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Creating a BGP Session Between the CE Router and the Parent VR

The fallback global option enables traffic that arrives at a VRF from the CE router to be sent out on the uplink determined to be optimal by using the full Internet routing table present in the parent VR.

If a CE router is multihomed to multiple PE routers, it must receive a full Internet routing table from each of the PE routers so that the CE router can determine which of the PE routers is optimal for a given Internet prefix.

You can easily create a BGP session from the VRF to the CE router to advertise routes in the VRF to the CE router. However, doing this is insufficient because the VRF does not contain the full Internet routing table, which is present only in the parent VR.

This situation requires a BGP session from the parent VR to the CE router (Figure 104). This BGP session in turn requires a route in the VRF to the loopback interface in the parent VR that is used for BGP peering with the CE router. To achieve this configuration, you must do both of the following:

In the parent VR, create a shared IP interface for the PE-CE interface and point a static route to the loopback of the CE router to the shared interface.
Use a global import map in the VRF to import into the VRF the route to the loopback interface in the parent VR.
Figure 104: BGP Session Between CE Router and Parent VR

The following commands configure a shared IP interface in the parent VR and point a static route for the loopback in the CE router to it:



host1(config)#virtual-router pe1 

host1:pe1(config)#interface ip ce1-cust 

host1:pe1(config-if)#ip share-interface atm2/0.1 

host1:pe1(config-if)#ip address 10.1.1.3 255.255.255.255 

host1:pe1(config-if)#exit 

host1:pe1(config)#ip route 10.4.4.4 255.255.255.255
ip ce1-cust

The following commands make the loopback in the parent VR reachable from the VRF by means of a global import map:



host1(config)#virtual-router pe1 

host1:pe1(config)#prefix-list VRloop permit
10.2.2.2/32 

host1:pe1(config)#route-map globimaploop 

host1:pe1(config-route-map)#match ip address
prefix-list VRloop 

host1:pe1(config-route-map)#exit 

host1:pe1(config)#ip vrf pe11 

host1:pe1(config-vrf)#rd 100:1 

host1:pe1(config-vrf)#route-target both 100:1 

host1:pe1(config-vrf)#global import map globimaploop

The following commands create a BGP session between the CE router and the parent VR.

On host 1, VR PE 1:



host1(config)#virtual-router pe1 

host1:pe1(config)#router bgp 100 

host1:pe1(config-router)#neighbor 10.4.4.4
remote-as 200 

host1:pe1(config-router)# neighbor 10.4.4.4
ebgp-multihop 

host1:pe1(config-router)#neighbor 10.4.4.4
update-source loopback1 

host1:pe1(config-router)#exit

On host 2, VR CE 1:



host2(config)#virtual-router ce1 

host2:ce1(config)#interface loopback 1 

host2:ce1(config-if)#ip address 10.4.4.4 255.255.255.255 

host2:ce1(config-if)#exit 

host2:ce1(config)#ip route 10.2.2.2 255.255.255.255
atm2/1.1 

host2:ce1(config)#router bgp 200 

host2:ce1(config-router)#neighbor 10.2.2.2
remote-as 100 

host2:ce1(config-router)#neighbor 10.2.2.2
ebgp-multihop 

host2:ce1(config-router)#neighbor 10.2.2.2
update-source loopback1 

host2:ce1(config-router)#exit

You must also configure either fallback global or a default route to a manually created shared interface in the VRF. See Configuring a Fallback Global Option or Configuring a Default Route to a Shared Interface for details.

You can use the BGP session between the CE router and the parent VR to enable the CE router to advertise prefixes within the VPN site that can be reachable from the global Internet. An alternative configuration is to use a global export map as described in Setting Import and Export Maps for a VRF .

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