Redistribution: Preventing Suboptimal Paths via Administrative Distance

One way to prevent the problem of suboptimal routes on redistribution routers is to flag the redistributed routes with a higher AD.  The routes AD is not advertised by the routing protocol, however you can assign different AD values to redistributed routes which will impact the routers best path decision to the destination.

For example when a route from a RIP domain and OSPF are considered, a router will learn both paths via both protocols, when you configure OSPF external routes to have a higher AD than RIP, the router will determine best path as:

  • OSPF Internal Type Route
  • RIP Route
  • OSPF External Type route

Under normal circumstances RIP routes AD is HIGHER than OSPF External type routes, this allows for optimal paths via RIP to be chosen.

This concept can also be used for RIP and EIGRP, where EIGRP Internal routes have an AD of 90 and EIGRP external routes have an AD of 170, setting RIP lower in this scenario would allow the same outcome as well.

Sometimes there may be instances where setting all External routes AD to be lower would cause sub-optimal routing, in those instances you would define the specific subnet to have an explicit AD using the following distance command:

distance { distance-value ip-address { wildcard-mask } [ ip-standard-list ] [ ip-extended-list ]

The logic of this command defines the Ad value for all routes, learned from a router that is defined by the IP address and wildcard mask, and for which the ACL permits the route.

 

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