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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
IS-IS LSP Fragment Extension

IS-IS LSP Fragment Extension

If the LSP capacity is insufficient, newly imported routes and new TLVs fail to be added to LSP fragments. In this case, you can use LSP fragment extension to increase the LSP capacity, restoring the LSP space. When the LSP space is restored, the system automatically attempts to re-add these routes and TLVs to LSP fragments.

When the LSPs to be advertised by IS-IS contain a large amount of information, they are advertised in multiple Link State PDUs (LSP) fragments belonging to the same system.

Virtual system IDs can be configured, and virtual LSPs that carry routing information can be generated for IS-IS.

IS-IS LSP fragment extension allows an IS-IS device to generate more LSP fragments and carry more IS-IS information.

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.

Table 8-3 IS Alias ID TLV

Field

Length

Description

Type

1 byte

TLV type. If the value is 24, it indicates the IS Alias ID TLV.

Length

1 byte

TLV length.

System ID

6 bytes

System ID.

Pseudonode number

1 byte

Pseudonode number.

Sub-TLVs length

1 byte

Length of sub-TLVs.

Sub-TLVs

0 to 247 bytes

Sub-TLVs.

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 8-16 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 8-16, 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 8-16, 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.

Table 8-4 Comparison between mode-1 and mode-2

LSP Field

Carried in Mode-1

Carried in Mode-2

IS Alias ID

Yes

Yes

Area

Yes

No

Overload bit

Yes

Yes

IS NBR/IS EXTENDED NBR

Yes

No

Routing

Yes

Yes

ATT bit

Yes, with value 0

Yes, with value 0

P bit

Yes, with value 0

Yes, with value 0

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

NOTE:

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.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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