Configuring IS-IS to Generate IPv6 Default Routes

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

Context

The destination address and mask of an IPv6 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 default routes using either of the following mode:

Command-triggered default route generation mode

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

As defined in IS-IS, a Level-1-2 router sets the ATT bit in the Level-1 LSPs to be advertised 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 receives the LSP with the ATT bit set, it generates a default route destined for the Level-1-2 router that sent this LSP. Based on the network requirements, you can configure whether a router sets the ATT bit carried in the LSP and whether a Level-1 router generates a default route after it receives the LSP with the ATT bit set.

This mode applies only to Level-1 routers.

Procedure

Configure command-triggered default route generation mode.
1. Run system-view
  The system view is displayed.
2. Run isis [ process-id ]
  The IS-IS view is displayed.
3. Run ipv6 default-route-advertise [ always | match default | route-policy route-policy-name | route-filter route-filter-name ] [ [ cost cost ] | [ tag tag ] | [ level-1 | level-2 | level-1-2 ] ] ^* [ avoid-learning | learning-avoid-loop ]
  IS-IS is configured to generate default IPv6 routes.
4. Run commit
  The configuration is committed.
Configure ATT bit (set)-triggered default route generation mode.
1. Run system-view
  The system view is displayed.
2. Run isis [ process-id ]
  The IS-IS view is displayed.
3. Run the following command 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. After receiving the LSPs with the ATT bit set, the Level-1 router generates a default route.
    - If the never parameter is specified, the ATT bit is not set. After receiving the LSPs in which the ATT bit is not set, 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 Level-1 LSPs in which the ATT bit is set, run the attached-bit avoid-learning command.
4. Run commit
  The configuration is committed.