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(OSPF) or Enhanced Interior Gateway Routing Protocol
(EIGRP) rather than relying on standard STP
convergence. Redirection of a packet after a link failure
via a routing protocol results in faster network
convergence than a solution that uses Layer 2 Spanning
Tree enhancements. Additionally, routed uplinks allow
better bandwidth utilization by implementing equal cost
routing (ECR) on the uplinks to perform load balancing.
This results in dynamic load balancing in a part of the
network that often acts as the bottleneck. And, routed
uplinks optimize the utility of uplinks out of the wiring
closet by eliminating unnecessary broadcast data flows
into the network backbone. The Catalyst 3550-12T
can also offer dramatic bandwidth savings as a
top-of-the-stack wiring closet aggregator switch in a
multicast environment. Using routed uplinks to the
network core will eliminate the requirement to transmit
multiple streams of the same multicast from the upstream
content servers to the wiring closet aggregator switch.
For example, if three users are assigned to three
separate virtual LANs (VLANs) and they all want to
view multicast ABC, then three streams of multicast
ABC are required to be transmitted from the upstream
router to the wiring closet aggregator switch—assuming
the aggregator switch is not capable of routed uplinks.
With the Catalyst 3550-12T, a scalable solution in a
multicast-rich network becomes attainable.
Customers who have an existing copper wiring
infrastructure in their buildings now have a truly
compelling option to migrate to Gigabit speeds in their
LANs. The Catalyst 3550-12T can be placed in the
network backbone or used as a top-of-the-stack
aggregator delivering up to 12 Gbps forwarding rate
to a stack of single IP managed switches. In conjunction
with the new Catalyst 2950T-24 and the 1000BaseT
GBIC, the Catalyst 3550-12T provides an integrated
Gigabit Ethernet over Copper solution for those
midmarket customers who have outgrown their Fast
Ethernet backbones.
Enhanced Quality of Service
The Catalyst 3550-12T offers superior Layer 3 granular
quality of service (QoS) features to ensure that network
traffic is classified, prioritized, and congestion is avoided
in the best possible manner. The Catalyst 3550-12T can
classify, reclassify, police, and mark the incoming packets
before the packet is placed in the shared buffer. Packet
classification allows the network elements to discriminate
between various traffic flows and enforce policies based
on Layer 2 and Layer 3 QoS fields. First, the Catalyst
3550-12T identifies “aggregates,” or packet groups, and
classifies or reclassifies these groups using the
Differentiated Services Code Point field (DSCP) and/or the
802.1p Class of Service (CoS) field. Classification and
reclassification can be based on criteria as specific as the
source/destination IP address, source/destination Media
Access Control (MAC) address or the Layer 4 TCP/UDP
ports. At the ingress, the Catalyst 3550-12T will also
perform policing and marking of the packet. Control
plane and data plane Access Control Lists (ACLs) are
supported on all ports to ensure proper policing and
marking on a per packet basis.
After the packet goes through classification, policing, and
marking, it is then assigned to the appropriate queue
before exiting the switch. The Catalyst 3550-12T
supports four egress queues per port, which allows the
network administrator to be more discriminating and
specific in assigning priorities for the various applications
on the LAN. At egress, the switch performs scheduling
and congestion control. Scheduling is an algorithm/
process that determines the order in which the queues are
processed. The Catalyst 3550-12T supports Weighted
Round Robin (WRR) scheduling with future support for
strict priority scheduling. Strict priority queuing ensures
that the highest priority packets will always get serviced
first, ahead of all other traffic. The WRR queuing
algorithm ensures that the lower priority packets are not
entirely starved for bandwidth and are serviced without
compromising the priority settings administered by the
network manager. In conjunction with scheduling, the
Catalyst 3550-12T supports congestion control via
Weighted Random Early Detection (WRED). WRED
avoids congestion by setting thresholds at which packets
are dropped before over-congestion occurs.
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