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 updates for IPv6 are specified as OSPF Version 3 in RFC 5340. For more information on these OSPF configuration procedures, go to: See the following sections for configuration tasks for the sham-link feature. Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2, Cisco IOS Configuration Fundamentals Command Reference, Release 12.2, Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2, http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/1rfospf.htm, http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t5/vpn.htm, http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm, http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm, 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, Determining Platform Support Through Cisco Feature Navigator. Thanks for the request Christian! When the VPN backbone has a sham intra-area link, this sham link can be preferred over the backup link if the sham link has a lower OSPF metric than the backup link. O 172.16.0.0 [110/65] via 10.15.0.1, 00:03:19, Serial0/1.15 OSPF Network Design Solutions (2nd Edition)One of my reader asked a question,"Normally customers require sham link to prefer back door link in case of MPLS VPN environment, when customer run same area. This doesnt change a bit in the discussion, anyway. What they are, how they. This video demonstrates configuration a DMVPN Hub and Spoke in Phase 3 Configuration . OSPF then selects the best path based on the metrics of the links and selects the sham link path, ensuring that the backdoor link is not used. Removes the IP address. %OSPF-5-ADJCHG: Process 1, Nbr 10.12.0.2 on OSPF_SL0 from LOADING to FULL, Loading Done. Associates the loopback interface with a VRF. To access Cisco Feature Navigator, you must have an account on Cisco.com. area 120 sham-link 10.0.0.1 10.44.0.1 cost 1, network 10.120.0.0 0.255.255.255 area 120, network 10.140.0.0 0.255.255.255 area 120, area 120 sham-link 10.44.0.1 10.0.0.1 cost 1. Router2(config)# interface loopback interface-number. Displays information about how the sham-link is advertised as an unnumbered point-to-point connection between two PE routers. 10.0.0.0/24 is subnetted, 2 subnets Apply Now Nezar Lourens Enterprise Wireless Certification. The metric is used on the remote PE routers to select the correct route. VPN. Applicants are expected to participate in after-hours work and an on-call rotation. The goal was to connect it over MPLS and leverage OSPF for the PE to CE connection. Notice that R1 and R5 can see each others Fa0/0 and Fa0/1 connected networks. If these sites are connected over a backdoor link in addition to the VPN backbone, all traffic passes over the backdoor link instead of over the VPN backbone. When sending traffic to a particular destination, the PE router uses the MP-BGP forwarding information. To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL: http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml. The sham link is a logical link, similar to a virtual link. A sham-link represents an intra-area (unnumbered point-to-point) connection between PEs. 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 Figure4. Great Courses, Lessons and Learning Material. All logos, trademarks and registered trademarks are the property of their respective owners. It is amazing how much there is to learn. Router2(config-if)# area area-id sham-link source-address destination-address cost number. By configuring OSPF Domain-ID using as below we can change the route type from OSPF External to Inter-Area. It creates a link that makes the MPLS PE's participating in the sham link appear as a point to point link within OSPF. Before you can configure a sham-link in an MPLS VPN, you must first enable OSPF as follows: Specify the range of IP addresses to be associated with the routing process. When we enable these interfaces, R1 and R5 will become neighbors, and see each others routes to the Fa0/0 and Fa0/1 networks as Intra-Area routes. Thats correct. Notice that the Sham-links have been advertised through as a Type 5 external LSA link type. dont know if youre aware but in Chrome, the screen captures just show white bars. 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. As shown in bold in this example, the loopback interface is learned via BGP from PE-2 and PE-3. Enters global configuration mode on the second PE router. MPLS Layer 3 VPN PE-CE OSPF Sham Link _ NetworkLessons.com - Free download as PDF File (.pdf), Text File (.txt) or read online for free. Cisco Modeling Labs - Personal. A VPN contains geographically dispersed sites that can communicate securely over a shared backbone. A secure IP-based network that shares resources on one or more physical networks. 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. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature. Your email address will not be published. 2022 INE. All routing neighbors are available when verified from the device via CLI or GUI. To correct this default OSPF behavior in an MPLS VPN, use the area sham-link cost command to configure a sham-link between two PEs to connect the sites through the MPLS VPN backbone. configures the OSPF cost for sending an IP packet on the PE-2 sham-link interface. Pretty cool. We can do this with backup tunnels that repair the LSP of a primary (protected) tunnel. Router1(config-if)# ip address ip-address mask. The cost of the sham link can be modified using a command similar to the following: PE1(config-router)#area 0 . Scribd is the world's largest social reading and publishing site. Router1(config)# router ospf process-id vrf vrf-name. SPARK: VMware PEX and I am presenting four sessions. Configures the specified OSPF process with the VRF associated with the sham-link interface on PE-2 and enters interface configuration mode. I have connection from the loopbacks on C1 to the loopbacks on C2. Want to take a look for yourself? Lets add a backdoor link between CE1 and CE2. This is because OSPF always prefers intra-area routes over inter-area routes. These links are able to fool or trick routers in the OSPF domain that this is a better path thus preserving the LSAs as type 1 or type 3. As a result, the desired intra-area connectivity is created. Because the sham-link is seen as an Intra-Area link between PE routers (R2 and R4), an OSPF adjacency is created and database exchange takes place across the sham-link. Flexible Routing in an MPLS VPN Configuration I built out a simple MPLS cloud and had one customer joining two sites across it. The 5.5.5.5 network is now a type 1 LSA along with 192.168.35.3. By using two loopbacks on the respective devices advertised into the BGP address family that corresponds with the customer VRF, OSPF can create a link that is more appealing. Right now, the MPLS backbone is the only way for the CE routers to reach each other. For this reason, you should not modify the metric value when OSPF is redistributed to BGP, and when BGP is redistributed to OSPF. When the primary LSP is broken, we can continue to forward traffic down the backup tunnel until the headend router figures out a new best path. Figure4 shows a sample MPLS VPN topology in which a sham-link configuration is necessary. If you modify the metric value, routing loops may occur. Webinars & Videos. !!!!! This is the topology currently. Because I have tried redistribution other routing protocols into OSPF on CE without Sham Link and result is PE which connected directly with CE got the routes but other PEs didnt got it. For this reason, OSPF backdoor links between VPN sites must be taken into account so that routing is performed based on policy. To remove the sham-link, use the no form of this command. OSPF - NetworkLessons.com OSPF Course Description OSPF (Open Shortest Path First) is a popular link-state routing protocol. Redistributed routes in OSPF on a CE router is no problem. This takes less than ~50 ms. The sham link is an unnumbered point-to-point intra-area link between PE devices. Examples of common IGPs include IGRP, OSPF, and RIP. CE routercustomer edge router. R4(config-router)#area 1 sham-link 11.11.11.4 11.11.11.2 cost 5 We are actually going to pull a fast one, or a sham, on OSPF because the MPLS network is really acting as a superbackbone for OSPF, and therefore routes between the CEs are indeed Inter-Area by default. Associate the sham-link with an existing OSPF area. Cisco Feature Navigator is a web-based tool that enables you to quickly determine which Cisco IOS software images support a specific set of features and which features are supported in a specific CiscoIOS image. This could be a backup link that you want to use in case the MPLS VPN provider has issues: Lets enable OSPF on this interface and advertise it in area 0: The total cost through the MPLS VPN network is 4. (PE routers advertise OSPF routes learned over the VPN backbone as interarea paths.) 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 Figure2) may exist. OSPF has a lower administrative distance (AD) than internal BGP (BGP running between routers in the same autonomous system). Notice there are no longer any inter-area routes / type 3 summary LSAs? OSPF Sham Links 1,743 views Feb 12, 2021 49 Dislike Share Save Michael O'Brien's CCIE Journey 3.23K subscribers In this video I demonstrate OSPF sham-links. The team is responsible for running customers' mission critical applications on hybrid environments. Sham Link. 172.16.0.0/24 is subnetted, 1 subnets Even though the metric of 65 is worse than before, and using the slower serial link, the routers prefer these routes instead of using the PE learned routes, because Intra-Area routes are preferred over Inter-Area routes. So lets now take a look at the Sham link adjusted OSPF database. Router#show ip ospf data router ip-address. Here's an example. Get Full Access to our 751 Cisco Lessons Now, OSPF Point-to-Multipoint Non-Broadcast Network Type, How to configure OSPF NSSA (Not So Stubby) Area, How to configure OSPF Totally NSSA (Not So Stubby) Area, OSPF Loop-Free Alternate (LFA) Fast Reroute (FRR), OSPF Remote Loop-Free Alternate (LFA) Fast Reroute (FRR). OSPF always selects intra-area routes over interarea (external) routes. kind of weird. By default bgp learned routes do not get a label assigned (only the next hop). By using the commandarea sham-link cost it is possible to build this link. Ps. OSPF running on a PE device can use the routing information to generate inter-area routes from the PE to CE devices. 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. smart-discover Hello . Note that customer routers receive information from Ethernet0/0 the upward link to the ISP for the Customer device. OSPF sham-link. This will allow traffic to pass through the MPLS cloud as the preferred link and upon failure the backdoor link can be used to maintain connectivity. More fun times regarding MPLS, OSPF and MPBGP can be found in our workbooks for RS and SP. 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). And just to be sure, a ping to verify connectivity. Sign in|Recent Site Activity|Report Abuse|Print Page|Powered By Google Sites. O IA 172.16.0.0 [110/3] via 10.45.0.4, 00:01:49, FastEthernet0/1 ", describes how to configure a sham-link between two PE routers. You need to setup a sham-link if you want the traffic between the two sites to prefer the mpls backbone rather than the backdoor link. R5#show ip route ospf 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. Just a humble clarification: Routes traversing the mpls superbackbone wont be injected as external (E1/2) but as inter-area (O IA) routes. Valid values: numeric value or valid IP address. Reader's Digest version: MPLS networks aren't free. As a result, OSPF sees both the path over the backdoor link and the path over the backbone as intra-area paths. What the different OSPF LSA types are used for. R4(config-router)#address-family ipv4 vrf Vrf1 Proceeding to add a private link between the branch and HQ sites did I realise that OSPF no longer leveraged the MPLS cloud but used the private link. Looks like it is in place, but is it creating the desired result, of having the CE routers R1 and R5 see the Ethernet remote networks as reachable through the PE routers R2 and R4? 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. Expert in low latency network technologies - Including Multicast (IGMP, PIM), L2 /L3, WAN Design, expert in routing (BGP, OSPF ) Minimum of 10 years of experience in a network engineering, operations, and support. An OSPF sham-link will have a default cost of 1 ensuring that it is chosen as the best path over alternative possible paths. An advanced Layer 3 IP switching technology. Because the sham-link is seen as an Intra-Area link between PE routers (R2 and R4), an OSPF adjacency is created and database exchange takes place across the sham-link. --> OSPF Sham link is used when there is a backdoor link between two customer sites and MPLS VPN Connectivity. OSPF will always prefer an intra area route over an inter area route, this is regardless of the metric that is associated with that route. O 10.12.0.0 [110/65] via 10.15.0.1, 00:03:19, Serial0/1.15, Notice, that the remote customer networks attached to Fa0/0 and Fa0/1 are now reachable via the serial 0/1.15 interface, and they appear as Intra-Area routes. Looks like LSA type 5 & 7 are not exchanged cross MPLS backbone? This means upon redistribution out of BGP into OSPF, routes retain their external route marking. 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. Source: CCIE study: OSPF Sham . The PE routers are then able to flood LSAs across the MPLS VPN backbone. R5(config)#int ser 0/1.15 OSPF is often used by customers that run OSPF The networks from the other customer site are passed over this OSPF sham link as Type 1 router LSA. Interdomain routing protocol that exchanges reachability information with other BGP systems. No new or modified RFCs are supported by this feature. 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. Notify me of follow-up comments by email. Sending 5, 100-byte ICMP Echos to 172.16.0.1, timeout is 2 seconds: As a result, the desired Intra-Area routes are created. This command has no arguments or keywords. The following example shows how to configure a sham-link between two PE routers: This section documents new commands. The backdoor link becomes favoured and subsequently used. --> The problem with this scenario is CE routers will prefer path via back door compared to MPLS VPN Connection because of OSPF best path selection algorithm ( Intra Area vs Inter . The sham link is an unnumbered point-to-point link inside a routing-instance between two PE routers. 172.16.0.0/24 is subnetted, 1 subnets The trace route shows the path we are expecting to see and no hairpin routing is occurring. close menu Language. These links are able to fool/trick routers in the OSPF domain that this is a better path thus preserving the LSAs as type 1 or type 3. OSPF sham-link host interfaces MUST be advertised by BGP and not the ospf process. ID number of the Open Shortest Path First (OSPF) area assigned to the sham-link. Lets do some testing and verification of what is currently in place. Interesting post Anthony. The source and destination IP addresses must belong to the VRF and be advertised by Border Gateway Protocol (BGP) to remote PE routers. What Is Ospf Routing Protocol? Tracing the route to 172.16.0.1, 1 10.45.0.4 48 msec 92 msec 12 msec 2 10.34.0.3 [MPLS: Labels 16/24 Exp 0] 136 msec 180 msec 228 msec 3 10.12.0.2 [MPLS: Label 24 Exp 0] 124 msec 80 msec 88 msec 4 10.12.0.1 112 msec * 176 msec. To configure a sham-link interface on a provider edge (PE) router in a Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) backbone, use the area sham-link cost command in global configuration mode. Heres a quick example: 5 more replies! A traceroute. Thx. 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. If there is a backdoor link between R4 and R5, traffic will be routed over that backdoor link rather than going through MPLS cloud. (I dont think they will be providing a price break either). If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. 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. If no backdoor link exists between the sites, no sham-link is required. An Internet protocol used to exchange routing information within an autonomous system. cost number configures the OSPF cost for sending an IP packet on the PE-2 sham-link interface. Cisco IOS software is packaged in feature sets that support specific platforms. Next, we will enable the Serial0/1.15 interfaces of R1 and R5. A sham-link ensures that OSPF. IGPInterior Gateway Protocol. Routers will exchange pieces of information called LSAs (link state advertisement) in order to build a complete topology database which we call the LSDB (link state database). By default, OSPF external routes dont get redistributed into BGP but you can change that. It confirms Phase 3 connectivity between 2 Spokes and Hub to Spoke Conf. OSPF Sham Links are required when you try to use a backdoor link between two CE routers in an MPLS VPN PE CE scenario where you use OSPF as the PE-CE routing protocol. CE routers are not aware of associated VPNs. 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. This prefix is the loopback interface of the Winchester CE router. The sham link is advertised using Type 1 link-state advertisements (LSAs). The following example shows BGP routing table entries for the prefix 10.3.1.7/32 in the PE-1 router in Figure2. Well wait a few moments, to give the network time to converge, then take a look at the OSPF routes on the CE routers R1 and R5, just as we did earlier, and see if the routes are different. The routing table indicates that we are learning the other sites routes via the MPLS cloud. The Internet's global routing system is based on. The basics of link-state routing protocols and OSPF. To train the network to use the MPLS network as the primary transit path, we need to make the remote Ethernet customer networks look like Intra-Area routes via the PE routers, with a better metric than the serial interfaces, so they can be used instead of the slower serial link. All other commands used with this feature are documented in the Cisco IOS Release 12.2 command reference publications. The sham-link is configured on top of the MPLS VPN tunnel that connects two provider edge (PE) routers. Post was not sent - check your email addresses! In IE11, they show the text with scroll bars under each line of text? The /32 address must meet the following criteria: You can use the /32 address for other sham-links. Two sham-links have been configured, one between PE-1 and PE-2, and another between PE-2 and PE-3. Hence, the default Hello interval is 10 seconds and the default Router Dead interval is 40 seconds. All Webinars & Events. Success rate is 100 percent (5/5), round-trip min/avg/max = 120/130/148 ms. Thats cool, so we know we have connectivity, and based on the routing table output, we believe it is going through the SP MPLS network. All VPN processing occurs in the PE router. Community Impact. Emerging industry standard upon which tag switching is based. Rating: 4.7. To verify that the sham-link was successfully created and is operational, use the show ip ospf sham-links command in EXEC mode: To monitor a sham-link, use the following show commands in EXEC mode: Displays the operational status of all sham-links configured for a router. Open Shortest Pathway First ( OSPF ) is an Open Standard Link State routing protocol. 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. Figure3 shows a sample sham-link between PE-1 and PE-2. Now our transit traffic is moving through the MPLS network, and the serial 0/1.15 interfaces are available as a backup. In a Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) configuration, a virtual connection called a sham-link can be configured to interconnect between two VPN sites that want to be in the same OSPF area. sham-link VPNVPNOSPFVPNOSPF. In the following example, PE-2 shows how an MP-BGP update for the prefix is not generated. 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. Removes the IP address. Flexible Routing in an MPLS VPN Configuration. Is that correct? They are a type 5 external LSA. en Change Language. 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