Technomyths

This article discusses the campus protocols found in almost all enterprise networks today including spanning tree protocol, ethernet and gigabit optical fiber with a description of the current specifications and process.

Spanning Tree Protocol

Spanning Tree is an algorithm that runs on Layer 2 campus switches for preventing Layer 2 loops and broadcast storms on a network with at least 2 switches or bridges. The algorithm determines what ports at each switch or bridge must be blocked to create a loop free topology. Primary specifications are IEEE and 802.1d. The spanning tree port states are listening, learning, blocking, forwarding or disabled. Each group of switches will elect a root bridge as part of determining what switch ports must be blocked for a loop free topology. The root bridge is selected by default as the switch with the lowest priority (MAC address). Root bridge selection can be changed with the bridge priority command. It is important to consider your campus switch design and if necessary modify the root bridge selection for performance.

Newer switches that use VLAN's run one spanning tree instance per VLAN. This is important to note since Layer 2 loops occur per VLAN or segment. This allows a switch with dual connections to the same switch or different switches to load balance across those links without the concern of creating a broadcast storm. This is accomplished by assigning two VLAN's to a switch that has dual links to different campus core switches. Each link is configured to forward traffic from one VLAN and block it on the second link. For instance, consider a switch that is assigned 24 ports to VLAN 10 and 24 ports to VLAN 20 and a supervisor engine with 2 Gigabit ports that connect to different core switches. VLAN 10 is assigned to port A and VLAN 20 to port B. If traffic from VLAN 10 were allowed on port B as well, you would have a loop since traffic could leave on port A and return on port B assuming there is a trunk connection between the campus core switches.

Ethernet

This is the most popular campus data link protocol running today. The primary specifications today are 10BaseT, 100BaseT, and Gigabit Ethernet. Many networks today are comprised of 10BaseT and 100BaseT from the desktop and server to the campus switch. Gigabit Ethernet is used from switch to switch and building-to-building. 10BaseT is a specification for 10 Mbps over unshielded twisted pair category 3 cable (UTP). The distance from desktop to switch is 100 meters maximum. The 100BaseT specification is the same except the speed is 100 Mbps over unshielded twisted UTP) pair category 5 cable. Gigabit is a specification for 1000 Mbps across Fiber, STP and UTP cabling. Ethernet uses CSMA/CD as a method of dealing with collisions when two desktops are sending data simultaneously. Both workstations wait for a specified and different length of time before attempting re-transmission. The maximum packet size or MTU of an Ethernet packet is 1518 Bytes.

Optical Fiber Technologies

Gigabit Ethernet uses Multi-Mode Fiber (MMF) and Single Mode Fiber (SMF) for data transmission. Single Mode Fiber uses only one mode of light to travel across the fiber strand using a laser as the light source. Multimode allows for many modes of light to travel across the fiber strand at different angles using an LED as a light source. Modal dispersion results from different light modes transmitted across a fiber strand. The result is that Single Mode Fiber is higher bandwidth and transmits across greater distances. Multi-Mode Fiber is supported with 62.5 micron and 50 micron diameter fibers. The 50-micron fiber will transport across longer distances than 62.5 micron fiber. Multi Mode Fiber uses Short Wave Lasers (SX) and Long Wave Lasers (LX) for Gigabit transmission. Single Mode Fiber is supported with a 9 micron diameter fiber core. It uses long wave lasers to send data between buildings that are 10 kilometers apart.