Quick Reference Guide

662 PowerConnect B-Series TI24X Configuration Guide

53-1002269-02

Overview of OSPF

Figure 108 shows an example of the AS External LSA reduction feature. In this example, Layer 3

Switches D and E are OSPF ASBRs, and thus communicate route information between the OSPF

AS, which contains Routers A, B, and C, and another routing domain, which contains Router F. The

other routing domain is running another routing protocol, such as BGP4 or RIP. Routers D, E, and F,

therefore, are each running both OSPF and either BGP4 or RIP.

FIGURE 108 AS External LSA reduction

Notice that both Router D and Router E have a route to the other routing domain through Router F.

In earlier, 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

Router B

Router A

Router C

Router D

Router ID: 2.2.2.2

OSPF A

utonomous System (AS)

Router E

Router ID: 1.1.1.1

Routers D, E, and F

are OSPF ASBRs

and EBGP routers.

Another routing domain

(such as BGP4 or RIP)

Router F