OSPF: CONCEPT: ROUTE SELECTION


ROUTE SELECTION PREFERENCE

 HOW TO SELECT ROUTES TO BE INSTALLED WHEN LEARNING MULTIPLE ROUTES

 

TWO CASES:

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

Preference Rule:

Intra Area> Inter Area> External E1 > External E2

or,

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

https://networkproxy.wordpress.com/2014/03/26/ospf-lab-redistribution-among-different-processes/#more-569

 

 

2. Multiple routes available to the same network through same LSA

Example:

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.

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