Extreme Networks ExtremeWare XOS 10.1, Manual

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Overview of OSPF
ExtremeWare XOS 10.1 Concepts Guide 189
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.
Opaque LSAs
Opaque LSAs are a generic OSPF mechanism used to carry auxiliary information in the OSPF database.
Opaque LSAs are most commonly used to support OSPF traffic engineering.
Normally, support for opaque LSAs is auto-negotiated between OSPF neighbors. In the event that you
experience interoperability problems, you can disable opaque LSAs across the entire system using the
following command:
disable ospf capability opaque-lsa
To re-enable opaque LSAs across the entire system, use the following command:
enable ospf capability opaque-lsa
If your network uses opaque LSAs, we recommend that all routers on your OSPF network support
opaque LSAs. Routers that do not support opaque LSAs do not store or flood them. At minimum a
well-interconnected subsection of your OSPF network needs to support opaque LSAs to maintain
reliability of their transmission.
On an OSPF broadcast network, the designated router (DR) must support opaque LSAs or none of the
other routers on that broadcast network will reliably receive them. You can use the OSPF priority
feature to give preference to an opaque-capable router, so that it becomes the elected DR.
For transmission to continue reliably across the network, the backup designated router (BDR) must also
support opaque LSAs.
Areas
OSPF allows parts of a network to be grouped together into areas. The topology within an area is
hidden from the rest of the autonomous system. Hiding this information enables a significant reduction