Multicast and Routing Guide K/KA/KB.15.15

Adjusting virtual link performance by changing the interface settings
Optional: The OSPF interface parameters for this process are automatically set to their default
values for virtual links. No change to the defaults is usually required unless needed for specific
network conditions. These parameters are a subset of the parameters described under “Adjusting
performance by changing the VLAN or subnet interface settings” (page 155). (The cost and
priority settings are not configurable for a virtual link, and the commands for reconfiguring the
settings are accessed in the router OSPF context instead of the VLAN context.)
NOTE: The parameter settings for virtual links must be the same on the ABRs at both ends of a
given link.
Configuring OSPF authentication on a virtual link
OSPF supports the same two methods of authentication for virtual links as it does for VLANs and
subnets in an area—password and MD5. In the default configuration, OSPF authentication is
disabled. Only one method of authentication can be active on a virtual link at a time, and if one
method is configured on a virtual link, configuring the alternative method on the same link
automatically replaces the first method with the second. Both ends of a virtual link must use the
same authentication method (none, password, or MD5 key chain) and related credentials.(Any
interfaces that share a VLAN or subnet with the interface used on an ABR for a virtual link, including
intermediate routing switches, must be configured with the same OSPF authentication.)
About OSPF passive
OSPF sends LSAs to all other routers in the same AS. To limit the flooding of LSAs throughout the
AS, you can configure OSPF to be passive. OSPF does not run in the AS, but it does advertise the
interface as a stub link into OSPF. Routing updates are accepted by a passive interface, but not
sent out.
There is a limit of 512 total active and passive interfaces, but only a total of 128 can be active
interfaces.
About configuring shortest path first (SPF) scheduling
SPF scheduling (throttling) can be configured in intervals of seconds to potentially delay SPF
calculations when the network is unstable or there is a change in topology. It provides a granularity
of one to four seconds between SPF calculations as opposed to the current default of five seconds.
The interval for the SPF calculations is dynamically chosen, based on the frequency of topology
changes in the network. The chosen interval is within user-specified ranges of values. When the
network topology is unstable, SPF throttling calculates SPF scheduling intervals that are longer,
until the topology is again stable.
NOTE: It is guaranteed that no SPF will be calculated within the SPF currently in effect, however,
it is not guaranteed that the SPF will be calculated at the exact expiration of the timer if there have
been updates. The timer may be delayed due to system constraints.
Graceful shutdown of OSPF routing
OSPF routing can be gracefully shut down on HP switches without losing packets that are in transit.
OSPF neighbors are informed that the router should not be used for forwarding traffic, which allows
for maintenance on the switch without interrupting traffic in the network. There is no effect on the
saved switch configuration
Prior to a switch shutdown, the CLI/SNMP reload command or the CLI boot command is executed
to initiate the sending of OSPF "empty hello list" messages on the interfaces that are part of the
OSPF routing configuration. After a small delay (approximately 2 seconds) that allows the messages
to be transmitted on all applicable interfaces, the boot or reload command continues.
Adjusting virtual link performance by changing the interface settings 203