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BSCI – IS-IS review

Buried under dozens of definitions, values and parameters I try to wrap up my studies for the exam. I have to admit that it is as hard as they say and I wonder why am I doing this horrible thing to myself 🙂

IS-IS is one of the topics that I could avoid by taking the ROUTE exam but it is a fascinating protocol that take link-state to another level and open your mind to other ideas. You can think of it as trying a new browser – you know it is working like your Firefox (which 54% of you use) but it has a different feeling, some other features or just a different look. You’ll keep using your favorite fox but it is nice to know what other options are available out there.

Other than my study notes and my study guide, I found Aragoen Celtdra’s BSCI: IS-IS Configuration and even more his BSCI: IS-IS Concepts I BSCI: IS-IS Concepts II posts great reading.

Here are few important points:
IS-IS has 3 types of routers:

  • L1 – intra-area, uses LSP
  • L2 – inter-area, the backbone of IS-IS network
  • L1/L2 – connectors between L1 and L2 areas

IS-IS has 4 OSI routing levels:

  • level 0 routing – ES-IS protocol, same subnet
  • level 1 routing – ISs in the same area
  • level2 routing – L1 or L2 routers find L1/L2 router
  • level 3 routing – L1/L2 pass traffic to other AS

IS-IS must be configured both at the router level and interface configuration mode.

The comparison between IS-IS to OSPF is a major topic as we try to understand which one is better for our network.
Here are some of IS-IS benefits over OSPF:

  • supports more routers in an area
  • produces fewer link state advertisements for a given network
  • supports network layer protocols other than IP
  • forms adjacencies with all neighbors

There are some problems with IS-IS and CLNS:

  • CLNS adjacency can have IP addresses on different subnets at different end of the connection. That would make troubleshooting much harder
  • IS-IS neighbor relationships are established over CLNS and again, the fact it is not using IP make it more complicated

One way to bypass this problem is using only IP routing. In order to make it happen each IS-IS router require NET address and all network devices must use CLNS.

Read about CLNS, NSEL, NET address and DIS, they all show on the exam and Aragoen cover them well on concepts part II.

You can find another great IS-IS document here and a Cisco presentation on IS-IS. Detailed command guide from Cisco is available here.

  1. March 11, 2010 at 11:25 am

    Another resource I’ll use when I get to that test, Rofi. Thanks!

    • March 11, 2010 at 2:38 pm

      how is your ONT going?
      I hope you’re not waiting for me 😉

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