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IPv6 Training (Cisco): Using the “area virtual-link” Command 

Added: 01/30/2009, Hits: 3,207, Rating: 0, Comments: 0, Votes: 0
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Now, remembering from your CCNA or early networking days; you learned that in an OSPF network, all additional OSPF areas must be connected to the “backbone” area or (Area 0).

But, what if one of those areas lost its connection to the “backbone” area (Area 0); how could you quickly repair the lost connection?

The “area virtual-link" command is a Cisco IOS command that can be used in either “Router Address Family Topology” or “Router” configuration mode; network administrators (like you) use the command to create a “virtual link” that can be used to repair a lost connection to the “backbone” area (Area 0).

Here’s an example of a Cisco router in “Router Address Family Topology” configuration mode. (Use this mode if you plan on configuring the Multi-Topology Routing (MTR) feature)

Example: Router(config-router-af-topology)#

And, here’s an example of a Cisco router in “Router” configuration mode.

Example: Router(config-router)#

Below is the “area virtual-link" command’s proper syntax:

area transit-area-id virtual-link transit-router-id [hello-interval seconds] [retransmit-interval seconds] [transmit-delay seconds] [dead-interval seconds]

Notice, that the command requires a transit “router ID” rather than an IPv6 prefix; the “router ID” is the “router ID” of the remote router.

Here are the default intervals when using the “area virtual-link" command:

hello-interval seconds: 10 seconds
retransmit-interval seconds: 5 seconds
transmit-delay seconds: 1 second
dead-interval seconds: 40 seconds

Having a smaller “hello interval” value, means faster topological changes will be detected, but, you’ll have more routing traffic.

Now if you need to reconfigure the “retransmit-interval” value be conservative; if not unwanted retransmissions will occur. Also, remember its recommend to increase the value of this interval when using serial lines and virtual links.

And when thinking about adjusting the “transmit-delay” value, make sure you take into consideration the transmission and propagation delays of the interface.

Now, once the virtual link has been configured on a router, you can use the word “no” in front of the “area virtual-link" command to remove the link or you can use the command “no area area-id” which will remove all area options.

The “area virtual-link” command Keywords and Arguments Explained:

area-id-- This argument is an identifier of the area assigned to the transit area for the virtual link. This can be either a decimal value or a valid IPv6 prefix. There is no default.

router-id-- This argument is the “router ID” associated with the virtual link neighbor. The router ID appears in the show ipv6 ospf display. There is no default.

hello-interval seconds— This optional keyword represents time (in seconds) between the hello packets that the Cisco IOS software sends on an interface. The hello interval is an unsigned integer value to be advertised in the hello packets. The value must be the same for all routers and access servers attached to a common network. The default is 10 seconds.

retransmit-interval seconds-- This optional keyword represents time (in seconds) between link-state advertisement (LSA) retransmissions for adjacencies belonging to the interface. The retransmit interval is the expected round-trip delay between any two routers on the attached network. The value must be greater than the expected round-trip delay. The default is 5 seconds.

transmit-delay seconds— This optional keyword represents the estimated time (in seconds) required to send a link-state update packet on the interface. The integer value that must be greater than zero. LSAs in the update packet have their age incremented by this amount before transmission. The default value is 1 second.

dead-interval seconds-- This optional keyword represents time (in seconds) that hello packets are not seen before a neighbor declares the router down. The dead interval is an unsigned integer value. The default is four times the hello interval, or 40 seconds. As with the hello interval, this value must be the same for all routers and access servers attached to a common network.

I hope this article was very informative and helped you quickly understand the usage, keywords, and arguments of the “area virtual-link” command. If you need to learn more about the command; I suggest you visit my website, were you’ll find the latest information regarding Cisco IPv6 Design and Implementation Techniques.

To your success,

Charles Ross
CCNP #CSCO10444244

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