Before I explain this week’s article, I would like to thank all of you readers whom read my previous articles; you are truly my inspiration.
O.K. let’s get down to business, in order for you to use the Cisco IOS “Router configuration” command named “variance” in an IPv6 network environment, your Cisco router(s) needs to be running Cisco IOS 12.4(6)T or higher.
Below is the proper syntax and example of using the command:
Syntax: variance multiplier value
The multiplier value – Is the metric value that is used for load balancing. It can be a value from 1 to 128. The default is 1, which means equal-cost load balancing.
Example:
router>enable
router#configure terminal
router(config)#ipv6 unicast-routing
router(config)#interface fastethernet 0/0
router(config-if)#ipv6 enable
router(config-if)#ipv6 eigrp 100
router(config-if)#no shutdown
router(config-if)#ipv6 router eigrp 100
router(config-router)#variance 4
router(config-router)#router-id {ip-address | ipv6-address}
router(config-router)#no shutdown
router(config-router)#end
router#copy run start
In the example above, the router’s variance is being set to the value of four; notice that the router’s prompt is in “Router configuration” mode when the “variance” (eigrp) command is being used. If you need to “reset”the variance of a router back to the default value of 1, use the word “no” in front of the command like you see in the example below:
Example:
router>enable
router#configure terminal
router(config)#ipv6 unicast-routing
router(config)#interface fastethernet 0/0
router(config-if)#ipv6 enable
router(config-if)#ipv6 eigrp 100
router(config-if)#no shutdown
router(config-if)#ipv6 router eigrp 100
router(config-router)#no variance
router(config-router)#router-id {ip-address | ipv6-address}
router(config-router)#no shutdown
router(config-router)#end
router#copy run start
Now for those who are not familiar with the “variance” (eigrp) command here’s a quick synopsis:
The “variance” (eigrp) command is used to control load balancing (sharing) in an EIGRP internetwork.
Meaning when a network administrator (like you) sets a “variance” value on a Cisco router, the Enhanced Interior Gateway Routing Protocol (EIGRP) uses that value to install multiple loop-free routes with unequal-cost into a local routing table. Then, the local routing table enables (activates) the switching hardware or software to load-share (balance) between the multiple paths.
EIGRP has two conditions that a route(s) must meet to be installed into the local routing table:
1. The route must be loop- free. A route is considered to be loop-free when the reported distance is less than the total distance or when the route is a feasible successor.
2. And, the metric of the route must be lower than the metric of the best route (the successor) multiplied by the variance configured on the router.
In other words, if the variance of a router is set to 1, then only the routes with the same metric as the successor (the best route) are installed into the local routing table. And, if the variance of a router is set to 2, that means any EIGRP-learned route with a metric less than 2 times the successor metric will be installed into the local routing table.
I hope this article was very informative and helped you quickly understand the usage of the “variance” (EIGRP) Router configuration command. If you need to learn more; I suggest you visit my website, were you’ll find the latest information regarding Cisco IPv6 Design and Implementation Techniques.
To your success.
