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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter38 Configuring IP Unicast Routing
Configuring OSPF
•Default Metrics: OSPF calculates the OSPF metric for an interface according to the bandwidth of
the interface. The metric is calculated as ref-bw divided by bandwidth, where ref is 10 by default,
and bandwidth (bw) is specified by the bandwidth interface configuration command. For multiple
links with high bandwidth, you can specify a larger number to differentiate the cost on those links.
•Administrative distance is a rating of the trustworthiness of a routing information source, an integer
between 0 and 255, with a higher value meaning a lower trust rating. An administrative distance of
255 means the routing information source cannot be trusted at all and should be ignored. OSPF uses
three different administrative distances: routes within an area (interarea), routes to another area
(interarea), and routes from another routing domain learned through redistribution (external). You
can change any of the distance values.
•Passive interfaces: Because interfaces between two devices on an Ethernet represent only one
network segment, to prevent OSPF from sending hello packets for the sending interface, you must
configure the sending device to be a passive interface. Both devices can identify each other through
the hello packet for the receiving interface.
•Route calculation timers: You can configure the delay time between when OSPF receives a topology
change and when it starts the shortest path first (SPF) calculation and the hold time between two
SPF calculations.
•Log neighbor changes: You can configure the router to send a syslog message when an OSPF
neighbor state changes, providing a high-level view of changes in the router.
Beginning in privileged EXEC mode, follow these steps to configure these OSPF parameters:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 router ospf process-id Enable OSPF routing, and enter router configuration mode.
Step3 summary-address address mask (Optional) Specify an address and IP subnet mask for redistributed
routes so that only one summary route is advertised.
Step4 area area-id virtual-link router-id
[hello-interval seconds]
[retransmit-interval seconds] [trans]
[[authentication-key key] |
message-digest-key keyid md5 key]]
(Optional) Establish a virtual link and set its parameters. See the
“Configuring OSPF Interfaces” section on page38-30 for parameter
definitions and Table38-5 on page 38-26 for virtual link defaults.
Step5 default-information originate [always]
[metric metric-value] [metric-type
type-value] [route-map map-name]
(Optional) Force the ASBR to generate a default route into the OSPF
routing domain. Parameters are all optional.
Step6 ip ospf name-lookup (Optional) Configure DNS name lookup. The default is disabled.
Step7 ip auto-cost reference-bandwidth ref-bw (Optional) Specify an address range for which a single route will be
advertised. Use this command only with area border routers.
Step8 distance ospf {[inter-area dist1] [inter-area
dist2] [external dist3]}
(Optional) Change the OSPF distance values. The default distance
for each type of route is 110. The range is 1 to 255.
Step9 passive-interface type number (Optional) Suppress the sending of hello packets through the
specified interface.