Configuring IS-IS Route Leaking

Context

If multiple Level-1-2 devices in a Level-1 area are connected to devices in the Level-2 area, a Level-1 LSP sent by each Level-1-2 device carries an ATT flag bit of 1. This Level-1 area will have multiple routes to the Level-2 area and to other Level-1 areas.

By default, routes in a Level-1 area can be leaked into the Level-2 area so that Level-1-2 and Level-2 devices can determine the topology of the entire network. Devices in a Level-1 area are unaware of the entire network topology because they only maintain LSDBs in the local Level-1 area. Therefore, a device in a Level-1 area can forward traffic to a Level-2 device only through the nearest Level-1-2 device. The route used may not be the optimal route to the destination.

To enable a device in a Level-1 area to select the optimal route, configure IPv4 IS-IS route leaking so that specified routes in the Level-2 area can be leaked into the local Level-1 area.

Routes of services deployed only in the local Level-1 area do not need to be leaked into the Level-2 area. A policy can be configured to leak only desired routes into the Level-2 area.

Procedure

Specify routes in the Level-2 area and other Level-1 areas that can be leaked into the local Level-1 area.
1. Run system-view
  The system view is displayed.
2. Run isis [ process-id ]
  The IS-IS view is displayed.
3. Run import-route isis level-2 into level-1 [ tag tag | filter-policy { acl-number | acl-name acl-name | ip-prefix ip-prefix-name | route-policy route-policy-name } | direct { allow-filter-policy | allow-up-down-bit } ^* ] ^*
  Routes that meet the specified conditions in the Level-2 areas are leaked into the local Level-1 area.
  
  By default, routes in the Level-2 area are not leaked into Level-1 areas.
  
  The command is run on the Level-1-2 device that is connected to an external area.
Configure routes in Level-1 areas to leak into the Level-2 area.
1. Run system-view
  The system view is displayed.
2. Run isis [ process-id ]
  The IS-IS view is displayed.
3. Run import-route isis level-1 into level-2 [ tag tag | filter-policy { acl-number | acl-name acl-name | ip-prefix ip-prefix-name | route-policy route-policy-name } | direct allow-filter-policy ] ^*
  Routes that meet the specified conditions in Level-1 areas are leaked into the Level-2 area.
  
  By default, all routes in a Level-1 area are leaked into the Level-2 area.
  
  The command is run on the Level-1-2 device that is connected to an external area.