IS-IS Routing

Adjacency requirements:

Areas must match (for level1)
Levels must match
System ID’s must be unique
Authentification must succeed
Interface MTU’s must match


TLV – Type-Length-Value
Variable-length modular dataset
LSP -Link State PDU
Carry TLVs encompassing link state information
SNP – Sequence Number Packet
Used to request and advertise LSPs
DIS – Designated Intermediate System pseudonode responsible for emulating point-to-point links across a multi-access segment 

Example ISIS address:

46.0004.7off.f800.000 .000.0c00.1324 .00

46.0004.7off.f800.000 = area (address used for routing)
000.0c00.1324 = system id (unique router identifier – often derived from mac address)
00 = Sel/NSEL (network selector) identifies network layer service, always 0x00 in a NET address


Link State
Dijkstra alogirithm
Default 10 metric
Administrative distance 115
Protocols IP/CLNS
Layer 2
Authentification plain, MD5


Level 0 used to locate end systems
Level 1 Routing within an area
Level 2 Backbone between areas
Level 3 Inter-AS routing

DIS ( Designated Intermediate System) election :

Highest priortiy interface elected
Default interface priotiy 64
Current DIS can be preempted