ISIS External Routes

[kkalaycioglu]

I've got two questions regarding ISIS (dual ISIS):

1. When I redistribute into ISIS as L2 external, how is metric calculated?

2. COmparing to OSPF, what are advantages and disadvantages of ISIS in the real world? And therefore where can it be preferred over OSPF (or maybe EIGRP)?

Thanks in advance and Best Regards.

[ruwhite]

The metric isn't calculated, it's simply implied. You can change it using the default metric command, if you want to.

The second question is a big one. It's all going to depend on what you're trying to do, etc. Come to Networkers, I'm doing a two hour session that touches the tip of the iceberg on that topic, specifically.

:-)

Russ.W

[n-saquib]

If you talk about metric than answer is already given but there is other point during redistribution and that is administrative distance in case of ISIS it always stay same if it is external or internal route. Whereas other routing protocols OSPF/EIGRP have high AD for external routes.

You have to be careful during redistribution in ISIS as it can create some routing loops due same AD of external routes as of internal routes.

[ruwhite]

Take a look here:

http://www.cisco.com/en/US/tech/tk364/tk871/technologies_tech_note09186a0080094195.shtml

for information on the administrative distance. OSPF only uses one admin distance, just as IS-IS does. IS-IS does have different types of metrics for "externals" and "internals," such that "internals" are always preffered, just like OSPF, as well, but the metric given to a route when it's redistributed into IS-IS as an external is determined by the default metric command, or the default metric, or the metric configured on the redistribution configuration.

:-)

Russ.W