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Size of ISIS level-2


We are redesigning a large enterprise network of 400-500 routers (2800, 7600, CRS-1) running flat EIGRP to MPLS/VPN/ISIS. Due to address plan and SDH topology, it is almost impossible to create a level-2 backbone with level-1 stub areas. From different sources I have heard level-2 can be from 400-1000 routers maximum. If anyone can share their experience with me that would be great.

2 Replies 2


Hi there,

Have a look at

I think that as long as you try to minimize the number of 2800s in level-2 and set your LSP lifetime to 65535 and lsp refresh to 65000 (to keep flooding to a minimum) and follow the other guidelines in this paper you should be OK. There are also several papers out there that detail the best way to migrate from another IGP (usually OSPF) to IS-IS (mostly involves running the two protocols in parallel and changing the administrative distances of each one so that no significant FIB/RIB changes are triggered by the migration and rollback is easy in the event of a problem). Best of luck.


Harold Ritter
Cisco Employee
Cisco Employee


Another thing that might help the scalability is to only carry the loopback interface IP address in the ISIS LSPs and carry the transit links in BGP. This can be accomplished using the following commands:

router isis

advertise passive-only

passive-interface Loopback0

I have seen large service providers scaling their level-2 to over 1000 nodes but there wasn't any 2800 in these networks.

Hope this helps,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
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