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The Catalyst 3550 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.
To implement QoS, first, the Catalyst 3550 switches identify traffic flows, 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 Transmission
Control Protocol (TCP)/User Datagram Protocol (UDP) port. At the ingress, the Catalyst 3550 will also perform policing and marking of the packet.
Control plane and data plane 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 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 switches support Weighted Round Robin (WRR) scheduling
and strict priority queuing. 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. Strict priority queuing ensures that the highest priority
packets will always get serviced first, ahead of all other traffic, and allows the other three queues to be serviced using WRR scheduling. In
conjunction with scheduling, the Catalyst 3550 Gigabit Ethernet ports support congestion control via Weighted Random Early Detection (WRED).
WRED avoids congestion by setting thresholds at which packets are dropped before congestion occurs.
These features allow network administrators to prioritize mission-critical and/or bandwidth-intensive traffic, such as Enterprise Resource Planning
(ERP) (Oracle, SAP, etc.), voice (IP telephony traffic) and CAD/CAM over less time-sensitive applications such as FTP or e-mail (Simple Mail
Transfer Protocol [SMTP]). For example, it would be highly undesirable to have a large file download destined to one port on a wiring closet switch
and have quality implications such as increased latency in voice traffic, destined to another port on this switch. This condition is avoided by ensuring
that voice traffic is properly classified and prioritized throughout the network. Other applications, such as Web browsing, can be treated as low
priority and handled on a best-efforts basis.
The Cisco Catalyst 3550 is capable of performing rate limiting via its support of the Cisco Committed Information Rate (CIR) functionality.
Through CIR, bandwidth can be guaranteed in increments as low as 8 Kbps. Bandwidth can be allocated based on several criteria including MAC
source address, MAC destination address, IP source address, IP destination address, and TCP/UDP port number. Bandwidth allocation is essential
in network environments requiring service-level agreements or when it is necessary for the network manager to control the bandwidth given to
certain users. Each Catalyst 3550 switch 10/100 port supports 8 aggregate or individual ingress policers and 8 aggregate egress policers. Each
Catalyst 3550 Gigabit Ethernet port supports 128 aggregate or individual policers and 8 aggregate egress policers. This gives the network
administrator very granular control of the LAN bandwidth.
NETWORK SCALABILITY THROUGH HIGH-PERFORMANCE IP ROUTING
With hardware-based IP routing and the Enhanced Multilayer Software Image, the Catalyst 3550 switches deliver high performance dynamic
IP routing. The Cisco Express Forwarding (CEF)-based routing architecture allows for increased scalability and performance. This architecture
allows for very high-speed lookups while also ensuring the stability and scalability necessary to meet the needs of future requirements. In addition
to dynamic IP unicast routing, the Catalyst 3550 Series is perfectly equipped for networks requiring multicast support. Multicast routing protocol
(PIM) and Internet Group Management Protocol (IGMP) snooping in hardware make the Catalyst 3550 Series switches ideal for intensive multicast
environments.
These switches offer several advantages to improve network performance when used as a stackable wiring closet switch or as a top-of-the-stack
wiring closet aggregator switch. For example, implementing routed uplinks from the top of the stack will improve network availability by enabling
faster failover protection and simplifying the Spanning-Tree Protocol algorithm by terminating all Spanning-Tree Protocol instances at the
