IS-IS Design, area address assignation and summarization

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Mar 21st, 2010
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Hi !

I'm currently studdying IS-IS for BSCI certification exam.  The author of my documentation told is it important to plan IP and area assignement to permit summarization.  I know how I can summazise IP networks. But I don't know how to doing for NSAP address and area address.  I had to find some documentation about OSI protocol and IS-IS protocol but it's looklike hard to find.  Someone can help me ?

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Giuseppe Larosa Mon, 03/22/2010 - 01:18
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Hello Christian,

see this thread I had attached a chapter about ISIS that had been published by Cisco press

Generally speaking:

an OSPF router can be a member of multiple areas

an ISIS router is a member of only one area (up to 3 nets can be configured but only for migration purposes for short time)

if two routers share the same Area-id they can build an ISIS Level 1 adajacency and eventually a level 2 adjacency.

System-ids have to unique in the ISIS domain.

if Area-id is different only an ISIS level-2 adjacency can be built (if both devices can work at level 2).

The ISIS backbone is made of the collection of contiguous Level 2 links.

Cisco implementation does not support a recovey feature for partitioned backbone that was in ISO standards.

This calls for appropriate redundancy of backbone links.

IP networks are seen as leafs for ISIS the need for appropriate IP address plan is in order to allow the use of summarizations.

A typical design uses an area-id per site allowing L2L1 routers to perform appropriate summarization on links towards core.

Routers internal to a site can be configured as L1 only ( Cisco default is L1L2 so this need an explicit command)

Hope to help



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