Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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236 ExtremeWare 7.2e Installation and User Guide
Interior Gateway Protocols
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.
NOTE
Opaque LSAs are supported in ExtremeWare version 6.2 and above.
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
in LSA traffic, and reduces the computations needed to maintain the LSDB. Routing within the area is
determined only by the topology of the area.
The three types of routers defined by OSPF are as follows:
• Internal Router (IR) —An internal router has all of its interfaces within the same area.
• Area Border Router (ABR)
—An ABR has interfaces in multiple areas. It is responsible for
exchanging summary advertisements with other ABRs. You can create a maximum of 7 non-zero
areas.
• Autonomous System Border Router (ASBR) —An ASBR acts as a gateway between OSPF and other
routing protocols, or other autonomous systems.
Backbone Area (Area 0.0.0.0)
Any OSPF network that contains more than one area is required to have an area configured as area
0.0.0.0, also called the backbone. All areas in an autonomous system must be connected to the backbone.
When designing networks, you should start with area 0.0.0.0, and then expand into other areas.