OSPF Metric Calculations & Tuning

• • 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.

Intra-Area

• 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

Inter-Area Routes

• 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

• Intra-Area
  • within the same area (via Type 1 LSA via Type 2 LSA).
• Inter-Area
  • From another area (via Type 3 LSA).
• External
  • 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 1.1.1.1/24, 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