IS-IS does not run over IP. What does that mean, and why does it matter?
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Say this first
IS-IS packets are carried directly in layer 2 frames, not inside IP packets. On Ethernet they use LLC with the OSI service access point 0xFE. Its packets are hellos (IIH), link state PDUs (LSPs), and complete and partial sequence number PDUs (CSNP, PSNP). IP prefixes are just TLVs inside the LSPs (RFC 1195 for IPv4, RFC 5308 for IPv6). Being outside IP means it cannot be attacked from a remote IP host, and adding IPv6 needed new TLVs rather than a new protocol.
Go deeper
The packet types
| PDU | Purpose |
|---|---|
| IIH (LAN L1, LAN L2, point-to-point) | Find and keep neighbors |
| LSP (L1, L2) | A router's links and prefixes for one level |
| CSNP | A summary of every LSP in the database; the DIS sends it every 10 s on a LAN |
| PSNP | Requests or acknowledges specific LSPs |
On Ethernet, L1 packets go to MAC 0180.C200.0014 and L2 packets to 0180.C200.0015.
Practical effects
- On IOS you enable it per interface with
ip router isis TAG. The hellos still list the interface IP addresses, because routers need them as next hops for the IP routes. - IS-IS pads hellos to the full interface MTU by default, so an MTU mismatch stops the adjacency. That is deliberate: it finds MTU problems before large LSPs are silently dropped.
- You cannot ping or traceroute IS-IS itself.
show isis neighborsandshow clns interfaceare your tools.
They will ask next
What does the DIS do, and how is it different from an OSPF DR?
The DIS creates the pseudonode LSP for the LAN and sends CSNPs every 10 seconds so others can spot missing LSPs. There is no backup DIS, election is preemptive (highest priority, then highest MAC), and every router on the LAN still forms an adjacency with every other one. An OSPF DR has a backup, is not preemptive, and other routers only go Full with the DR and BDR.
Why use point-to-point mode on an Ethernet link between two routers?
isis network point-to-pointskips DIS election and the pseudonode, so the database is smaller and the adjacency comes up faster. It is standard practice on router to router links.
Traps
- Giving IS-IS an IP protocol number. IANA does list 124 for IS-IS over IPv4, but real IS-IS deployments carry it directly in layer 2 frames, not in IP.