##
IS-IS
Link-state IGP that runs directly on Layer 2 (not IP). AD 115, Dijkstra SPF — common in large ISP cores.
// routing levels
| Level 1 | Intra-area routing (within an area) |
| Level 2 | Backbone routing between areas |
| L1 / L2 | Border router in both (like an OSPF ABR) |
// PDU types
| IIH (Hello) | Establish & maintain adjacencies |
| LSP | Link-State PDU with TLVs |
| CSNP | Complete sequence-number PDU (full list) |
| PSNP | Partial sequence-number PDU (request/ack) |
| IS-IS | OSPF |
|---|
| Runs on | Layer 2 (CLNS) | IP (proto 89) |
| Area boundary | On the link (router in one area) | On the router (per-interface) |
| Designated | DIS (preemptive) | DR/BDR (no preempt) |
| Metric | Default 10 | Cost by bandwidth |
| Hierarchy | Level 1 / Level 2 | Area 0 + areas |
// terminology
| NET / NSAP | Router address: AFI . area . system-ID . SEL |
| System ID | Unique 48-bit router ID (often a MAC) |
| DIS | Designated IS — pseudonode on multi-access |
| LSP | Link-State PDU carrying TLVs |
| TLV | Type-Length-Value modular data |
// facts
- Link-state, Dijkstra SPF, AD 115 — runs on Layer 2, not IP
- Default interface metric 10 (use wide metrics for scale)
- DIS replaces the OSPF DR; default priority 64, preemptive
- Highest priority wins DIS; highest SNPA/MAC breaks the tie
- Adjacency needs matching MTU, level, area (L1) & auth
configuration · cisco ios
router isis
net 49.0001.0000.0000.00a1.00
is-type level-2-only
!
interface GigabitEthernet0/0
ip router isis
isis circuit-type level-2-only
isis authentication mode md5
isis authentication key-chain KC
verify
show isis neighbors
show isis database
show clns interface
show ip route isis