Posted on June 17, 2020 5:12 pm
 |  Asked by Kevin Malone
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I built a basic GNS3 Lab to get a better understanding of ISIS.  I come from a BGP/OSPF background but we might be overtaking a network running ISIS and wanted to familiarize myself with it.  I have 4 routers setup, R1 and R2 are L2 routers simulating my datacenter (area 0), R3 is a L1L2 router simulating a pop site (ABR), and R4 is a L1 router simulating a PE (normal area).  Everything is working with the exception I was expecting a default route be injected in the L1 database as L2 routes are not advertised to L1 routers.  I beleive this is default behavoir on cisco but doesn’t appear to be on arista nor do I see a “default information originate” i can envoke on the R3 for the L1 routers.  I assume I have something missconfigure, i read the arista ISIS documentation and search the forums but couldn’t find my answer.  I’m sure its something simple.  I included all relivant info.  Appreciate any help!

Posted by Dmytro Shypovalov
Answered on June 17, 2020 6:29 pm

Default route in IS-IS is normally created by L1L2 router setting attached bit in its LSP. The condition for this to happen is to have an L2 adjacency to a router in a different area. Looks like in your config, R1, R2 and R3 are in the same area (99.9999), hence R3 does not set attached bit.

Try to change area number on R1 or R2 and check if attached bit appears in R3's LSP.

Alternatively, you can try to configure a static nullroute on R3 and redistribute it into IS-IS. This is not a very good idea, as default route will be advertised in L1 and L2, which is probably not what you want.

Answered on June 17, 2020 6:56 pm

Hi Kevin,

The problem explanation is pretty detailed with relevant outputs making it easier to understand. Thanks for that. Please find our response as below:

The level 1-2 router does not generate a default route. Rather once a level 1-2 router is connected to a router in another area, it will set a special bit in its level 1 LSP called the attached bit. When a level 1 router sees this, it will generate a default route that is pointed to the level 1-2 router.

In our case, all the routers are in the same area(Highlighted part in the NET ID is the area):

net 99.9999.1111.1111.1111.00
net 99.9999.2222.2222.2222.00
net 99.9999.3333.3333.3333.00
net 99.9999.4444.4444.4444.00

And hence we don't see the default route in R4. Once R3(level 1-2 router) forms a neighbourship with a router in another area we should see the default route in R4.

Hope this helps.

Posted by Kevin Malone
Answered on June 17, 2020 8:09 pm

that was it, figured it was something simple I missed.  I changed the area on R1/R2 and now i'm getting a default route on R4.


Thanks for the help!

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