Queuing Overview
Queuing configuration usually acts on outbound traffic. Each interface has a
hardware queue, or transmit ring (TxQ), that holds traffic ready to be serialized
onto the interface media. This queue is always First In/First Out (FIFO).
When outbound traffic arrives at an interface, the interface scheduler sends it
to the transmit ring to be placed on the media. If the transmit ring is full,
other traffic must wait in some buffer memory space assigned to that interface
called a software queue. When traffic must be placed into queues, the
interface is said to be congested.
Causes of congestion include:
■ Speed mismatches—Typically LAN traffic needing to go across a
much slower WAN link (persistent congestion). It can also be
GigEthernet traffic bound out a FastEthernet interface (transient
congestion).
■ Link aggregation—For WAN links, this occurs when multiple remote
sites access a hub site across an oversubscribed link. For LAN links,
this typically occurs on a distribution switch with multiple access
switches feeding in to it on the uplink port(s) to the core switch.