Configuring Ospf; Understanding Graceful Restart - Enterasys Matrix DFE-Gold Series Configuration Manual

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Configuring OSPF

OSPF is an advanced routing feature that must be enabled with a license key. If you have purchased an
advanced license key, and have enabled routing on the device, you must activate your license as described
back in "Activating Licensed
purchase an advanced routing license, contact Enterasys Networks Sales.

Understanding Graceful Restart

OSPF graceful restart, sometimes referred to as non‐stop forwarding, provides for an OSPF router
to remain on the forwarding path during a restart of its OSPF software. Graceful‐restart has four
elements to its configuration: enabling, helper router, restart interval, and strict LSA checking.
Enabling graceful restart instructs the firmware to perform a graceful restart, rather than a
standard OSPF restart. Restart is only initiated by a fail‐over. Grace LSAs are sent when OSPF is
restarted on another module. Whether the failover is intentional or not, the failed router protocol
is restarted on another module, and upon startup, OSPF sends grace LSAs out to its neighbors
using existing link aggregation groups. Use the graceful‐restart enable command to enable the
graceful restart ability on this router.
The helper relationship with the restarting router is on a per network segment basis. The helper
monitors the network for topology changes. If no changes occur, the helper router continues to
advertise its LSAs as though no restart was occurring. If the restarting router was the designated
router, the helper continues to treat it as such. If a topology change does occur, graceful restart is
terminated on the restarting router and a standard restart occurs. Helper mode can be disabled on
a restarting router neighbor using the graceful‐restart helper‐disabled command. If the restarting
router receives an LSA indicating a disabled helper, the graceful restart terminates and a standard
restart occurs.
A restart interval provides for a maximum time in seconds after which the graceful restart will
terminate should it not complete or terminate for other reasons within the interval. Use the
graceful‐restart restart‐interval command to change the restart interval setting.
Strict LSA checking assures that graceful restart will terminate if there is a change to an LSA that
would be flooded to the restarting router. Strict LSA checking can be disabled. If disabled, a
change to an LSA that would be flooded to the restarting router will not cause the graceful restart
to terminate. Use the graceful‐restart strict‐lsa‐checking‐disabled command to disable strict LSA
checking.
View the router OSPF section of the show running‐config display to verify any non‐default
graceful restart settings.
Graceful Restart and High Availability
The DFE supports single router high availability failover using the following components:
• OSPF graceful restart
• Non‐stop router frame forwarding on each module
• Single router configuration
• Router protocol process failover to another module
• Link Aggregate Group (LAG) connectivity to neighboring routers
The DFE is a distributed routing system. The routing protocol process resides on a single router
module. Information such as access list rules, policy routing rules, interface configuration and best
routes are calculated by the protocol process and distributed to all modules. Each module has its
Important Notice
Features" on page 2-58 in order to enable the OSPF command set. If you wish to
Enterasys Matrix DFE-Gold Series Configuration Guide 21-19
Configuring OSPF