ISIS – Intermediate System-Intermediate System
- Seen inside the core SP Network
- NET=Network Entity Title=ISO NSAP Format – comprises Area, System ID & N-Selector
- Level 1 [Intra-Area, Like NSSA in OSPF] & Level 2 [Intra/Inter-Area, Like Area 0 in OSPF] Adjancencies – Seperate LSP Databases – Extensive use in SP Lab.
- CLNS Resolution required for multipoint networks to ensure adjacency established as it is not an IP Protocol
- Additional ISIS Features include authentication, redistribution, route leaking, performance, metrics, priority, multiple NET addresses, etc should be reviewed – see CCO link below.
- Level-1-2 router have to be in the same area as level-1 routers otherwise level-1 routers will not be able to form an adjacency with them
- The level-1-2 router must have a level-2 adjacency with a router in another area for the att bit to be set in outgoing level-1 updates
- By default level-1 routes are advertised to level-2 routers
- By default level-2 routes are NOT advertised to level-1 routers
- The passive-interface command under the router isis process includes the interface address(es) in the isis database and advertisements while preventing isis/clns protocol packets on the passive-interface.
- When you use the passive-interface command under router isis, it is NOT necessary (or desireable) to use the ip router isis command under the interface. Using isis the passive-interface command will cause the routing process to advertise the address(es) on the passive interface without configuring ip router isis on that interface.
- Interface level “isis circuit-type” commands will overwrite the same commands in global configuration mode.
- Finally remember L2 – L3 transport for Frame-Relay & ATM interfaces for ISIS in the SP Lab so “protocol clns_is broadcast” under ATM interfaces and “frame-relay map clns xxx bradcast” under the appropriate serial interface.
ip address 10.91.1.10 255.255.255.0
ip router isis
isis circuit-type level-2-only
Verification – Sh clns is-nei, sh ip route isis, sh isis database detail, sh isis hostname.