3Com S7902E, Configuration Manual

Page: 740 / 2621

1-29
To do… Use the command… Remarks
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.
Limiting LSP flooding
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.