Understanding OSPF Routes

by Tony DeSimone

Is your organization running the OSPF or Open Shortest Path First routing protocol for Internet Protocol networks? If so, your access to the Cisco router has been limited to a few “show” commands and you are wondering how to decipher some of those OSPF routes you see in the routing table.

When running a link-state routing protocol such as OSPF, it is customary that the AS (autonomous system) be broken into multiple groups of routers called areas. This design is used to keep the routing tables small and isolate changes in your network topology.

The first route you may notice looks similar to this:

O   131.108.79.208 [110/74] via 203.250.15.1, 00:00:10, Serial0

An “O” designation above indicates that the OSPF route is one that originated from a router within your router’s area; it is an intra-area route. It is either something we call an LSA type1 or LSA type2 route.  Too many intra-area routes may indicate a need to break the area up into multiple areas.

A second route may look similar to this:

O IA    203.250.14.0 [110/74] via 203.250.16.2, 00:06:31, Serial1

An “O IA” designation indicates that the route originated in a different area than your router’s; it is an inter-area route called an LSA type3 route.  Too many inter-area routes may indicate that you are in need of performing route summarization on the ABR (Area Border Router), the router in between 2 areas.

Another route may resemble this:

O E1   128.213.64.0 [110/10] via 203.250.15.1, 00:00:29, Serial0

or

O E2    128.213.64.0 [110/10] via 203.250.15.1, 00:03:57, Serial0

An “O E1” or “O E2” designation indicates that the route originated outside the OSPF AS; an external route redistributed into OSPF, possibly from an AS running a different routing protocol like EIGRP. These routes are called LSA type5 routes.

Too many external routes may indicate that you are in need of performing route summarization on the ASBR (Autonomous System Boundary Router), the router in between the 2 ASs. The difference between “E1” and “E2” is how the OSPF metric is calculated to determine best path to the external routes.

Training Resources:
Implementing Cisco Enterprise Advanced Routing and Services (ENARSI)
Enterprise Networking

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