Explain IS-IS levels. What are L1, L2 and L1/L2 routers?
Show the answer
Say this first
IS-IS has two levels. Level 1 routers route inside their own area and reach everything else through a default route. Level 2 routers form the backbone that connects areas. An L1/L2 router sits at the edge of an area, keeps a separate database for each level, and joins the area to the backbone, much like an OSPF ABR. Unlike OSPF, a whole router belongs to one area, so the area border runs between routers, not through them. Cisco routers are L1/L2 by default.
Go deeper
Who forms what adjacency
| Router A | Router B | Same area | Different areas |
|---|---|---|---|
| L1 | L1 | L1 | None |
| L1 | L1/L2 | L1 | None |
| L1/L2 | L1/L2 | L1 and L2 | L2 only |
| L2 | L2 or L1/L2 | L2 | L2 |
| L1 | L2 | None | None |
OSPF comparison
| IS-IS | OSPF equivalent |
|---|---|
| L1 area | A totally stubby area, by default |
| L2 backbone | Area 0 |
| L1/L2 router | ABR |
| Area border between routers | Area border inside the ABR, on its interfaces |
Setting the level
router isis CORE
net 49.0001.0000.0000.0001.00
is-type level-1
!
interface GigabitEthernet0/0/1
isis circuit-type level-2-onlyis-type sets what the router can be. isis circuit-type limits one interface, which saves sending
useless L1 hellos on backbone links.
They will ask next
Must the L2 backbone be contiguous?
Yes. All L2-capable routers must form one connected L2 network, like OSPF area 0. The difference is that the backbone is not an area: it is the chain of L2 adjacencies, which can cross many areas.
Why leave every router as L1/L2 if that is the default?
Many providers run a single level, all L2, across the whole network. It is simple and keeps every router's view complete. L1 areas are added only when the database or flooding becomes too large.
Traps
- Saying an interface belongs to an area, as in OSPF. In IS-IS the router belongs to the area.