OSPF Autonomous System (AS)
Router A
Router B
Router C
Routers D, E, and F | Another routing domain |
are OSPF ASBRs |
and EBGP routers. | (such as BGP4 or RIP) |
Router D
Router ID: 2.2.2.2
Router F
Router E
Router ID: 1.1.1.1
Figure 8.4 AS External LSA reduction
Notice that both Router D and Router E have a route to the other routing domain through Router F. In software releases earlier than 07.1.X, if Routers D and E have equal-cost routes to Router F, then both Router D and Router E flood AS External LSAs to Routers A, B, and C advertising the route to Router F. Since both routers are flooding equivalent routes, Routers A, B, and C receive multiple routes with the same cost to the same destination (Router F). For Routers A, B, and C, either route to Router F (through Router D or through Router E) is equally good.
OSPF eliminates the duplicate AS External LSAs. When two or more HP routing switches configured as ASBRs have equal-cost routes to the same next-hop router in an external routing domain, the ASBR with the highest router ID floods the AS External LSAs for the external domain into the OSPF AS, while the other ASBRs flush the equivalent AS External LSAs from their databases. As a result, the overall volume of route advertisement traffic within the AS is reduced and the routing switches that flush the duplicate AS External LSAs have more memory for other OSPF data. In Figure 8.4, since Router D has a higher router ID than Router E, Router D floods the AS External LSAs for Router F to Routers A, B, and C. Router E flushes the equivalent AS External LSAs from its database.
