Introduction
OSPF External routers typically receives multiple paths for the same destination ip address. The OSPF External route selection algorithm decides which is the best path to install in the IP routing table and use to forward traffic .
Requirements
OSPF external route selection algorithm shown in this document is based on RFC 3101 and replaces RFC 1587 ospf implementation starting from Cisco IOS release 15.1(2)S
Brief : What are External or redistributed Routes in OSPF ( E1 , E2 , N1 , N2 )
These types of routes are carried in Type 5 / Type 7 LSAs.
External Type 2 or E2 route
- E2 routes are default External route type in OSPF.
- E2 routes only reflects the cost of particular route from ASBR ( Autonomous Summary Border Route ) to destination
- E2 routes have default fixed cost of 20 sometimes referred to as seed metric.
- Uses Type 5 LSA
- With E2 routes there is another metric called "forward metric" ( internal OSPF cost to ASBR ) which is used to break tie in case of two routes with same E2 cost.
- If both E2 and forward metric of same route is same then equal cost loadbalancing will happen.
External Type 1 or E1 route
- While redistributing we could manually configure OSPF with argument "metric-type" to advertise external routes as E1 routes.
- E1 routes will have internal OSPF area cost ( cost to ASBR ) added to the seed metric.
- Uses Type 5 LSA
Not So Stubby Area
- NSSA is a Special type of area to allow ospf stub area to carry external routes ( learned using other routing protocols, redistributed) using Type 7 LSAs.
- Type 7 LSAs are only flooded within originated NSSA area
- The NSSA ABR translates type 7 LSA to type 5 LSA and flood into OSPF domain
- When there are multiple NSSA ABR routers for type 7 to type 5 translation , ABR with highest router ID does the task.
What is P bit or Propagate bit
- When P bit is on NSSA ABR translates type 7 to type 5 and flood into ospf domain.
- NSSA ABR always set P bit int options field of type 7 LSA . Only exception is when NSSA ASBR and NSSA ABR is the same router .
As per RFC 3101 , below is the sequence of OSPF External route selection algorithm :
1) E 1 and N1 routes will be preferred over E2 and N2 routes.
2) If same route is learning as both E2 and N2 , below will happen :
a) first preference is to route metric , route with better metric will win (
also consider forward metric )
3) If both routes cost are same (i.e., same destination, cost and non-zero forwarding address) then apply the following priorities in deciding which LSA is preferred:
a) P bit will be used to break the tie (
LSA with P bit on will be considered )
b) In case P bit is not on and cost is same , E2 route will be choosen .
c) The LSA with higher router ID
