AS that PE 1 belongs to and a number
that PE 1 assigns. The RD is prepended immediately before the IPv4 address.
PE routers assign MPLS labels to each VRF. In this example, the label for the VRF
associated with customer site 1 is 16. The MPLS label is prepended immediately
before the route distinguisher.
NOTE: The explicit null label is prepended only to routes that are being withdrawn
in the MP-REACH-NLRI attribute.
Some non–E Series implementations allocate a separate label for each prefix. By
default, the E Series router generates one label for all BGP routes advertised by the
VRF, thus reducing the number of stacked labels to be managed. The ip mpls
forwarding-mode label-switched command enables you to have the router generate
a label for each different FEC pointed to by a BGP route in a given VRF. However,
some routes always receive a per-VRF label; see “Creating Labels per FEC” on page 438
for more information.
Overview ■ 387
Chapter 5: Configuring BGP-MPLS Applications
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