OSPF Sham-Link Support for MPLS VPN
Feature Overview
3
Cisco IOS Release 12.2(8)T
Figure 2
Backdoor Paths Between OSPF Client Sites
For example,
Figure 2
shows three client sites, each with backdoor links. Because each site runs OSPF within
the same Area 1 configuration, all routing between the three sites follows the intraarea path across the
backdoor links, rather than over the MPLS VPN backbone.
The following example shows BGP routing table entries for the prefix 10.3.1.7/32 in the PE-1 router in
Figure 2
. This prefix is the loopback interface of the Winchester CE router. As shown in bold in this example,
the loopback interface is learned via BGP from PE-2 and PE-3. It is also generated through redistribution into
BGP on PE-1.
PE-1# show ip bgp vpnv4 all 10.3.1.7
BGP routing table entry for 100:251:10.3.1.7/32, version 58
Paths: (3 available, best #2)
Advertised to non peer-group peers:
10.3.1.2 10.3.1.5
Local
10.3.1.5 (metric 30) from 10.3.1.5 (10.3.1.5)
Origin incomplete, metric 22, localpref 100, valid, internal
Extended Community: RT:1:793 OSPF DOMAIN ID:0.0.0.100 OSPF
RT:1:2:0 OSPF 2
Local
10.2.1.38 from 0.0.0.0 (10.3.1.6)
Origin incomplete, metric 86, localpref 100, weight 32768,
valid, sourced, best
Extended Community: RT:1:793 OSPF DOMAIN ID:0.0.0.100 OSPF
RT:1:2:0 OSPF 2
Local
10.3.1.2 (metric 30) from 10.3.1.2 (10.3.1.2)
Origin incomplete, metric 11, localpref 100, valid, internal
Extended Community: RT:1:793 OSPF DOMAIN ID:0.0.0.100 OSPF
RT:1:2:0 OSPF 2
Within BGP, the locally generated route (10.2.1.38) is considered to be the best route. However, as shown in
bold in the next example, the VRF routing table shows that the selected path is learned via OSPF with a next
hop of 10.2.1.38, which is the Vienna CE router.
MPLS VPN Backbone
Area 1
Winchester
10.3.1.7
Brighton
70391
PE-3
10.3.1.2
PE-2
10.3.1.5
PE-1
10.3.1.6
Area 1
Stockholm
10.3.1.3
Area 1
Vienna
10.3.1.15
