Cisco Catalyst 4500 Series Configuration Manual page 895

Chapter 40 Configuring Quality of Service
Queue Memory
The number of queue entries that can be allocated has to be a multiple of 8 and can range from 16 to
8184. When a class-based queue is instantiated on a physical port, it is given a default number of entries.
This default queue size is based on the number of slots in the chassis and the number of front-panel ports
in each slot.
Catalyst 4900M, Catalyst 4948E, Supervisor Engine 6-E, and Supervisor Engine 6L-E have 512 K
(524,288) queue entries of which the system sets aside 100 K (102,400) queue entries in a free reserve
pool. Of the remaining 412 K (421,88), the drop port is provided 8184 entries and the CPU ports are
assigned 11704 entries. Supervisor Engine 7-E has 1M (1,048,576) queue entries of which the system
sets aside 100K (102,400) queue entries in a free reserve pool. Of the remaining queue entries, the drop
port is provided 8184 entries, 24576 entries for recirculation ports and the CPU ports are assigned 8656
entries. Supervisor Engine 7L-E has a 512 K queue entries.
The remaining entries are divided equally among the slots in the chassis. In a redundant chassis the two
supervisor slots are treated as one for the purpose of this entries distribution. Within each slot the number
of queue entries are equally divided among the front-panel ports present on the line card in that slot.
When the user configuration for queue entries on an interface exceeds its dedicated quota, the system
attempts to satisfy the configuration from the free reserve pool. The entries from the free reserve pool
are allocated to interfaces on a first-come first-served basis.
Service Policy Association
When a QoS service-policy with queuing actions is configured, but no explicit queue-limit command is
attached in the egress direction on a physical interface, each of the class-based queues gets the same
number of queue entries from within the dedicated quota for that physical port. When a queue is
explicitly given a size using the queue-limit command, the switch tries to allocate all the entries from
within the dedicated quota for the interface. If the required number of entries is greater than the
dedicated quota for the interface, the switch tries to allocate the entries from the free reserve.
The queue entries associated with a queue always have to be consecutive. This requirement can result in
fragmentation of the 512K of the queue entries that are shared across the switch. For example, an
interface may not have enough entries for a queue in its dedicated quota and thus have to use the free
reserve to set up that queue. In this case, the queue entries from the dedicated quota remain unused
because they cannot be shared with any other port or slot.
When the QoS service-policy associated with an interface is removed, any queue entries taken from the
free reserve are returned to the free reserve pool. The interface queuing configuration reverts to two
queues — class-default and the control-packet queue with default shape, bandwidth, and size. The
control-packet queue is set up with size 16 whereas the default queue is set up with the maximum size
possible based on the dedicated quota for that interface.
Queue Allocation Failure
The switch might not be able to satisfy the explicit queue size required on one or more queues on an
interface because of fragmentation of queue memory or lack of enough free reserve entries. In this
scenario, the switch logs an error message to notify you of the failure. The QoS service-policy is left
configured on the interface. You can fix the error by removing the QoS service-policy and examining the
current usage of the queue entries from the free reserve by other ports on the switch.
To configure class-level queue-limit in a service policy, perform this task:
OL-25340-01
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
Configuring QoS
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