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IP Routing

OSPF virtual link

OSPF virtual link is a tunnel that extends backbone area through a non-backbone area. It is interpreted by the router as unnumbered point-to-point network. Virtual links must be configured between two Area Border Routers. These ABRs are considered neighbours, by establishing the virtual link between them, although they are not linked physically. The transit area (the area through which the virtual link is configured) must have full routing information and cannot be a stub area.

Virtual links add complexity to a network and should be avoided. Use them only as a temporary fix to OSPF topology problem.

Virtual links are used for the following purposes:

  • To link an area to the backbone area through a non-backbone area (most common case). On the picture below Area2 was connected to the Area0 via virtual link, which extends Area0 through transit Area1.

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BGP peering with loopback interfaces

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how does BGP select routes

When there are multiple routes to the same destination, BGP (Border Getaway Protocol)  on Huawei devices:

  • Prefers the route with the largest PreVal. PrefVal is a Huawei-specific parameter. It is valid only on the device where it is configured.
  • Prefers the route with the highest Local_Pref.
  • Prefers a locally originated route. A locally originated route takes precedence over a route learned from a peer.
  • Prefers the aggregated route. The preference of an aggregated route is higher than a non-aggregated route.
  • Prefers the local route that is manually aggregated. The preference of the local route that is manually aggregated is higher than that of automatically aggregated.
  • Prefers the local route that is imported by using network command. The preference of the route that is imported by using the network command is higher than that imported by  import-route command.
  • Prefers the route with the shortest AS_Path.
  • Compares Origin attributes, and selects routes whose origin types are IGP, EGP, and Incomplete respectively.
  • Prefers the route with the smallest MED.
  • Prefers the routes learned from EBGP. The preference of an EBGP route is higher than an IBGP.
  • Prefers the route of an IGP with the lawest metric in an AS. If load balancing is configured and there are multiple external routes with the same AS_Path, load balancing is performed according to the number of configured routes.
  • Prefers the route with the shortest Cluster_List.
  • Prefers the route with the smallest Originator_ID.
  • Prefers the route advertised by the router with the smallest router ID.
  • Compares IP addresses of its peers, and prefers the route that is learnt from the peer with the smallest IP address.
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    Huawei route preferences

    Routing protocols are used to discover routes to a destination. Only one routing protocol at one time determines the optimal route to a destination.

    What happens if we have different routes to the same destination learnt by different routing protocols?

    When multiple routing information sources coexist on the same router, the router with the highest preference is selected as the optimal route.

    Below you can find the default route preferences by each routing protocol (the smaller the value, the higher the preference is):

    Of course you can change these default values manually.

    But what happens if different routing protocols are configured with the same preference?

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