IS-IS is an IGP link-state protocol that uses an SPF-algorithm similar to OSPF. ISO developed IS-IS for ISO's Connectionless Network Protocol (CLNP). Extensions to IS-IS were written to support IP forming integrated IS-IS.
An IS-IS network is a single AS that consists of end-systems (ESs) and intermediate systems (ISs). IS-IS packets are referred to as PDU's.
IS-IS is hierarchically divided into areas:
Level 1: route within an area or towards a level 2 system.
Level 2: route between areas and towards another AS.

IS-IS & OSPF common features:

- maintain LSDB and construct tree; use SPF.
- use hello to form & maintain adjacencies.
- use two level hierarchy.
- address summarization between areas.
- elect DR.
- use authentication.

IS-IS uses the following PDUs:

- IS-IS Hello: broadcast to discover neighbor and see if it's level 1 or 2.
- Link-state PDU (LSP): flooded through area, informs about state of adjacency to neighboring II-systems.
- Complete Sequence Number PDU (CSNP): contains a complete description of all link-state PDUs (LSP headers).
- Partial Sequence Number PDU (PSNP): used to request missing link-state PDU.
- TLVs: II-PDU use TLV encoding.

Neighbors & Adjacencies.

- lvl 1 router never forms adjacency with lvl 2 router & vice versa.
- all lvl 1 adjacency area-IDs must match.
- lvl 2 adjacency area-IDs can be different.


         Level 1 internal routes are announced to level 2 by default
         Level 2 routes are not leaked to level 1 by default
         Level 1/2 routers identifies itself by sending attach-bit in level 1 LSPs. Level 1 routers will install default route to nearest router with this bit set.
II-DIS election based on interface priority (0-127), default is 64. With priority 0, router cannot be DR. Highest router wins. Upon tie, MAC is used.
Multi-access network is considered Pseudo node. Each router advertises link to Pseudo node. The DR represents the Pseudo node to the rest of the network. The Pseudo Node is the only one that sends CSNPs periodically. All routers have an adjacency to the Pseudo node.
If DIS fails, new DIS is elected. New & higher prio router becomes DIS.


II uses a single, required, default metric with a maximum path value of 1023. A single link max value is 63. Sum of link values is the path metric.
II has three optional metrics/costs;
- delay
- expense
- error

Wide metrics can be used to increase ranges of metric values.

Beside metrics, II routes have different preference values. These values are the following;

         IS-IS Level 1 internal route: 15
         IS-IS Level 2 internal route: 18
         IS-IS Level 1 external route: 160
         IS-IS Level 2 external route: 165

Configuring II.

When II is enabled on an interface, it is level 1&2. Disable what is not needed.
Random examples;
Group level 2 disabled, nothing configured on interface: interface is level 1 enabled and level 2 disabled
Group level 2 disabled, interface level 1 disabled: interface is level 1 disabled and level 2 disabled

Some configuration commands examples;

set protocols isis interface ge-0/0/0.0 level 1 disable
set interfaces ge-0/1/0 unit 0 family iso
set interfaces lo0 unit 0 family iso address 49.0001.0192.0152.0102.00

Random handy commands;
show isis database
show isis adjacency
show isis spf log
show isis statistics
show isis route

Troubleshooting adjacency.

If no adjacency, check;
- Physical & data link layer.
- mismatched areas & levels.
- failure to support minimum MTU (1492).
- Lack of, or malformed, ISO-NET