OSPF Sham-Link Support for MPLS VPN Feature Overview
5
Cisco IOS Release 12.2(8)T
Figure 3 Using a Sham-Link Between PE Routers to Connect OSPF Client Sites
Because the sham-link is seen as an intra-area link between PE routers, an OSPF adjacency is created and
database exchange (for the particular OSPF process) occurs across the link. The PE router can then flood
LSAs between sites from across the MPLS VPN backbone. As a result, the desired intra-area connectivity is
created.
The section, “Creating a Sham-Link”, describes how to configurea sham-link between two PE routers.
For more information about how to configure OSPF, refer to:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_c/1cprt1/1cospf.htm
Sham-Link Configuration Example
The example in this section is designed to show how a sham-link is used only to affect the OSPF intra-area
path selection of the PE and CE routers. The PE router also uses the information received from MP-BGP to
set the outgoing label stack of incoming packets, and to decide to which egress PE router to label switch the
packets.
Figure 4 shows a sample MPLS VPN topology in which a sham-link configuration is necessary. A VPN client
hasthree sites, each with a backdoor link. Two sham-links have been configured, one between PE-1 and PE-2,
and another between PE-2 and PE-3. A sham-link between PE-1 and PE-3 is not necessary in this
configuration because the Vienna and Winchester sites do not share a backdoor link.
MPLS VPN Backbone
Area 1
Winchester
10.3.1.7
Brighton
70392
Net=10.3.1.7
Type-1 LSA
PE-3
10.3.1.2
MP-BGP
Area 1
Stockholm
10.3.1.3
Net=10.3.1.7
Type-1 LSA
Area 1 Vienna
10.3.1.15
Net=10.3.1.7
Type-1 LSA
Net=10.3.1.7
Route-type 1:2:0
Net=10.3.1.7
Route-type 1:2:0
Sham-linkSham-link
PE-2
10.3.1.5
PE-1
10.3.1.6