10G Core Routing Switch User Manual
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Configuring OSPF Virtual Link
According to RFC2328, after the area division of OSPF, not all the areas are equal. In which, an area is different from all
the other areas. Its area-id is 0.0.0.0 and it is usually called the backbone Area. The OSPF routes between
non-backbone areas are updated with the help of the backbone area. OSPF stipulates that all the non-backbone areas
should maintain the connectivity with the backbone area. That is, at least one interface on the ABR should fall into the
area 0.0.0.0. If an area does not have a direct physical link with the backbone area 0.0.0.0, a virtual link must be
created.
If the physical connectivity cannot be ensured due to the network topology restriction, a virtual link can satisfy this
requirement. The virtual link refers to a logic channel set up through the area of a non-backbone internal route
between two ABRs. Both ends of the logic channel should be ABRs and the connection can take effect only when both
ends are configured. The virtual link is identified by the ID of the remote router. The area, which provides the ends of
the virtual link with a non-backbone area internal route, is called the transit area. The ID of the transit area should be
specified when making configuration.
The virtual link is activated after the route passing through the transit area is calculated,which is equivalent to a p2p
connection between two ends. Therefore, similar to the physical interfaces, you can also configure various interface
parameters on this link, such as hello timer.
The "logic channel" means that the multiple routers running OSPF between two ABRs only take the role of packet
forwarding (the destination addresses of the protocol packets are not these routers, so these packets are transparent
for them and the routers forward them as common IP packets). The routing information is directly transmitted
between the two ABRs. The routing information herein refers to the type-3 LSAs generated by the ABRs, for which the
synchronization mode of the routers in the area will not be changed.
Beginning in OSPF configuration mode, follow these steps to configure OSPF Virtual Link.
Command
Purpose
Step 1
area
area-id
virtual-link
router-id
[
authentication
|
CR
|
hello-interval
seconds
|
retransmit-interval
seconds
|
transmit-delay
seconds
|
dead-interval
seconds
|
Create and configure a virtual link
authentication-key
auth_key
|
message-digest-key
key_id
md5
key
]
Step 2
exit
Return to global configuration mode.
Step 3
exit
Return to privileged EXEC mode.
Step 4
show ip ospf
Verify your entries.
Step 5
Copy
running-config
startup-config
(Optional) Save your entries in the configuration file.
To remove the created virtual link, use
no area
area-id
virtual-link
router-id
[
CR | hello-interval
seconds
|
retransmit-interval
seconds
|
transmit-delay
seconds
|
dead-interval
seconds
|
authentication-key
auth_key
|
message-digest-key
key_id
md5
key
] OSPF configuration command.
area-id and router-id have no default value. By default, hello timer is 10 seconds, retransmit 5 seconds, trans-delay 1
second, and the dead 40 seconds.
