Cisco Systems 1400 series, 1600R manual Sham-Link Configuration Example

OSPF Sham-Link Support for MPLS VPN

Feature Overview

Figure 3 Using a Sham-Link Between PE Routers to Connect OSPF Client Sites

	MPLS VPN Backbone
	Net=10.3.1.7	PE-3				Winchester
	Route-type 1:2:0
						10.3.1.7	Area 1
		10.3.1.2
						Net=10.3.1.7
	Net=10.3.1.7					Type-1 LSA
						Brighton
	MP-BGP Route-type 1:2:0
	Net=10.3.1.7
PE-1	Sham-link Type-1 LSA	PE-2
					Net=10.3.1.7
10.3.1.6		10.3.1.5
					Type-1 LSA
Area 1	Vienna		Stockholm			Area 1
	10.3.1.15			10.3.1.3

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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 configure a 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 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.

Cisco IOS Release 12.2(8)T

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