3.3.k Identify and troubleshoot sub-optimal routing

When you are redistributing between two routing protocol e.g. OSPF /EIGRP , if you don’t configure filters or administrative distance modification then suboptimal routing may occur.

As an example, redistributing between different OSPF processes in multiple points on the network, it is possible to get into situations of suboptimal routing or even worse, a routing loop. When enabling multiple OSPF processes on a router, from the software perspective , the processes are independent. OSPF protocol, inside one OSPF process , always prefers the Internal route over the External route. However, OSPF does not do any OSPF route selection between processes (for instance, OSPF metrics and route types are not taken into account, when deciding the route of which process should be installed into the routing table).

There is no interaction between different OSPF processes, and the tie-breaker is the administrative distance. Thus, since both OSPF processes have a default administrative distance of 110, the first process trying to install that route makes it into the routing table. Therefore, administrative distance for routes from different OSPF processes must be configured, so that routes of certain OSPF processes are preferred over routes of another process by human intention, and not as a matter of chance.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2807-2810).  . Kindle Edition.