IS-IS and Default Routes
The purpose of the default route in any routing protocol is to forward traffic to destinations that are not in the router's routing table. It is not possible for all the routers in a network to have full Internet routes. For this purpose, routers without full routes to all the destinations forward traffic to the default originating router.
Level 1 routers never maintain information about any destination that is outside their area, so all level 1 routers merely send packets to the nearest level 2 router for any destination outside their local area.
The default-information originate command is used with level 2 routers for sending traffic to destinations not found in the local routing table. This command is used to send a default route in the backbone, and it creates an external entry into the L2 LSP. Unlike OSPF, this command does not require a default route to be present in the router that is originating the default route.
If you compare this command with the OSPF default-information command, it behaves similar to the way that the default-information originate always command behaves in OSPF. This means that, regardless of the default route's presence in the routing table of the originating router, the command still propagates a default route.
Level 1 routers never maintain information about any destination that is outside their area, so all level 1 routers merely send packets to the nearest level 2 router for any destination outside their local area.
The default-information originate command is used with level 2 routers for sending traffic to destinations not found in the local routing table. This command is used to send a default route in the backbone, and it creates an external entry into the L2 LSP. Unlike OSPF, this command does not require a default route to be present in the router that is originating the default route.
If you compare this command with the OSPF default-information command, it behaves similar to the way that the default-information originate always command behaves in OSPF. This means that, regardless of the default route's presence in the routing table of the originating router, the command still propagates a default route.
