IS-IS LSP Fragment Extension

When an IS-IS router needs to advertise the LSPs that contain much information, the IS-IS router generates multiple LSP fragments to carry more IS-IS information.

IS-IS LSP fragments are identified by the LSP Number field in their LSP IDs. This field is of 1 byte. An IS-IS process can generate a maximum of 256 LSP fragments; therefore, only a limited number of routes can be carried.

As defined in RFC 3786, virtual system IDs can be configured and virtual LSPs that carry routing information can be generated for IS-IS.

Concepts

Originating system: is a router that runs the IS-IS protocol. A single IS-IS process can function as multiple virtual routers to advertise LSPs, and the originating system refers to the IS-IS process.
Normal System-ID: is the system ID of the originating system.
Virtual System: is the system identified by the additional system ID to generate extended LSP fragments. These fragments carry additional system IDs in their LSP IDs.
Additional System-ID: is assigned by network administrators to identify a virtual system. A maximum of 256 extended LSP fragments can be generated for each additional system ID.

Like a normal system ID, an additional system ID must be unique in a routing domain.
TLV 24 (IS Alias ID TLV): describes the relationship between the originating system and virtual system.

Principles

In IS-IS, each system ID identifies a system, which can generate a maximum of 256 LSP fragments. With more additional system IDs (up to 50 virtual systems can be configured), an IS-IS process can generate a maximum of 13,056 LSP fragments.

After LSP fragment extension is configured, the system prompts you to restart the IS-IS process if information is lost because LSPs overflow. After being restarted, the originating system loads as much routing information to LSPs, adds the overloaded information to the LSPs of the virtual system for transmission, and uses TLV 24 to notify other routers of its relationship with the virtual system.

Operating Modes

An IS-IS router can run the LSP fragment extension feature in two modes.

Figure 1 IS-IS LSP fragment extension

Operating Mode	Usage Scenario	Principles	Example	Precautions
Mode-1	Some routers on the network do not support LSP fragment extension.	Virtual systems participate in SPF calculation. The originating system advertises LSPs containing information about links to each virtual system. Similarly, each virtual system advertises LSPs containing information about links to the originating system. Virtual systems look like the physical routers that connect to the originating system. Mode-1 is a transitional mode for the earlier versions that do not support LSP fragment extension. In earlier versions, IS-IS cannot identify the IS Alias ID TLV and processes the received LSP that is advertised by a virtual system as an LSP advertised by an IS-IS process.	In Figure 1, RouterB does not support LSP fragment extension, and RouterA is configured to support LSP fragment extension in mode-1. RouterA1 and RouterA2 are virtual systems of RouterA and send LSPs carrying some routing information of RouterA. After receiving LSPs from RouterA, RouterA1, and RouterA2, RouterB considers that there are three individual routers at the remote end and calculates routes. Because the cost of the route from RouterA to RouterA1 and the cost of the route from RouterA to RouterA2 are both 0, the cost of the route from RouterB to RouterA is the same as the cost of the route from RouterB to RouterA1.	The LSP sent by a virtual system contains the same area address and overload bit as those in a common LSP. If the LSPs sent by a virtual system contain TLVs specified in other features, these TLVs must be the same as those in common LSPs. The virtual system carries neighbor information indicating that the neighbor is the originating system, with the metric equal to the maximum value minus 1. The originating system carries neighbor information indicating that the neighbor is the virtual system, with the metric 0. This ensures that the virtual system is the downstream node of the originating system when other routers calculate routes.
Mode-2	All the routers on the network support LSP fragment extension.	Virtual systems do not participate in SPF calculation. All the routers on the network know that the LSPs generated by virtual systems actually belong to the originating system. An IS-IS router working in mode-2 can identify the IS Alias ID TLV, which is used as a reference for calculating the SPT and routes.	In Figure 1, RouterB supports LSP fragment extension, and RouterA is configured to support LSP fragment extension in mode-2. RouterA1 and RouterA2 are virtual systems of RouterA and send LSPs carrying some routing information of RouterA. When receiving LSPs from RouterA1 and RouterA2, RouterB obtains the IS Alias ID TLV and knows that the originating system of RouterA1 and RouterA2 is RouterA. RouterB then considers that information advertised by RouterA1 and RouterA2 belongs to RouterA.	-

Operating Mode

Usage Scenario

Principles

Example

Precautions

Mode-1

Some routers on the network do not support LSP fragment extension.

Virtual systems participate in SPF calculation. The originating system advertises LSPs containing information about links to each virtual system. Similarly, each virtual system advertises LSPs containing information about links to the originating system. Virtual systems look like the physical routers that connect to the originating system.

Mode-1 is a transitional mode for the earlier versions that do not support LSP fragment extension. In earlier versions, IS-IS cannot identify the IS Alias ID TLV and processes the received LSP that is advertised by a virtual system as an LSP advertised by an IS-IS process.

In Figure 1, RouterB does not support LSP fragment extension, and RouterA is configured to support LSP fragment extension in mode-1. RouterA1 and RouterA2 are virtual systems of RouterA and send LSPs carrying some routing information of RouterA. After receiving LSPs from RouterA, RouterA1, and RouterA2, RouterB considers that there are three individual routers at the remote end and calculates routes. Because the cost of the route from RouterA to RouterA1 and the cost of the route from RouterA to RouterA2 are both 0, the cost of the route from RouterB to RouterA is the same as the cost of the route from RouterB to RouterA1.

The LSP sent by a virtual system contains the same area address and overload bit as those in a common LSP. If the LSPs sent by a virtual system contain TLVs specified in other features, these TLVs must be the same as those in common LSPs.

The virtual system carries neighbor information indicating that the neighbor is the originating system, with the metric equal to the maximum value minus 1. The originating system carries neighbor information indicating that the neighbor is the virtual system, with the metric 0. This ensures that the virtual system is the downstream node of the originating system when other routers calculate routes.

Mode-2

All the routers on the network support LSP fragment extension.

Virtual systems do not participate in SPF calculation. All the routers on the network know that the LSPs generated by virtual systems actually belong to the originating system.

An IS-IS router working in mode-2 can identify the IS Alias ID TLV, which is used as a reference for calculating the SPT and routes.

In Figure 1, RouterB supports LSP fragment extension, and RouterA is configured to support LSP fragment extension in mode-2. RouterA1 and RouterA2 are virtual systems of RouterA and send LSPs carrying some routing information of RouterA. When receiving LSPs from RouterA1 and RouterA2, RouterB obtains the IS Alias ID TLV and knows that the originating system of RouterA1 and RouterA2 is RouterA. RouterB then considers that information advertised by RouterA1 and RouterA2 belongs to RouterA.

When the originating system and virtual system send the LSPs with fragment number 0, the LSPs must carry the IS Alias ID TLV to indicate the originating system regardless of the operation mode (mode-1 or mode-2).