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IS-IS Router Levels affecting routing tables?

I need to find out how changing Router's Levels on IS-IS network affects routing tables. I am having troubles finding any information about this on the internet. I was given an example network and told how the levels would change on it. I need to explain in theory how it would affect the routing tables. I would be very grateful if someone could explain me how the routing tables woudl change on the next network:

As default the routers are all on L1/L2 state. Now we would leave ROOPE TOUHO, TUPU and LEENU routers as L1/L2 and change all routers in areas 0002-0005 to L1. And finally the KROISOS router would be changed to L2. Now how would the routing tables change from the starting position where all the routers were L1/L2? 

Picture of network attached

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Accepted Solutions
Cisco Employee

Re: IS-IS Router Levels affecting routing tables?

Hello Niko,

Key rule to remember: The routing table of an L1 router will contain routes only from the router's own area. An L1 router has no information about routes in other areas - instead, they will be represented by a default route pointing towards the nearest L1/2 router that has an active adjacency to a neighbor in different area. L1/2 and L2 routers will know routes from all areas in the routing domain.

The changes you will encounter:

  1. Before making any changes, all routers know all routes from all areas, and these routes are most probably visible as L1/L2 routes in your routing tables.
  2. After the Roope, Touho, Tupu, Leenu and Kroisos routers are left L1/L2 and all other routers are configured as L1 routers, internal routers in areas 0002 - 0005 will see only routes from their own areas, learnt as L1. Routes from all other areas will be removed and automatically replaced by default routes. Routers Roope, Touho, Tupu, Leenu and Kroisos will contain complete routing tables. As in IS-IS, a router as a whole belongs to a single area only, these five routers will see the routes from their own areas as L1 and routes from all other areas as L2.
  3. After the Kroisos is reconfigured as L2, all routes it knew will remain in its routing table but all will be learnt as L2

Does this help a little? Please feel welcome to ask further!

Best regards,

Peter

P.S.: Is this a real network scenario or a school project?

2 REPLIES
Cisco Employee

Re: IS-IS Router Levels affecting routing tables?

Hello Niko,

Key rule to remember: The routing table of an L1 router will contain routes only from the router's own area. An L1 router has no information about routes in other areas - instead, they will be represented by a default route pointing towards the nearest L1/2 router that has an active adjacency to a neighbor in different area. L1/2 and L2 routers will know routes from all areas in the routing domain.

The changes you will encounter:

  1. Before making any changes, all routers know all routes from all areas, and these routes are most probably visible as L1/L2 routes in your routing tables.
  2. After the Roope, Touho, Tupu, Leenu and Kroisos routers are left L1/L2 and all other routers are configured as L1 routers, internal routers in areas 0002 - 0005 will see only routes from their own areas, learnt as L1. Routes from all other areas will be removed and automatically replaced by default routes. Routers Roope, Touho, Tupu, Leenu and Kroisos will contain complete routing tables. As in IS-IS, a router as a whole belongs to a single area only, these five routers will see the routes from their own areas as L1 and routes from all other areas as L2.
  3. After the Kroisos is reconfigured as L2, all routes it knew will remain in its routing table but all will be learnt as L2

Does this help a little? Please feel welcome to ask further!

Best regards,

Peter

P.S.: Is this a real network scenario or a school project?

New Member

Re: IS-IS Router Levels affecting routing tables?

Thank you Peter, this helped me perfectly. Your answer cleared all my questions.

It is part of pre-study for school project we have next week with IS-IS.

Thank you again, you helped me a lot

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