Dell PowerConnect FCS624S Configuration Guide - Page 969

FIGURE 141 AS External LSA reduction OSPF Autonomous System (AS) Overview of OSPF 29 Routers D, E, and F are OSPF ASBRs and EBGP routers. Another routing domain (such as BGP4 or RIP) Router A Router D Router ID: 2.2.2.2 Router B Router C Router E Router ID: 1.1.1.1 Router F Notice that both Router D and Router E have a route to the other routing domain through Router F. In earlier software releases, 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 Layer 3 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 Layer 3 Switches PowerConnect B-Series FCX Configuration Guide 927 53-1002266-01