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Bandwidth Sharing on Nonqueuing Packet Forwarding Engines Overview

You can configure bandwidth sharing rate limits, excess rate, and excess priority at the queue level on the following Juniper Networks routers and switches:

  • EX Series switches

  • M120 Multiservice Edge Router (rate limit and excess priority only; excess rate is not configured by the user)

  • M320 router with Enhanced FPCs (rate limit, excess rate, and excess priority)

  • MX Series 5G Universal Routing Platform with nonqueuing DPCs (rate limit, excess rate, and excess priority)

You configure rate limits when you have a concern that low-latency packets (such as high or strict-high priority packets for voice) might starve low-priority and medium-priority packets. In Junos OS, the low latency queue is implemented by rate-limiting packets to the transmit bandwidth. The rate-limiting is performed immediately before queuing the packet for transmission. All packets that exceed the rate limit are not queued, but dropped.

By default, if the excess priority is not configured for a queue, the excess priority will be the same as the normal queue priority. If none of the queues have an excess rate configured, then the excess rate will be the same as the transmit rate percentage. If at least one of the queues has an excess rate configured, then the excess rate for the queues that do not have an excess rate configured will be set to zero.

When the physical interface is on queuing hardware such as the IQ, IQ2, or IQE PICs, or MX Series routers queuing DPCs or EX Series switches, these features are dependent on the PIC (or queuing DPC in the case of the MX Series router) configuration.

You cannot configure both rate limits and buffer sizes on these Packet Forwarding Engines.

Four levels of excess priorities are supported: low, medium-low, medium-high, and high.


Rate limiting is implemented differently on Enhanced Queuing DPCs and non-queuing Packet Forwarding Engines. On Enhanced Queuing DPCs, rate-limiting is implemented using a single rate two color policer. On non-queuing Packet Forwarding Engines, rate-limiting is achieved by shaping the queue to the transmit rate and keeping the queue delay buffers small to prevent too many packets from being queued once the shaping rate is reached.