3com S7906E Manuel De Montage

 

1-29 

Enter IS-IS view 

isis [ process-id ] [ vpn-instance 
vpn-instance-name ] 

–– 

Enable LSP fragment 
extension and specify the 
working mode 

lsp-fragments-extend [ [ level-1 | 
level-1-2 | level-2 ] | [ mode-1 | 
mode-2 ] ] * 

Required 

Not enabled by default 

Configure a virtual system 
ID 

virtual-system virtual-system-id 

Required 

Not configured by default

 

z 

After LSP fragment extension is enabled for an IS-IS process, the MTUs of all the interfaces 

running the IS-IS process must not be less than 512; otherwise, LSP fragment extension will not 

take effect. 

z 

At least one virtual system needs to be configured for the router to generate extended LSP 

fragments. An IS-IS process allows 50 virtual systems at most. 

 

In well connected ATM, FR and NBMA networks, many P2P links exist. The following figure shows a 

fully meshed network, where Routers A, B, C and D run IS-IS. When Router A generates an LSP, it 

floods the LSP out Ethernet 1/1, Ethernet 1/2 and Ethernet 1/3. After receiving the LSP from Ethernet 

1/3, Router D floods it out Ethernet 1/1 and Ethernet 1/2 to Router B and Router C, which however has 

received the LSP from Router A. In this case, LSP flooding consumes extra bandwidth. 

Figure 1-14 Network diagram of a fully meshed network 

 

 

To avoid this, you can configure some interfaces as a mesh group or/and configure the blocked 

interfaces. 

z 

After receiving an LSP, a member interface in a mesh group floods it out the interfaces that does 

not belong to the mesh group. 

z 

If an interface is blocked, it does not send LSPs unless the neighbor sends LSP requests to it. 

Before configuring this task, you need to consider redundancy for interfaces to avoid the fact that LSP 

packets cannot be flooded due to link failures.