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IS-IS: L1L2 Router

Hi:

Is it possible to have a L1L2 router connected to two different areas at the same time?

Theory says that all ISO areas are like OSPF stub areas, please clarify.

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New Member

Re: IS-IS: L1L2 Router

Hi,

I'm not sure if I'm looking at the right picture.I could not reach the URL you sent.I'm looking at a white paper called "Introduction to Intermediate System-to-Intermediate System Protocol.In Fig. 2, the scenario is as you describe. By "connected" I mean that this L1L2 router has L2 links to area 2 and 3.Still, all of the L1L2s interfaces belong to Area 4. Unlike OSPF, where an ABR would have 1 interface in the backbone area, belonging to a specific network in that area. 1 interface in area 2, belonging to a specific network in area 2.The same goes for area 3.Now, this ABR would belong to 3 different areas. The L1L2 router in area 4 only have 1 network address, the NSAP NET address.So the border between areas in IS-IS lies On the links rather than in the router.That's how IS-IS works.The SPF calculations and forming of neighbor adjacencies are done with CLNS packets and rules, not IP, even if Integrated IS-IS for IP only is used.

Since the L1L2 router in area 4 is the exit out of area 4, it could be considered the equivalent of an OSPF ABR. The L1 router in area 4 has only information about routers and ESs in its own area.To get to ESs and routers in another area it knows that it must use the L1L2 router as an exit. In an OSPF stub area, no external routes are allowed, therefore the routers in that area doesn't know how to reach these external networks.The only way is through the default route pointing to an ABR.

//H

4 REPLIES
New Member

Re: IS-IS: L1L2 Router

Yes, an L1L2 router can be connected to several areas, but it can only belong to one area.I guess that L1 routers and ESs in an isis area only knows about the other routers and ESs in its area.If an ES or L1 node wants to reach another node in a different area it will send the packet to the nearest exit point, which will be an L1L2 router.In an ospf totally stubby area no summary LSAs are allowed, so the routes to other areas and ASs are sent to the exit point (ABR) that announced the default route.

New Member

Re: IS-IS: L1L2 Router

Change the word "routes" to "packets" in the last sentence.

New Member

Re: IS-IS: L1L2 Router

Hans:

Sorry, I dont understand what you mean by : an L1L2 router can be "connected" to several areas, but it can only "belong" to one area. In URL:

http://www.cisco.com/en/US/partner/tech/tk365/tk381/technologies_white_paper09186a00800a3e6f.shtml (figure 2), I see L1L2 router in area 4 is "connected" to area 2 and 3. So, how (or why) can this router be similar to a stub router ? It´s confusing this comparison. Please explain.

New Member

Re: IS-IS: L1L2 Router

Hi,

I'm not sure if I'm looking at the right picture.I could not reach the URL you sent.I'm looking at a white paper called "Introduction to Intermediate System-to-Intermediate System Protocol.In Fig. 2, the scenario is as you describe. By "connected" I mean that this L1L2 router has L2 links to area 2 and 3.Still, all of the L1L2s interfaces belong to Area 4. Unlike OSPF, where an ABR would have 1 interface in the backbone area, belonging to a specific network in that area. 1 interface in area 2, belonging to a specific network in area 2.The same goes for area 3.Now, this ABR would belong to 3 different areas. The L1L2 router in area 4 only have 1 network address, the NSAP NET address.So the border between areas in IS-IS lies On the links rather than in the router.That's how IS-IS works.The SPF calculations and forming of neighbor adjacencies are done with CLNS packets and rules, not IP, even if Integrated IS-IS for IP only is used.

Since the L1L2 router in area 4 is the exit out of area 4, it could be considered the equivalent of an OSPF ABR. The L1 router in area 4 has only information about routers and ESs in its own area.To get to ESs and routers in another area it knows that it must use the L1L2 router as an exit. In an OSPF stub area, no external routes are allowed, therefore the routers in that area doesn't know how to reach these external networks.The only way is through the default route pointing to an ABR.

//H

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