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Understanding Link-State PDU Throttling for IS-IS Interfaces

 

Link-state PDU throttling by use of the lsp-interval statement is a mechanism to control the flooding pace to neighboring routing devices to prevent overloading them.

Control traffic (link-state PDU and related packets) might cause delays in user traffic (information packets) because control traffic always has precedence in terms of scheduling on the interface cards.

Unfortunately, the control traffic transmission rate does not get lower on low-bandwidth interfaces such as DS-0 or fractional T1/E1 lines. Control traffic stays the same, regardless of line bandwidth.

Junos OS does not support automated calculation of link-state PDU throttling based on available bandwidth because the lowest-speed interface cards on a Juniper Networks routing device starts at T1/E1 speeds (1.5 and 2 Mbps). It is assumed that even with link-state PDU pacing of 20 ms, the control traffic will not consume more than half of the interface bandwidth.

However, there might be fractional T1/E1 circuits (less than the full bandwidth) configured as well, where link-state PDU pacing might have to be adjusted.

Thus, the lsp-interval statement helps to resolve two issues: regulating the control-traffic-to-user-traffic ratio, and protecting neighbors during transient situations.

The traffic subject to this pacing is non-self-originated traffic, which is traffic that has been originated by other routers, not the local router. Junos OS has hard-coded rate limiting for locally generated link-state PDUs. All the link-state PDUs are paced using a 20 ms timer. Additionally, there is logic that makes sure that the adjacency is reliably up for some time before advertising the adjacency.