Priority Scheduling Overview
Junos supports multiple levels of transmission priority, which in order of increasing priority
are low, low-medium, low-high,
medium-low, medium-high, high,
strict-high, and low-latency. This allows the
software to service higher-priority queues before lower-priority queues.
Priority scheduling determines the order in which an output interface transmits traffic
from its queues, thus ensuring that queues containing important traffic have better
access to the outgoing interface. Junos accomplishes priority scheduling by examining
the assigned priority of each individual queue and whether each individual queue is
within its defined bandwidth profile. Junos determines whether an individual queue is
within its bandwidth profile by comparing, at regular intervals, the amount of data
transmitted by the queue against the amount of bandwidth allocated to it by the
configured scheduler transmission rate (transmit-rate)
defined at the [edit class-of-service schedulers
scheduler-name] hierarchy level. When the transmitted
amount is less than the allocated amount, the queue is considered to be in
profile. A queue is out of profile when its transmitted amount is larger
than its allocated amount.
The queues for a given output physical or logical interface are divided into sets based on their priority. Any such set contains queues of the same priority.
Junos traverses the sets in descending order of priority. If at least one of the queues in the set has a packet to transmit, the software selects that set. A queue from the set is selected based on the weighted round robin (WRR) algorithm, which operates within the set.
Junos performs priority queuing using the following steps:
The software locates all high-priority queues that are currently in profile. These queues are serviced first in a weighted round-robin fashion.
The software locates all medium-high priority queues that are currently in profile. These queues are serviced second in a weighted round-robin fashion.
The software locates all medium-low priority queues that are currently in profile. These queues are serviced third in a weighted round-robin fashion.
The software locates all low-priority queues that are currently in profile. These queues are serviced fourth in a weighted round-robin fashion.
The software locates all high-priority queues that are currently out of profile and are not rate limited. The weighted round-robin algorithm is applied to these queues for servicing.
The software locates all medium-high priority queues that are currently out of profile and are not rate limited. The weighted round-robin algorithm is applied to these queues for servicing.
The software locates all medium-low priority queues that are currently out of profile and are not rate limited. The weighted round-robin algorithm is applied to these queues for servicing.
The software locates all low-priority queues that are currently out of profile and are also not rate limited. These queues are serviced last in a weighted round-robin manner.
Strict-High Priority Configuration Overview
You can configure one queue per interface to have strict-high
priority, which works the same as high priority, but provides
unlimited transmission bandwidth. As long as the queue with
strict-high priority has traffic to send, it receives
precedence over all other queues, except queues with high priority.
Queues with strict-high and high priority take
turns transmitting packets until the strict-high queue is empty,
the high priority queues are empty, or the high
priority queues run out of bandwidth credit. Only when these conditions are met can
lower priority queues send traffic.
When you configure a queue to have strict-high priority, you do not
need to include the transmit-rate statement in the queue
configuration at the [edit class-of-service schedulers
scheduler-name] hierarchy level because the
transmission rate of a strict-high priority queue is not limited by
the WRR configuration. If you do configure a transmission rate on a
strict-high priority queue, it does not affect the WRR
operation. The transmission rate does, however, affect the calculation of the delay
buffer and also serves as a placeholder in the output of commands such as the
show interface queue command.
strict-high priority queues might starve low
priority queues, and under certain circumstances might limit high
priority queues. The high priority allows you to protect traffic
classes from being starved by traffic in a strict-high queue. For
example, a network-control queue might require a small bandwidth allocation (say,
5 percent). You can assign high priority to this queue to prevent
it from being underserved.
A queue with strict-high priority supersedes bandwidth guarantees
for queues with lower priority; therefore, we recommend that you use the
strict-high priority to ensure proper ordering of special
traffic, such as voice traffic. You can preserve bandwidth guarantees for queues
with lower priority by allocating to the queue with strict-high
priority only the amount of bandwidth that it generally requires by applying the
rate-limitoption to the strict-high queue’s
transmission rate. For example, consider the following allocation of transmission
bandwidth:
-
Q0 BE—20 percent, low priority
-
Q1 EF—30 percent, strict-high priority
-
Q2 AF—40 percent, low priority
-
Q3 NC—10 percent, low priority
This bandwidth allocation assumes that, in general, the EF forwarding class requires
only 30 percent of an interface’s transmission bandwidth. However, if short bursts
of traffic are received on the EF forwarding class, and the
rate-limitoption is not applied, 100 percent of the bandwidth
is given to the EF forwarding class because of the strict-high
setting.