How to configure Routing IS-IS optional metric?

[aldindha17]

Hi,

I just want to know is there any way to enable IS-IS optional metric like delay, expense, and error? I know that Cisco only implemented default metric (cost) as stated in the document, but I just wondering is there any way to enable its optional metric on Cisco router? Btw I'm using Cisco router 7200 and IOS 12.4 (22)T.

Thank you.

[Peter Paluch · Accepted solution]

Hi,

There is no way of enabling the optional IS-IS metrics simply because the entire IOS ignores their value and advertises them as unsupported. Even if an IOS IS-IS implementation received an LSP that contained non-zero values in the delay/expense/error metrics, it would not use them during its SPF computation. To my best knowledge, no major IS-IS implementation takes the optional metrics into account. In today's networks, the need for such metrics has been largely obsoleted by Multi Topology Routing that basically achieves the same effect with different means.

Best regards,
Peter

[aldindha17]

Hi Peter thanks for the answer. So it can't enabling IS-IS optional metric. So, basically cisco can only implemented default metric (cost value) with default cost is 10 per link right? So If I want to change the default metric, is there any consideration to do this? Do you think I need some consideration to change the default cost to the right cost based on today's networks? Because you know if I use default cost it would be same with RIP hop count and no special about it.

*Sorry for my bad english anyway

Thank you

[Peter Paluch]

Hi,

Don't worry about your English - it is good. I am not a native English speaker, either.

You are correct - Cisco's IS-IS has no internal support for optional metrics. The only metric value that is going to be used in best path selection is the default metric. Regarding considerations about metrics in IS-IS, the only consideration I find important is that all new IS-IS deployments should use wide metrics. These can be activated using the metric-style wide in the router isis configuration. Wide metrics allow you to use a significantly wider metric range than the original IS-IS standard: 24 bits for interface metric, and 32 bits for total path metric. It is important to say that all L1 routers within an area, or all L2 routers in the domain must use the same metric type, either the classic (also called narrow) or the new wide metrics.

Apart from that, there are no special considerations I am aware of. The choice of metric values for a particular interface is completely up to you. Of course you might want to configure lower metrics for faster interfaces (and vice versa), but what values you choose is up to you.

Best regards,
Peter

[aldindha17]

About wide metric I know about it too. But is there any difference between wide style metric with the old style metric besides its range? Maybe wide style metric use bandwidth reference or something?

Thank you

[Peter Paluch]

Hi,

No, wide metrics won't cause your router to assign different metrics to interfaces based on their different bandwidths. This is a fairly plain fact: in IOS, interface bandwidth is never automatically reflected in its IS-IS metric. All interfaces have the same default IS-IS metric of 10.

Some automation - perhaps TCL scripts or similar - could be used to go over the interface bandwidth values and calculate a scaled IS-IS metric value. However, the IOS itself does not do this.

Best regards,
Peter

[aldindha17]

Thanks for the answer. It helped me a lot.

Regards,

Aldindha