Configuration Guide User guide

BigIron RX Series Configuration Guide 757

53-1002484-04

Overview of OSPF (Open Shortest Path First)

FIGURE 114 AS external LSA reduction

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

OSPF eliminates the duplicate AS External LSAs. When two or more BigIron RX switches are

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 BigIron RX switches that flush the duplicate AS External LSAs have more memory for other

OSPF data. In Figure 114, 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.

Algorithm for AS external LSA reduction

Figure 114 shows an example in which the normal AS External LSA reduction feature is in effect.

The behavior changes under the following conditions:

• There is one ASBR advertising (originating) a route to the external destination, but one of the

following happens:

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