What is an OSPF sham link and what is the function of an OSPF sham link

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In Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) configuration, OSPF is used as the routing protocol between PEs and CEs so that the sites in a VPN can be connected through the MPLS backbone network. Though the connectivity between OSPF PEs and CEs ensures the connectivity between VPN sites, the backdoor link between VPN sites should also be considered. For two sites to the same site, the path through the backdoor link is always preferred because, according to OSPF specifications, an intra-area path is always preferred to an inter-area path. Therefore, when a backdoor link exists, consider controlling routes through policies.

If the backdoor link is used for backup only but is not used to provide VPN services, the default processing flow will be unacceptable. For connections to be re-established between sites through the MPLS VPN backbone area, a logical intra-area link must be established between the ingress and egress Virtual Routing and Forwarding (VRF) interfaces of the related PEs. This feature provides a solution to establish an OSPF sham link between two sites as an intra-area channel so that the two sites communicate with each other through the MPLS backbone area, while the backdoor link is used for backup. If no backdoor link exists between the two sites, no sham link is needed.

Other related questions:
Why configure sham links for OSPF on S series switches
In Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) configuration, OSPF is used as the routing protocol between PEs and CEs so that the sites in a VPN can be connected through the MPLS backbone network. Though the connectivity between OSPF PEs and CEs ensures the connectivity between VPN sites, the intra-area link between VPN sites should also be considered. For two sites that belong to the same site, the path through the intra-area link is always preferred because, according to OSPF specifications, an intra-area path is always preferred to an inter-area path. Therefore, when an intra-area link exists, you can control routes through a policy. If the intra-area link is used for backup only, the default processing flow will be unacceptable. For connections to be re-established between sites through the MPLS VPN backbone area, a logical intra-area link must be established between the ingress and egress Virtual Routing and Forwarding (VRF) interfaces of the related PEs. An OSPF sham link can be established between two sites as an intra-area channel to enable the two sites to communicate with each other through the MPLS backbone area, and the intra-area link is used for backup. If no intra-area link exists between the two sites, no sham link is required.

What are the functions of OSPF virtual links
From the perspective of a protocol, the functions of OSPF virtual links are as follows: Areas not physically connected to the backbone area can be connected to the backbone area through virtual links so that these areas route traffic normally. This function is mainly used to merge networks. Virtual links improve network reliability by allowing for normal routing after the physical connection to the backbone area is down. On actual networks, virtual links are seldom used. Generally, networks are planned properly and backbone areas are seldom physically disconnected from the backbone areas. It is rare that networks are merged without network re-planning. In addition, on actual networks, virtual links are seldom used to enhance the robustness of backbone areas.

Why configure OSPF virtual links on S series switches
Question: What are the functions of OSPF virtual links configured on S series switch supporting OSPF? Answer: OSPF virtual links can connect areas that are not physically connected to the backbone area to the backbone area so that these areas can route traffic normally. This function is mainly used to merge networks. Virtual links also improve network reliability by allowing normal routing after the physical connection to the backbone area is down. On actual networks, virtual links are seldom used. Generally, networks are planned properly and areas are seldom physically disconnected from the backbone areas. It is rare that networks are merged without network re-planning. In addition, virtual links are seldom used to enhance the robustness of backbone areas.

The USG configures the OSPF link cost method
The USG2000 & 5000 & 6000 configures the OSPF link cost as follows: 1. Run: system-view The system view is displayed. 2. Run: interface interface-type interface-number The interface view is displayed. 3. Run the ospf cost cost command to set the cost of the OSPF interface. The OSPF protocol cost of the interface is valid for the OSPF route learned from the interface. That is, the cost of the route generated in the device routing table is equal to the cost of the route advertised by the interface. The cost of the interface is set by the ospf cost command.

What is the purpose of an OSPF route tag
OSPF route tags are used in only Virtual Private Network (VPN) scenarios to prevent Type 5 link-state advertisement (LSA) loops in Customer Edge (CE) dual-homing networks. When OSPF detects that the route tag of a Type 5 LSA is the same as a route tag on the Provider Edge (PE) router, this route is ignored. When a CE router is connected to two PEs, PE1 sends the Type 5 LSA generated based on the redistributed Border Gateway Protocol (BGP) route. The CE then forwards this LSA to PE 2. Because an OSPF route has a higher priority than a BGP route to the CE, PE 2 replaces the BGP route with the OSPF route. Thus, a routing loop occurs. With a route tag configured, when the PE detects that the route tag of the LSA is the same as that of its route tag, the PE ignores the LSA, thereby avoiding routing loops. The default route tag is calculated based on the AS numbers in the BGP. If BGP is not configured, the default value of route tag is 0.

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