IP Routing: OSPF Configuration Guide, Cisco IOS Release 15SY
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This module describes how to configure and use a sham-link to connect Virtual Private Network (VPN) client sites that run
the Open Shortest Path First (OSPF) protocol and share backdoor OSPF links in a Multiprotocol Label Switching (MPLS) VPN configuration.
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Feature Overview
Using OSPF in PE-CE Router Connections
In an MPLS VPN configuration, the OSPF protocol is one way you can connect customer edge (CE) routers to service provider
edge (PE) routers in the VPN backbone. OSPF is often used by customers that run OSPF as their intrasite routing protocol,
subscribe to a VPN service, and want to exchange routing information between their sites using OSPF (during migration or on
a permanent basis) over an MPLS VPN backbone.
The figure below shows an example of how VPN client sites that run OSPF can connect over an MPLS VPN backbone.
When OSPF is used to connect PE and CE routers, all routing information learned from a VPN site is placed in the VPN routing
and forwarding (VRF) instance associated with the incoming interface. The PE routers that attach to the VPN use the Border
Gateway Protocol (BGP) to distribute VPN routes to each other. A CE router can then learn the routes to other sites in the
VPN by peering with its attached PE router. The MPLS VPN superbackbone provides an additional level of routing hierarchy to
interconnect the VPN sites running OSPF.
When OSPF routes are propagated over the MPLS VPN backbone, additional information about the prefix in the form of BGP extended
communities (route type, domain ID extended communities) is appended to the BGP update. This community information is used
by the receiving PE router to decide the type of link-state advertisement (LSA) to be generated when the BGP route is redistributed
to the OSPF PE-CE process. In this way, internal OSPF routes that belong to the same VPN and are advertised over the VPN backbone
are seen as interarea routes on the remote sites.
For basic information about how to configure an MPLS VPN, refer to the "MPLS Virtual Private Networks Configuration" module.
Using a Sham-Link to Correct OSPF Backdoor Routing
Although OSPF PE-CE connections assume that the only path between two client sites is across the MPLS VPN backbone, backdoor
paths between VPN sites (shown in grey in the figure below) may exist. If these sites belong to the same OSPF area, the path
over a backdoor link will always be selected because OSPF prefers intraarea paths to interarea paths. (PE routers advertise
OSPF routes learned over the VPN backbone as interarea paths.) For this reason, OSPF backdoor links between VPN sites must
be taken into account so that routing is performed based on policy.
For example, the figure above 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 the figure above.
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.
PE-1# show ip route vrf ospf 10.3.1.7
Routing entry for 10.3.1.7/32
Known via "ospf 100", distance 110, metric 86, type intra area
Redistributing via bgp 215
Advertised by bgp 215
Last update from 10.2.1.38 on Serial0/0/0, 00:00:17 ago
Routing Descriptor Blocks:
* 10.2.1.38
, from 10.3.1.7, 00:00:17 ago, via Serial0/0/0
Route metric is 86, traffic share count is 1
This path is selected because:
The OSPF intra-area path is preferred over the interarea path (over the MPLS VPN backbone) generated by the PE-1 router.
OSPF has a lower administrative distance (AD) than internal BGP (BGP running between routers in the same autonomous system).
If the backdoor links between sites are used only for backup purposes and do not participate in the VPN service, then the
default route selection shown in the preceding example is not acceptable. To reestablish the desired path selection over the
MPLS VPN backbone, you must create an additional OSPF intra-area (logical) link between ingress and egress VRFs on the relevant
PE routers. This link is called a sham-link.
A sham-link is required between any two VPN sites that belong to the same OSPF area and share an OSPF backdoor link. If no
backdoor link exists between the sites, no sham-link is required.
The figure below shows a sample sham-link between PE-1 and PE-2. A cost is configured with each sham-link and is used to
decide whether traffic will be sent over the backdoor path or the sham-link path. When a sham-link is configured between PE
routers, the PEs can populate the VRF routing table with the OSPF routes learned over the sham-link.
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 the "Configuring OSPF" module.
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.
The figure below 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.
The following example shows the forwarding that occurs between sites from the standpoint of how PE-1 views the 10.3.1.7/32
prefix, the loopback1 interface of the Winchester CE router in the figure above.
PE-1# show ip bgp vpnv4 all 10.3.1.7
BGP routing table entry for 100:251:10.3.1.7/32, version 124
Paths: (1 available, best #1)
Local
10.3.1.2 (metric 30) from 10.3.1.2
(10.3.1.2)
Origin incomplete, metric 11, localpref 100, valid, internal,
best
Extended Community: RT:1:793 OSPF DOMAIN ID:0.0.0.100 OSPF
RT:1:2:0 OSPF 2
PE-1# show ip route vrf ospf 10.3.1.7
Routing entry for 10.3.1.7/32
Known via "ospf 100
", distance 110, metric 13, type intra area
Redistributing via bgp 215
Last update from 10.3.1.2 00:12:59 ago
Routing Descriptor Blocks:
10.3.1.2 (Default-IP-Routing-Table), from 10.3.1.7, 00:12:59 ago
The next example shows forwarding information in which the next hop for the route, 10.3.1.2, is the PE-3 router rather than
the PE-2 router (which is the best path according to OSPF). The reason the OSPF route is not redistributed to BGP on the PE
is because the other end of the sham-link already redistributed the route to BGP and there is no need for duplication. The
OSPF sham-link is used only to influence intra-area path selection. When sending traffic to a particular destination, the
PE router uses the MP-BGP forwarding information.
PE-1# show ip bgp vpnv4 all tag | begin 10.3.1.7
10.3.1.7/32 10.3.1.2
notag/38
PE-1# show tag-switching forwarding 10.3.1.2
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
31 42 10.3.1.2/32
0 PO3/0/0 point2point
PE-1# show ip cef vrf ospf 10.3.1.7
10.3.1.7/32, version 73, epoch 0, cached adjacency to POS3/0/0
0 packets, 0 bytes
tag information set
local tag: VPN-route-head
fast tag rewrite with PO3/0/0, point2point, tags imposed: {42 38
}
via 10.3.1.2
, 0 dependencies, recursive
next hop 10.1.1.17, POS3/0/0 via 10.3.1.2/32
valid cached adjacency
tag rewrite with PO3/0/0, point2point, tags imposed: {42 38}
If a prefix is learned across the sham-link and the path via the sham-link is selected as the best, the PE router does not
generate an MP-BGP update for the prefix. It is not possible to route traffic from one sham-link over another sham-link.
In the following example, PE-2 shows how an MP-BGP update for the prefix is not generated. Although 10.3.1.7/32 has been
learned via OSPF across the sham-link as shown in bold, no local generation of a route into BGP is performed. The only entry
within the BGP table is the MP-BGP update received from PE-3 (the egress PE router for the 10.3.1.7/32 prefix).
PE-2# show ip route vrf ospf 10.3.1.7
Routing entry for 10.3.1.7/32
Known via "ospf 100
", distance 110, metric 12, type intra area
Redistributing via bgp 215
Last update from 10.3.1.2 00:00:10 ago
Routing Descriptor Blocks:
* 10.3.1.2 (Default-IP-Routing-Table), from 10.3.1.7, 00:00:10 ago
Route metric is 12, traffic share count is 1
PE-2# show ip bgp vpnv4 all 10.3.1.7
BGP routing table entry for 100:251:10.3.1.7/32, version 166
Paths: (1 available, best #1)
Not advertised to any peer
Local
10.3.1.2 (metric 30) from 10.3.1.2 (10.3.1.2)
Origin incomplete, metric 11, localpref 100, valid, internal,
best
Extended Community: RT:1:793 OSPF DOMAIN ID:0.0.0.100 OSPF
RT:1:2:0 OSPF 2
The PE router uses the information received from MP-BGP to set the ongoing label stack of incoming packets, and to decide
to which egress PE router to label switch the packets.
Benefits
Client Site Connection Across the MPLS VPN Backbone
A sham-link overcomes the OSPF default behavior for selecting an intra-area backdoor route between VPN sites instead of an
interarea (PE-to-PE) route. A sham-link ensures that OSPF client sites that share a backdoor link can communicate over the
MPLS VPN backbone and participate in VPN services.
Flexible Routing in an MPLS VPN Configuration
In an MPLS VPN configuration, the OSPF cost configured with a sham-link allows you to decide if OSPF client site traffic will
be routed over a backdoor link or through the VPN backbone.
Restrictions
When OSPF is used as a protocol between PE and CE routers, the OSPF metric is preserved when routes are advertised over the
VPN backbone. The metric is used on the remote PE routers to select the correct route. For this reason, you should not modify
the metric value when OSPF is redistributed to BGP, and when BGP is redistributed to OSPF. If you modify the metric value,
routing loops may occur.
Related Features and Technologies
MPLS
OSPF
BGP
Related Documents
Cisco IOS IP Routing: OSPF Command Reference
"MPLS Virtual Private Networks" module
"Configuring OSPF" module
Cisco IOS IP Routing: BGP Configuration Guide, Release 15.0
RFC 1163, A Border Gateway Protocol
RFC 1164, Application of the Border Gateway Protocol in the Internet
RFC 2283, Multiprotocol Extensions for BGP-4
RFC 2328, Open Shortest Path First, Version 2
RFC 2547, BGP/MPLS VPNs
Supported Platforms
Cisco 1400 series
Cisco 1600
Cisco 1600R
Cisco 1710
Cisco 1720
Cisco 1721
Cisco 1750
Cisco 1751
Cisco 2420
Cisco 2600
Cisco 2691
Cisco 3620
Cisco 3631
Cisco 3640
Cisco 3660
Cisco 3725
Cisco 3745
Cisco 7100
Cisco 7200
Cisco 7500
Cisco 7700
URM
Cisco uBR7200
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Before you create a
sham-link between PE routers in an MPLS VPN, you must:
Configure a new
interface with a /32 address on the remote PE so that OSPF packets can be sent
over the VPN backbone to the remote end of the sham-link. The /32 address must
meet the following criteria:
Belong to a VRF.
Not be advertised by OSPF.
Be
advertised by BGP.
You can use the /32
address for other sham-links.
Associate the
sham-link with an existing OSPF area.
Configures
the sham-link on the PE-1 interface within a specified OSPF area and with the
loopback interfaces specified by the IP addresses as endpoints.
costnumber
configures the OSPF cost for sending an IP packet on the PE-1 sham-link
interface.
Step 13
Router2(config)#
routerospfprocess-idvrfvrf-name
Configures
the specified OSPF process with the VRF associated with the sham-link interface
on PE-2 and enters interface configuration mode.
Configures
the sham-link on the PE-2 interface within a specified OSPF area and with the
loopback interfaces specified by the IP addresses as endpoints.
costnumber
configures the OSPF cost for sending an IP packet on the PE-2 sham-link
interface.
Verifying Sham-Link Creation
To verify that the sham-link was successfully created and is operational, use the showipospfsham-links command in EXEC mode:
Router1# show ip ospf sham-links
Sham Link OSPF_SL0 to address 10.2.1.2 is up
Area 1 source address 10.2.1.1
Run as demand circuit
DoNotAge LSA allowed. Cost of using 40 State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40,
Hello due in 00:00:04
Adjacency State FULL (Hello suppressed)
Index 2/2, retransmission queue length 4, number of
retransmission 0
First 0x63311F3C(205)/0x63311FE4(59) Next
0x63311F3C(205)/0x63311FE4(59)
Last retransmission scan length is 0, maximum is 0
Last retransmission scan time is 0 msec, maximum is 0 msec
Link State retransmission due in 360 msec
Monitoring and Maintaining a Sham-Link
Command
Purpose
Router# showipospfsham-links
Displays the operational status of all sham-links configured for a router.
Router# showipospfdatarouterip-address
Displays information about how the sham-link is advertised as an unnumbered point-to-point connection between two PE routers.
Configuration Examples
The following example shows how to configure a sham-link between two PE routers:
Router1(config)
# interface loopback 1
Router1(config-if)# ip vrf forwarding ospf
Router1(config-if)# ip address 10.2.1.1 255.255.255.255
!
Router2(config)# interface loopback 1
Router2(config-if)# ip vrf forwarding ospf
Router2(config-if)# ip address 10.2.1.2 255.255.255.255
!
Router1(config)# router ospf 100 vrf ospf
Router1(config-if)# area 1 sham-link 10.2.1.1 10.2.1.2 cost 40
!
Router2(config)# router ospf 100 vrf ospf
Router2(config-if)# area 1 sham-link 10.2.1.2 10.2.1.1 cost 40
Glossary
BGP--Border Gateway Protocol. Interdomain routing protocol that exchanges reachability information with other BGP systems. It
is defined in RFC 1163.
CErouter--customer edge router. A router that is part of a customer network and that interfaces to a provider edge (PE) router. CE
routers are not aware of associated VPNs.
CEF--
Cisco Express Forwarding. An advanced Layer 3 IP switching technology. CEF optimizes network performance and scalability for
networks with large and dynamic traffic patterns.
OSPF--Open Shortest Path First protocol.
IGP--Interior Gateway Protocol. An Internet protocol used to exchange routing information within an autonomous system. Examples
of common IGPs include IGRP, OSPF, and RIP.
LSA--link-state advertisement. A broadcast packet used by link-state protocols. The LSA contains information about neighbors
and path costs and is used by the receiving router to maintain a routing table.
MPLS--Multiprotocol Label Switching. Emerging industry standard upon which tag switching is based.
PErouter--provider edge router. A router that is part of a service provider network connected to a customer edge (CE) router. All
VPN processing occurs in the PE router.
SPF--shortest path first calculation.
VPN--Virtual Private Network. A secure IP-based network that shares resources on one or more physical networks. A VPN contains
geographically dispersed sites that can communicate securely over a shared backbone.
VRF--VPN routing and forwarding instance. A VRF consists of an IP routing table, a derived forwarding table, a set of interfaces
that use the forwarding table, and a set of rules and routing protocols that determine what goes into the forwarding table.
In general, a VRF includes the routing information that defines a customer VPN site that is attached to a PE router.