Configuring Levels on IS-IS Interfaces
You can administratively divide a single AS into smaller groups called areas. You configure each routing device interface to be in an area. Any interface can be in any area. The area address applies to the entire routing device; you cannot specify one interface to be in one area and another interface in a different area. In order to route between areas you must have two adjacent Level 2 routers that communicate with each other.
Level 1 routers can only route within their IS-IS area. To send traffic outside their area, Level 1 routers must send packets to the nearest intra-area Level 2 router. A routing device can be a Level 1 router, a Level 2 router, or both. You specify the router level on a per-interface basis, and a routing device becomes adjacent with other routing devices on the same level on that link only.
You can configure one Level 1 routing process and one Level 2 routing process on each interface, and you can configure the two levels differently.
To configure an area, include the level statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
The statements within the level statement allow you to perform the following tasks when configuring the following optional level-specific properties:
- Disabling IS-IS at a Level on IS-IS Interfaces
- Advertising Interface Addresses Without Running IS-IS
- Configuring Authentication for IS-IS Hello Packets
- Configuring the Transmission Frequency for IS-IS Hello Packets
- Configuring the Delay Before IS-IS Neighbors Mark the Routing Device as Down
- Configuring the Metric Value for IS-IS Routes
- Configuring the IS-IS Metric Value Used for Traffic Engineering
- Configuring the Designated Router Priority for IS-IS
- Advertising Interface Addresses Without Running IS-IS
Disabling IS-IS at a Level on IS-IS Interfaces
By default, IS-IS is enabled for IS-IS areas on all enabled interfaces on which the ISO protocol family is enabled (at the [edit interfaces interface unit logical-unit-number] hierarchy level). To disable IS-IS at any particular level on an interface, include the disable statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Enabling IS-IS on an interface (by including the interface statement at the [edit protocols isis] hierarchy level), disabling it (by including the disable statement), and not actually having IS-IS run on an interface (by including the passive statement) are mutually exclusive states.
Example: Disabling IS-IS at a Level
On SONET/SDH interface so-0/0/0, enable IS-IS for Level 1 only. With this configuration, tracing messages periodically indicate that IS-IS is creating Level 2 link-state PDUs. However, because IS-IS for Level 2 is disabled, these link-state PDUs are never distributed to neighboring routers.
Advertising Interface Addresses Without Running IS-IS
By default, IS-IS must be configured on an interface or a level for direct interface addresses to be advertised into that level. To advertise the direct interface addresses without actually running IS-IS on that interface or level, include the passive statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Enabling IS-IS on an interface (by including the interface statement at the [edit protocols isis] hierarchy level), disabling it (by including the interface disable statement), and not actually having IS-IS run on an interface (by including the passive statement) are mutually exclusive states.
 | Note: If neither passive mode nor family ISO are configured on the IS-IS interface, then the routing device treats the interface as not being operational and no direct IPv4/IPv6 routes are exported into IS-IS. |
Configuring Authentication for IS-IS Hello Packets
You can configure authentication for a given IS-IS level on an interface. On a point-to-point link, if you enable hello authentication for both IS-IS levels, the password configured for Level 1 is used for both levels.
 | Caution: If no authentication is configured for Level 1 on a point-to-point link with both levels enabled, the hello packets are sent without any password, regardless of the Level 2 authentication configurations. |
By default, hello authentication is not configured on an interface. However, if IS-IS authentication is configured, the hello packets are authenticated using the IS-IS authentication type and password.
To enable hello authentication for an IS-IS level on an interface and define the password, include the hello-authentication-type and hello-authentication-key statements:
For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.
Configuring the Transmission Frequency for IS-IS Hello Packets
Routing devices send hello packets at a fixed interval on all interfaces to establish and maintain neighbor relationships. This interval is advertised in the hello interval field in the hello packet. By default, a designated intersystem (DIS) router sends hello packets every 3 seconds, and a non-DIS router sends hello packets every 9 seconds.
To modify how often the routing device sends hello packets out of an interface, include the hello-interval statement:
The hello interval range is from 1 through 20,000 seconds.
You can send out hello packets in sub-second intervals. To send out hello packets every 333 milliseconds, set the hello-interval value to 1.
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Configuring the Delay Before IS-IS Neighbors Mark the Routing Device as Down
The hold time specifies how long a neighbor should consider this routing device to be operative without receiving another hello packet. If the neighbor does not receive a hello packet from this routing device within the hold time, it marks the routing device as being unavailable. The default hold-time value is three times the default hello interval: 9 seconds for a DIS router and 27 seconds for a non-DIS router.
To modify the hold-time value on the local routing device, include the hold-time statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Configuring the Metric Value for IS-IS Routes
All IS-IS routes have a cost, which is a routing metric that is used in the IS-IS link-state calculation. The cost is an arbitrary, dimensionless integer that can be from 1 through 63, or from 1 through 16,777,215 (224 – 1) if you are using wide metrics. The default metric value is 10 (with the exception of the lo0 interface, which has a default metric of 0). To modify the default value, include the metric statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
For more information about IS-IS interface metrics, see Configuring the Reference Bandwidth Used in IS-IS Metric Calculations.
Configuring the IS-IS Metric Value Used for Traffic Engineering
When traffic engineering is enabled on the routing device, you can configure an IS-IS metric that is used exclusively for traffic engineering. The traffic engineering metric is used for information injected into the traffic engineering database. Its value does not affect normal IS-IS forwarding.
To modify the default value, include the te-metric statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Configuring the Designated Router Priority for IS-IS
A routing device advertises its priority to become a designated router in its hello packets. On all multiaccess networks, IS-IS uses the advertised priorities to elect a designated router for the network. This routing device is responsible for sending network link-state advertisements, which describe all the routing devices attached to the network. These advertisements are flooded throughout a single area.
The priority value is meaningful only on a multiaccess network. It has no meaning on a point-to-point interface.
A routing device’s priority for becoming the designated router is indicated by an arbitrary number from 0 through 127; routing devices with a higher value are more likely to become the designated router. By default, routing devices have a priority value of 64.
To modify the interface’s priority value, include the priority statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
Advertising Interface Addresses Without Running IS-IS
The routing device can advertise the direct interface addresses on an interface or on a sub-level of the interface without actually running IS-IS on that interface or at that level. This occurs in passive mode.
To enable an interface as passive, include the passive statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
