ROUTE SELECTION PREFERENCE
HOW TO SELECT ROUTES TO BE INSTALLED WHEN LEARNING MULTIPLE ROUTES
1. Multiple routes available to the same network through different LSA types
2. Multiple routes available to the same network from same LSA type
1. Multiple routes available to the same network through different LSA
Example: we are at Router, R1 in Area 0. From R1, we have to reach network, 192.168.10.0/24. we are learning routes to reach network, 192.168.10.0/24 from different LSA’s.
In a Single Process
Intra Area> Inter Area> External E1 > External E2
LSA-1 or LSA-2 > LSA-3 > LSA-5 or LSA-7
Example: Learning network 192.168.10.0/24 through LSA-1(Inter-Area), LSA-3(Intra-Area) and LSA-5(E1 & E2)
Order of Preference: INTRA > INTER > E1 > E2 OR LSA-1 > LSA-3 > LSA-5(E1) > LSA-5(E2)
Case: Can we change order of preference. In a single process, we cannot change order of preference.
In Multiple Process (Above Preference rule is not applicable)
Multiple processes do not exchange routes by itself, until and unless configured to do so.
Multiple processes are almost like two different domains. So, there is no comparision inbetween them. LSA’s of two different processes are not compared. No Preference rule is applicable.
In multiple process, route comparison occurs only if metrics and administrative distances (if they have been changed) of the OSPF processes are changed
Reference Lab: To modify AD value of two different processes
2. Multiple routes available to the same network through same LSA
Learning network 192.168.10.0/24 through two different LSA-5, means learning from two different ASBR.
The OSPF metric calculated as cost ( based on the bandwidth) is used for selecting the best route. The route with the lowest value for cost is chosen as the best route. If cost is equal here, all the routes get installed in the routing table, and the router does equal cost load balancing across multiple paths.