IS-IS LSP Fragment Extension

Terms

• Originating system

  The originating system is a device that runs the IS-IS protocol. A single IS-IS process advertises LSPs as virtual devices do, except that the originating system refers to a real IS-IS process.

• Normal system ID

  The normal system ID is the system ID of the originating system.

• Additional system ID

  The additional system ID, assigned by the network administrator, is used to generate additional or extended LSP fragments. A maximum of 256 additional or extended LSP fragments can be generated. Like a normal system ID, an additional system ID must be unique in a routing domain.

• Virtual system

  The virtual system, identified by an additional system ID, is used to generate extended LSP fragments. These fragments carry additional system IDs in their LSP IDs.

Principles

IS-IS LSP fragments are identified by the LSP Number field in their LSP IDs. The LSP Number field is 1 byte. Therefore, an IS-IS process can generate a maximum of 256 fragments. With fragment extension, more information can be carried.

Each system ID represents a virtual system, and each virtual system can generate 256 LSP fragments. In addition, another virtual systems can be configured. Therefore, an IS-IS process can generate more LSP fragments.

After a virtual system and fragment extension are configured, an IS-IS device adds the contents that cannot be contained in its LSPs to the LSPs of the virtual system and notifies other devices of the relationship between the virtual system and itself through a special TLV in the LSPs.

IS Alias ID TLV

Standard protocol defines a special Type-Length-Value (TLV): IS Alias ID TLV.

LSPs with fragment number 0 sent by the originating system and virtual system carry IS Alias ID TLVs to indicate the originating system.

Operation Modes

IS-IS devices can use the LSP fragment extension feature in the following modes:

Figure 1 Networking for IS-IS LSP fragment extension

• Mode-1

  Mode-1 is used when some devices on the network do not support LSP fragment extension.

  In this mode, virtual systems participate in SPF calculation. The originating system advertises LSPs containing information about links to each virtual system and each virtual system advertises LSPs containing information about links to the originating system. In this manner, the virtual systems function the same as the actual devices connected to the originating system on the network.

  Mode-1 is a transitional mode for earlier versions that do not support LSP fragment extension. In the earlier versions, IS-IS cannot identify Alias ID TLVs. Therefore, the LSP sent by a virtual system must look like a common IS-IS LSP.

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

  LSPs sent by a virtual system carry information of the neighbor (the originating system), and the carried cost is the maximum value minus 1. LSPs sent by the originating system carry information of the neighbor (the virtual system), and the carried cost is 0. This mechanism ensures that the virtual system is a node downstream of the originating system when other devices calculate routes.

  In Figure 1, Device B does not support LSP fragment extension; Device A supports LSP fragment extension in mode-1; Device A1 and Device A2 are virtual systems of Device A. Device A1 and Device A2 send LSPs carrying partial routing information of Device A. After receiving LSPs from Device A, Device A1, and Device A2, Device B considers there to be three devices at the peer end and calculates routes normally. Because the cost of the route from Device A to Device A1 or Device A2 is 0, the cost of the route from Device B to Device A is equal to that from Device B to Device A1.

• Mode-2

  Mode-2 is used when all the devices on the network support LSP fragment extension. In this mode, virtual systems do not participate in SPF calculation. All the devices on the network know that the LSPs generated by the virtual systems actually belong to the originating system.

  IS-IS working in mode-2 identifies IS Alias ID TLVs, which are used to calculate the SPT and routes.

  In Figure 1, Device B supports LSP fragment extension, and Device A supports LSP fragment extension in mode-2; Device A1 and Device A2 send LSPs carrying some routing information of Device A. After receiving LSPs from Device A1 and Device A2, Device B obtains IS Alias ID TLV and learns that the originating system of Device A1 and Device A2 is Device A. Device B then considers information advertised by Device A1 and Device A2 to be about Device A.

Whatever the LSP fragment extension mode, LSPs can be resolved. However, if LSP fragment extension is not supported, only LSPs in mode-1 can be resolved.

Process

After LSP fragment extension is configured, if information is lost because LSPs overflow, the system restarts the IS-IS process. After being restarted, the originating system loads as much routing information as possible. Any excessive information beyond the forwarding capability of the system is added to the LSPs of the virtual systems for transmission. In addition, if a virtual system with routing information is deleted, the system automatically restarts the IS-IS process.

Usage Scenario

If there are non-Huawei devices on the network, LSP fragment extension must be set to mode-1. Otherwise, these devices cannot identify LSPs.

Configuring LSP fragment extension and virtual systems before setting up IS-IS neighbors or importing routes is recommended. If IS-IS neighbors are set up or routes are imported first and the information to be carried exceeds the forwarding capability of 256 fragments before LSP fragment extension and virtual systems are configured, you have to restart the IS-IS process for the configurations to take effect.