Configuring LSP Packet Attributes

LSP messages are used to exchange link status information. By configuring the basic properties of LSP, the size and maximum valid time of LSP packets can be controlled. It is also possible to adjust the interval between LSP generations, which can enable the network to converge quickly. Additionally, you can configure configuring the refresh period for LSPs to send LSP packets and the refresh cycle of LSP packets.

Configure the Maximum Size of Generated LSPs

When the link state information becomes larger, the length of the generated LSP message can be increased, allowing each LSP to carry more information. Users can control the size of LSP generation and reception through the following commands.

set protocols isis area-tag <text> [vrf <vrf-name>]  lsp-mtu <max-size>

The following example configures the maximum size of generated LSPs to 1024 bytes.

Configure the Minimum Interval between Regenerating LSP

When there are changes in local routing information, the device needs to generate new LSPs to announce these changes. When changes in local routing information occur frequently, immediately generating new LSPs consumes a large amount of system resources. To speed up network convergence without affecting system performance, you can adjust the interval between LSP generations using set protocols isis area-tag <text> [vrf <vrf-name>]  lsp-timers gen-interval <lsp-gen-interval> command.

Configure the Refresh Period for LSPs

The IS-IS network primarily achieves synchronization of link states through the flooding of LSPs. Flooding entails a device sending its LSP to neighbor devices, which then forward the same LSP to all neighbors except the one from which they received it, thus progressively disseminating the LSP throughout the entire hierarchy. This ensures that every device within the hierarchy possesses identical LSP information, thereby maintaining synchronization of the Link State Database (LSDB).

The flooding of LSPs requires periodic refreshing because when a device generates its system LSP, it includes the maximum valid time for the LSP. As other devices receive this LSP, its valid time decreases over time. If a device does not receive an updated LSP, once the valid time of the LSP reaches 0, it will be retained for an additional 60 seconds. If a new LSP is not received within this time, the LSP will be deleted.

Use the command set protocols isis area-tag <text> [vrf <vrf-name>] lsp-timers refresh-interval <lsp-refresh-interval> to configure the refresh period for LSPs. This interval determines how often the router will re-advertise its LSPs to ensure that other routers in the network have up-to-date information about the network topology. Adjusting this interval can help balance the need for up-to-date information with the desire to minimize unnecessary network traffic.

Configure the Maximum Valid Time for the LSPs

When the switch generates system LSP, the maximum valid time of this LSP will be filled in the LSP. When this LSP is received by other switches, its valid time will continuously decrease over time. If the switch has not received an updated LSP and the valid time of this LSP has decreased to 0, the LSP will remain for another 60 seconds. If a new LSP has not been received yet, it will be deleted.
The set protocols isis area-tag <text> [vrf <vrf-name>] lsp-timers max-lifetime <lsp-refresh-interval> command can adjust the maximum valid time of LSP to ensure the validity of old LSP before receiving updated LSP.