Configuring IS-IS to Generate IPv6 Default Routes

This section describes how to configure Intermediate System to Intermediate System (IS-IS) to generate IPv6 default routes to control the advertising of IS-IS routing information.

Context

The destination address and mask of a default route are all 0s. If the destination address of a packet does not match any entry in the routing table of a device, the device sends the packet along the default route. If neither the default route nor the destination address of the packet exists in the routing table, the device discards the packet and informs the source end that the destination address or network is unreachable.

IS-IS can generate IPv6 default routes using either of the following mode:

Command-triggered IPv6 default route generation mode

You can run the ipv6 default-route-advertise command on a device so that the device adds an IPv6 default route to the LSP before sending the LSP to a neighbor. Therefore, the neighbor can learn this IPv6 default route.
ATT bit 1-triggered IPv6 default route generation mode

IS-IS defines that a Level-1-2 router sets the ATT bit to 1 in the LSP to be advertised to a Level-1 area if the Level-1-2 router can reach more Level-1 areas through the Level-2 area than through the Level-1 area. After a Level-1 router in the Level-1 area receives the LSP, it generates a default route destined for the Level-1-2 router. Based on the network requirements, you can configure whether the Level-1-2 router sets the ATT bit carried in the LSP and whether a Level-1 router generates a default route after it receives the LSP carrying ATT bit 1.

This mode applies only to Level-1 routers.

Procedure

Configure command-triggered IPv6 default route generation mode.
1. Access the system view.
  system-view
2. Access the IS-IS process view.
  isis [ process-id ]
3. Configure IS-IS to generate IPv6 default routes.
  ipv6 default-route-advertise [ always | match default | route-policy route-policy-name ] [ cost cost | tag tag | [ level-1 | level-1-2 | level-2 ] ] ^* [ avoid-learning ]
Configure ATT bit 1-triggered IPv6 default route generation mode.
1. Access the system view.
  system-view
2. Access the IS-IS process view.
  isis [ process-id ]
3. Run either of the following commands as required:
  - To set the ATT bit in the LSPs sent by the Level-1-2 router, run the attached-bit advertise { always | never } command.
    - If the always parameter is specified, the ATT bit is set to 1. After receiving the LSPs carrying the ATT bit 1, the Level-1 router generates a default route.
    - If the never parameter is specified, the ATT bit is set to 0. After receiving the LSPs carrying the ATT bit 0, the Level-1 router does not generate a default route, which reduces the size of a routing table.
  - To disable the Level-1 router from generating default routes even though it receives the LSPs carrying ATT bit 1, run the attached-bit avoid-learning command.