ISIS area id

[Jon Berres]

Hello,

I have a question concerning the area id in an ISIS NET ID. According to documentation I know that a level-1 router can only become neighbors with a router with the same area but level-2 routers do not care about area ids. Using area ids a network can be broken into smaller areas to provide better scalability.

The problem I have is understanding the need for area ids to segment a network when any level-1 routers are already segmented from other level-1 routers when there are level-2 links inbetween. So for instance if I have a few level-1 routers connected to a level-1-2 and that router is connected to a level-2 router, the level-1 routers are segmented from any level-1 or level-2 routers that may connect beyond the level-2 router. Below is a diagram showing this. But since level-1 LSPs do not flood past the level-1-2 router I do not see the need for area ids to segment a network.

[lvl-1 rtr] -----L1-    \                                                                                       /-L1-------[lvl-1 rtr]

[lvl-1 rtr] -----L1----   [lvl-1-2 rtr] -----L2 link ----[lvl-2 rtr] -----L2 link -------[lvl-1-2 rtr] ---L1------[lvl-1 rtr]

[lvl-1 rtr] -----L1-    /                                                                                       \-L1-------[lvl-1 rtr]  

In this diagram the level-1 routers on the right are not going to know about the level-1 topology on the left because the L2 links will not forward L1-LSPs accross. I am just wondering because if I can keep the complexity of area ids down and still maintain a scalable network.

If anyone has any thoughts on this I would appreciate it.

thanks,

jb

[Mohamed Sobair]

Jon,

This initially depends on your design. Surely in ISIS , L1 & L2 maintain different Link state database. an L1/L2 enabled interface sends a default route attachment for L1 router to reach the opposite L1 routers. So when L1 Area travels across L1/L2 towards L2 Only and Back again towards L1/L2 back to L1, it maintains Two Seperate Databases.

This Design is not complex, it depends on the requirement, IF for example you have spread Large Network environment where you require Access to all branches be through the Backbone, Your Above design is absolutely the right approach/way to go.

If you have a Small Network where You can maintain a single Database, a complexity of different Area is not required.

The Idea behind your design is actually similar to the OSPF Stub Area, Where you need to maintain L1 Small routing table and just have a default through the Backbone to reach another L1 Area.

Let me know if this answers your question,

Regards,

Mohamed

[Jon Berres]

Hi Muhamed,

Thanks for the reply. Your assessment aligns with what I was thinking. The one question I still have is whether a different area id is actually needed in the router NET ID. For instance in the example I gave before the routers on the right would have area 49.0001 and the routers on the left could have 49.0002. I understand that the L2 routers will segment the network for scalabiliy, however, do I still need the different area ids or could I just use 49.0001 on all routers and get the same effect.

thanks,

jb

[Mohamed Sobair · Accepted solution]

Hello Jon,

All routers could have the same Area-id, its not a problem and this would have the same effect. However each router in the ISIS domain MUST have different ISIS NET-ID, OR a conflict could occur & result in an unexpected behaviour.

Regards,

Mohamed

[Jon Berres]

Hi Mohamed,

Ok, that helps a lot. I understand the NET-ID needs to be unique in any given area domain, I think that basing the NET ID on the router loopback makes the most sense.

The part that I kept wondering was what the purpose of the area ID is if a level-2 link between two routers effectively creates an L1 LSP flooding boundary between two level 1 areas. This will limit the scope of LSP floods and limit the impact of routers running SPF on those updates.

The only reason I see to actually set the area ID (i.e. 49.0001 or 49.0002) would be for cosmetics and to make it clear when you look at two routers that they are in different areas.

thanks,

jb