- Analyse the LSDB to find all possible routes to reach the subnet.
- Per route, add the interface cost for all outgoing interfaces in that route.
- Pick the route with the lowest cost.
- Cost is derived from the egress interface bandwith.
- Locates subnets via interface Type 1 LSA and any Type 2 LSA
- Uses SPF to find all paths for each subnet
- Calculates the OSPF interface cost for the routes, and picks the lowest cost
- From the router’s point of view it only knows the subnet via Type 3 LSA. Doesn’t know the topology
- ABR sends the cost of the subnet in the area
- With multiple ABRs between area’s you can modify the cost to control outgoing routing
Priority of route Selection
- within the same area (via Type 1 LSA via Type 2 LSA).
- From another area (via Type 3 LSA).
- Redistribution, doesn’t live in the OSPF domain (via Type 5 LSA).
- Route Hirarchy:
- O > O IA > E1 > E2 > N1 > N2
OSPF Metric tuning
show ip ospf interface
show ip ospf interface brief
- change reference bandwidth
- Reference-bandwith / interface-bandwith
- default reference-bandwith 100Mbps/ (Default is 100/100 = 1)
- auto-cost reference-bandwidth bandwidth
- Setting bandwidth
- (config-if)#bandwidth (value)
- Not recommended because of other protocols and QoS.
- Config cost directly
- (config)#ip ospf cost (value)
- More controllable but more work
R1#sh ip ospf int loopback 1
Loopback1 is up, line protocol is up
Internet Address 126.96.36.199/24, Area 1
Process ID 1, Router ID 10.10.10.1, Network Type LOOPBACK, Cost: 1
Enabled by interface config, including secondary ip addresses
Loopback interface is treated as a stub Host