Occasionally I stumble across information on the Internet that's slightly misleading since it doesn't tell the whole story. For example, this post about OSPF external routes should tell you more about the OSPF route selection process. Just in case you haven't been deeply involved in it yet, here are the rules:
- Intra-area routes are preferred over inter-area or external routes regardless of their cost; see Section 16.2 paragraph (6) of the OSPF RFC.
- Inter-area routes are preferred over external routes; see Section 16.2 paragraph (5) of the OSPF RFC.
- E1 routes are preferred over E2 routes; see Section 16.4 paragraph (6) of the OSPF RFC.
- When comparing E1 routes, the route metric is the external cost added to the internal cost; see Section 16.4 paragraph (6.d) of the OSPF RFC.
- The internal cost of an E1/E2 route is the cost between the current router and the forwarding address specified in the Type-5 LSA (or originating ASBR if the forwarding address is set to 0.0.0.0); see Section 16.4 paragraph (3) of the OSPF RFC.
- When comparing E2 routes, only the external costs are compared; see Section 16.4 paragraph (6.b) of the OSPF RFC.
- If multiple E2 routes have the same external cost, the internal cost (as above) is compared; see Section 16.4 paragraph (6.d) of the OSPF RFC.
Posted to Cisco IOS Hints and Tricks by IvanPepelnjak
©Copyright Ivan Pepelnjak
