Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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Overview of OSPF
ExtremeWare 7.2e Installation and User Guide 235
Link-State Database
Upon initialization, each router transmits a link-state advertisement (LSA) on each of its interfaces.
LSAs are collected by each router and entered into the LSDB of each router. Once all LSAs are received,
the router uses the LSDB to calculate the best routes for use in the IP routing table. OSPF uses flooding
to distribute LSAs between routers. Any change in routing information is sent to all of the routers in the
network. All routers within an area have the exact same LSDB. Table 41 describes LSA type numbers.
OSPF passive adds the interface to the Type 1 LSA, but it does not send hellos or establish adjacencies
on that interface.
Database Overflow
The OSPF database overflow feature allows you to limit the size of the LSDB and to maintain a
consistent LSDB across all the routers in the domain, which ensures that all routers have a consistent
view of the network.
Consistency is achieved by:
• Limiting the number of external LSAs in the database of each router.
• Ensuring that all routers have identical LSAs.
To configure OSPF database overflow, use the following command:
configure ospf ase-limit <number> {timeout <seconds>}
where:
•
<number>
—Specifies the number of external LSAs that the system supports before it goes into
overflow state. A limit value of zero disables the functionality.
When the LSDB size limit is reached, OSPF database overflow flushes LSAs from the LSDB. OSPF
database overflow flushes the same LSAs from all the routers, which maintains consistency.
•
timeout
—Specifies the timeout, in seconds, after which the system ceases to be in overflow state. A
timeout value of zero leaves the system in overflow state until OSPF is disabled and re-enabled.
Table 41: LSA Type Numbers
Type Number Description
1 Router LSA
2 Network LSA
3 Summary LSA
4 AS summary LSA
5 AS external LSA
7 NSSA external LSA
9 Link local—Opaque
10 Area scoping—Opaque
11 AS scoping—Opaque