Extreme Networks ExtremeWare XOS 11.3, Manual

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Overview of OSPF
ExtremeWare XOS 11.3 Concepts Guide
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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 autonegotiated between OSPF neighbors. In the event that you
experience interoperability problems, you can disable opaque LSAs across the entire system using the
following command:
enable 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, Extreme Networks recommends 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 must support opaque LSAs to
maintain reliability of their transmission.
Graceful OSPF Restart
RFC 3623 describes a way for OSPF control functions to restart without disrupting traffic forwarding.
Without graceful restart, adjacent routers will assume that information previously received from the
restarting router is stale and won’t be used to forward traffic to that router. However, in many cases,
two conditions exist that allow the router restarting OSPF to continue to forward traffic correctly. The
first condition is that forwarding can continue while the control function is restarted. Most modern
router system designs separate the forwarding function from the control function so that traffic can still
be forwarded independent of the state of the OSPF function. Routes learned through OSPF remain in
the routing table and packets continue to be forwarded. The second condition required for graceful
restart is that the network remain stable during the restart period. If the network topology is not
changing, the current routing table remains correct. Often, networks can remain stable during the time
for restarting OSPF.
Restarting and Helper Mode
Routers involved with graceful restart fill one of two roles: the restarting router or the helper router.
With graceful restart, the router that is restarting sends out Grace-LSAs informing its neighbors that it is
in graceful restart mode, how long the helper router should assist with the restart (the grace period),
and why the restart occurred. If the neighboring routers are configured to help with the graceful restart
(helper-mode), they will continue to advertise the restarting router as if it was fully adjacent. Traffic
continues to be routed as though the restarting router is fully functional. If the network topology
changes, the helper routers will stop advertising the restarting router. The helper router will continue in
helper mode until the restarting router indicates successful termination of graceful restart, the
Grace-LSAs expire, or the network topology changes. A router can be configured for graceful restart,
and for helper-mode separately. A router can be a helper when its neighbor restarts, and can in turn be
helped by a neighbor if it restarts.