Reference Guide

Configuring Virtual Links
Areas within OSPF must be connected to the backbone area (Area ID 0.0.0.0).
If an OSPF area does not have a direct connection to the backbone, at least one virtual link is required. Configure virtual
links on an ABR connected to the backbone.
• hello-interval — help packet
• retransmit-interval — LSA retransmit interval
• transmit-delay — LSA transmission delay
• dead-interval — dead router detection time
• authentication-key — authentication key
• message-digest-key — MD5 authentication key
To configure virtual links, use the following command.
• Configure the optional parameters of a virtual link.
CONFIG-ROUTEROSPF- id mode
area area-id virtual-link router-id [hello-interval seconds | retransmit-
interval seconds | transmit-delay seconds | dead-interval seconds |
authentication-key key | message-digest-key keyid md5 key]
– area ID: assigned earlier (the range is from 0 to 65535 or A.B.C.D).
– router ID: IP address associated with the virtual link neighbor.
– hello interval seconds: the range is from 1 to 8192 (the default is 10).
– retransmit interval seconds: the range is from 1 to 3600 (the default is 5).
– transmit delay seconds: the range is from 1 to 3600 (the default is 1).
– dead interval seconds: the range is from 1 to 8192 (the default is 40).
– authentication key: eight characters.
– message digest key keyid: the range is from 1 to 255.
– md5 key: 16 characters.
If you do not enter other parameters, the defaults are used.
Only the area ID and router ID require configuration to create a virtual link.
Use EITHER the Authentication Key or the Message Digest (MD5) key.
Use the show ip ospf process-id virtual-links command to view the virtual link.
Example of Viewing Virtual Links
FTOS#show ip ospf 1 virtual-links
Virtual Link to router 192.168.253.5 is up
Run as demand circuit
Transit area 0.0.0.1, via interface GigabitEthernet 13/16, Cost of using 2
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:02
FTOS#
Creating Filter Routes
To filter routes, use prefix lists. OSPF applies prefix lists to incoming or outgoing routes.
Incoming routes must meet the conditions of the prefix lists. If they do not, OSPF does not add the route to the routing
table. Configure the prefix list in CONFIGURATION PREFIX LIST mode prior to assigning it to the OSPF process.
494