Cisco 1400 series Specification Sheet page 5

OSPF Sham-Link Support for MPLS VPN
Figure 3
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,
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
has three 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.
Using a Sham-Link Between PE Routers to Connect OSPF Client Sites
MPLS VPN Backbone
Net=10.3.1.7
Route-type 1:2:0
MP-BGP
Route-type 1:2:0
Sham-link Sham-link
PE-1
10.3.1.6
Area 1
Vienna
10.3.1.15
"Creating a Sham-Link", describes how to configure a sham-link between two PE routers.
shows a sample MPLS VPN topology in which a sham-link configuration is necessary. A VPN client
PE-3
10.3.1.2
Net=10.3.1.7
Net=10.3.1.7
Type-1 LSA
PE-2
10.3.1.5
Stockholm
10.3.1.3
Feature Overview
Winchester
10.3.1.7
Net=10.3.1.7
Type-1 LSA
Brighton
Net=10.3.1.7
Type-1 LSA
Area 1
Cisco IOS Release 12.2(8)T
Area 1
5