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IPv6 Training (Cisco): Using the auto-cost (IPv6) Command 

Added: 03/14/2009, Hits: 4,440, Rating: 0, Comments: 0, Votes: 0
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This article is for the present and future OSPF (IPv6) network administrators.

Now remembering from your CCNA days; by default, the OSPF for IPv6 protocol uses the default Mbps (megabit per second) reference value of 100 when calculating metrics (costs) for interfaces (links). But, let’s say you didn’t want to use the default Mbps reference value; how could you change the reference value of 100 Mbps to a different value?

The Cisco IOS “Router” configuration command named “auto-cost” is used to control (change) the Mbps reference value used (by the OSPF for IPv6 protocol) when calculating the metrics (costs) for interfaces (links). Below is the syntax and example of using the command.


auto-cost reference-bandwidth Mbps

The keyword “reference-bandwidth Mbps” represents the rate in Mbps (megabit per second). The range is from 1 to 4294967; and the default is 100.


router#configure terminal
router(config)#ipv6 router ospf 1
router(config-router)#auto-cost reference-bandwidth 1000

In the above example, the auto-cost reference bandwidth is being set to 1000 Mbps (megabit per second).

Now, if after setting the Mbps reference value to a particular value; you want to return the reference value back to the default value (100 Mbps); all you need to do is type the word “no” in front of the command like so.

router(config-router)#no auto-cost reference-bandwidth 1000

OSPF for IPv6 uses the following calculation (formula) to determine the metric (cost) for an interface (link):

Mbps reference value divided by the bandwidth

Note: The “bandwidth” is determined by the use of the bandwidth (interface) command.

Below are the default metrics (costs) for certain types of interfaces (links):

  • 56-kbps serial link—Default cost is 1785.

  • 64-kbps serial link—Default cost is 1562.

  • T1 (1.544-Mbps serial link)—Default cost is 64.

  • E1 (2.048-Mbps serial link)—Default cost is 48.

  • 4-Mbps Token Ring—Default cost is 25.

  • Ethernet—Default cost is 10.

  • 16-Mbps Token Ring—Default cost is 6.

  • Fast Ethernet—Default cost is 1.

  • X25—Default cost is 5208.

  • Asynchronous—Default cost is 10,000.

  • ATM—Default cost is 1.

  • Lastly, if you have multiple links with high bandwidth (such as Fast Ethernet or ATM), you might want to use a larger number to differentiate the cost on those links.

    I hope this article was very informative and helped you quickly understand the usage of the “auto-cost” (IPv6) 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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