Focus on Switches

Introduction. Switches operate in a fundamentally different way than hubs. Signals that enter a hub's port are indiscriminately repeated to the hub's other ports. Switches read both the source port and the destination port of each frame and switch messages between the two.

A switch is able to perform more router like duties than hubs within LANs because

They have more powerful electronics than hubs

They operate at the data link layer using the MAC-Media Access Control address

In fact, by operating at layer 2 they don't need to dig as deep to get the location to which they should be sending the frames/message as routers do.

It is turning out that switches are turning out to be viable replacements for hubs at the low end and routers at the higher end. Switches are replacing hubs because switches implement higher bandwidth. They are also often replacing routers because they are inherently faster there is a push to replace routers with switches when viable. This seems to be most frequently occurring on backbones that connect LANs within a building or campus. In other locations it occurs selectively because switches are usually about 30 to 40 percent more expensive than hubs. The rule of thumb is becoming, "Switch where you can, route when you must."

When a switch receives a frame/message seeking an address it doesn't know, instead of dropping the message it broadcasts it to all of its ports. While switches rely on MAC addresses to locate the destination, which are essentially like serial numbers, they are often within the switch's dynamic address table. If the destination MAC address isn't there then a switch can be set up to explore and map the surrounding neighborhood or access the dynamic address tables of other nearby switches. It is also possible to configure switch servers that support extremely large dynamic MAC address tables and best routes. So particularly if you are moving messages within certain pats of an organization you are more likely to get better performance from switches. Routers are required for more open ended moves using IP addresses to more remote locations.

We are likely get much more into how switches operate and how they operate differently from routers and when each should be used. The following table presents CISCO's offerings for switches.

Cisco Switches
Product Series	Description
Micro Switches	Two models with eight ports. These are desktop devices designed to create high performance LANs. This is the only Cisco series with unmanaged models.
Catalyst 1900 Series	Four models with 12 or 24 10BaseT ports and two uplink ports for 100BaseTX or 100BaseFX. All models hake 1K MAC address cache but aren't stackable.
Catalyst 2820 Series	Four models designed for aggregating 10BaseT hubs to 100BaseT, FDDI or ATM backbones or servers. Contains 24 10BaseT ports plus two slots to accommodate a choice of 100BaseT, FDDI or ATM high speed modules Has 1Gbps backplane and 2K or 8K MAC address cache. Not stackable.
Catalyst 2900 Series	Four models with 12 - 48 10BaseTX, 100BaseTX 100BaseFX ports and two uplink ports for 100BaseX. Have 1.2 - 2.4Gbps backplane. Not stackable.
Catalyst 2900XL Series	Five models in two basic packages with 12 100BaseFX or 24 ports for 10BaseTX/100BaseTX and two uplinks for 100BaseFX. Up to 16K MAC address cache. Has 3.2Gbps backplane. Not stackable.
Catalyst 3100 Series	Has 24 ports for 10BaseT/100BaseT, one StackPort slot for scalability, and one expansion FlexSlot for broad media support. Scalable up to 224 switched Ethernet ports. Stackable up to eight switches.
Catalyst 3200 Series	Has 24 ports for 10BaseT/100BaseT, seven expansion slots. Scalable up to 224 switched Ethernet ports. Stackable up to eight switches.
Catalyst 3500XL Series	Three models, with 12, 24 or 48 10BaseT/100BaseT Ethernet ports, and one model with Gigabit Ethernet ports. 3500 XLs are stackable up to nine units running a switch fabric up to 10Gbps. This series is being positioned as the premier solution for low end Gigabit Ethernet connectivity.
Catalyst 3900 Series	Two models with 20 or 24 fixed Token Ring ports and two slots for expansion modules, each with four ports for additional Token Ring user ports or one ATM OC-3 or two 100BaseX uplink(s). Has 520Mbps backplane. Stackable up to eight switches.
Catalyst 4000 Series	One model with three slot modular chassis supporting 10/100/100 Ethernet. One module has 48 10/100 ports and another has 32 10/100 ports with a variety of 1000BaseX uplink options. 524Gbps backplane. Not stackable.
Catalyst 4840G Series	One model with 40 10/100 ports and two 1000BaseX Gigabit Ethernet ports. Optimized for server farms. Includes QoS and load balancing features.
Catalyst 5000/5500 Family	Two series family with five models, with two to five slots modular chassis with 48 - 528 ports supporting 100BaseX, 1000X, ATM, FDDI, or Token Ring. 1.2 - 3.6 Gbps backplane. Not stackable.
Catalyst 6000 Family	Four model multilayer switch family with six to nine slots supporting 384 10/100 ports, 192 100BaseFX ports or 130 1000BaseX ports. Has 150Mbps throughput and 32Gbps backplane scalable to 256Gbps. Not stackable.
Catalyst 8500 Series	Two models with 5 - 13 slots supporting multiservice ATM switching, optimized for aggregating multiprotocol traffic. Has 6 - 24 Mbps throughput and 10 or 40 Gbps backplane. Not stackable.