Chapter 4: Managing LAN Traffic

Frames Frame is the term used to describe data transmitted at Layer 2 of the OSI model. The source and destination addresses are MAC addresses. Frames contain information which helps receiving hosts determine if the frame has become corrupted during transmission. Hosts receiving frames drop the destination address of the frame that does not match their MAC address.

Frame Traffic Unicast frames are sent from one host to another single host. Unicast frames are the most common type of LAN traffic. Broadcast frames are sent from one host to all hosts and are not passed across routers. Broadcast frames are addressed to MAC address FF-FF-FF-FF-FF-FF. Multicast frames are sent from one host to a small group of hosts. There can be several multicast groups on a single LAN.

Datagram Also known as a packet, datagrams represent traffic at Layer 4 of the OSI model. Datagrams differ from frames in that they have Layer 4 addressing information, such as a source and destination IPv4 address. As a datagram is passed from router to router, it retains its original IPv4 addressing information. The MAC address information, however, is changed as the router that was once the frame’s destination becomes its source as it travels further across the network.

ARP Request Stands for Address Resolution Protocol. Way that a host determines a computer’s MAC address from its IP address. Broadcast frame sent out with the query “who owns this IP address?” Unicast frame is sent back with MAC address information of target host.

Collision and Broadcast Domains A collision domain is a section of network where packet collisions can occur if two nodes attempt to communicate at the same time. A broadcast domain includes all of the hosts that a broadcast frame transmitted by a single host can reach. As routers do not pass broadcast traffic, they form a boundary of a broadcast domain. All hosts in a broadcast domain share a common Layer 3 network address. In TCP/IP terminology this means that they are on the same subnet.

Bridges Unicast frames only cross the bridge if they are addressed for a host on the other side. No hosts on Segment B will receive unicast frames from host to host communication on Segment A as they will be blocked by the bridge. Bridges are usually used on RG58 networks and are not commonly used today except in wireless networks.

Switches A switch divides a LAN so that each host has its own segment. You connect twisted pair cable with RJ45 connectors to ports on a switch. A switch can have as many as 96 ports. Switches filter traffic based on MAC address. A unicast frame will only be forwarded to the port that connects to the host with the destination MAC address. To hosts on all other ports of the switch, it appears as though there is no network traffic. Only when two separate hosts attempt to communicate with the same third host do collisions occur. Switches learn which MAC addresses are associated with particular ports by noting source addresses as frames enter the switch. Broadcast frames are forwarded to all ports of a switch.

Switching Methods A benefit of switches is that they can provide error correction, dropping corrupt frames before they reach the target host. There are three switching methods. Each is a trade off between error correction and speed. Store and forward switching. Stores the entire frame and performs a CRC check before forwarding. Best error correction, but takes the most time, impacting on speed. Cut through switching. No error correction. Switch only reads address information. Provides best transmission speed, though on networks with a high number of frame errors this will end up slower than other methods. Fragment free switching. Most frame corruption appears in the first 64 bytes of a frame. Fragment free checks only those bytes, discarding frames with errors. A good balance between error correction and speed.

VLAN Stands for virtual local area network. A group of ports on a switch can be configured as a separate broadcast domain. This separate broadcast domain is called a VLAN. VLAN membership can include ports on other switches. Each separate VLAN must have a unique network address. In IPv4 terminology, hosts on each VLAN must all be on the same unique subnet. Traffic traveling between VLANs, even if it is between adjacent ports on the same switch, must be routed.

Routers Routers are used to move datagrams from one broadcast domain to another. Routers decide where to send datagrams based on IPv4 address. Routers have routing tables, which inform them of the location of other networks. When these tables can be programmed in, they are called static routes. Static routes can have problems when routes fail as they can’t adjust automatically. When tables are generated by listening to other routers broadcasting route information, they are called dynamic routes. Dynamic routes can adjust automatically when routes fail, rewriting the routing table as necessary.

Layer 3 and 4 Switching Some switches have the ability to shift traffic at Layer 3 and 4 of the OSI model. There are two Layer 3 switching methods: Packet by packet: The header of each packet is checked and then routed to the appropriate interface. Flow based routing: First packet is routed to appropriate interface. All subsequent packets in the data stream from the original transmitting host are switched to the same interface without examination of the packet header. Layer 4 switching allows traffic to be prioritized based on protocol.

Hop Count Hop count is a measure of the distance, in routers, to a remote network. Simple routing protocols calculate the best route based on the number of hops. More complex routing protocols also take into account things like link bandwidth and reliability. It may be that the quickest way to move data from one network to another is not via the route with the shortest hop count, but via a route with more hops but higher bandwidth.

Reducing Network Traffic Congestion Reducing congestion improves network performance. Reduce the number of nodes on a particular segment by implementing a switch. On networks where there is a lot of broadcast traffic, use VLANs to reduce the size of broadcast domains. Analyze switch logs to determine if you should use a different switching method. Increase the bandwidth of your cabling and switches.

Summary Frames can be unicast, multicast, or broadcast. Unicast frames are 1:1, multicast 1:Many and broadcast 1:All. Frames include Layer 2 addressing information. Datagrams include Layer 3 addressing information. Switches manage traffic at Layer 2. Some switches have Layer 3 and 4 functionality, but are not as suited as routers to managing traffic above Layer 3. Switches can use store and forward, cut through or fragment free switching methods. VLANs allow separate broadcast domains to be located on a single switch. Routers manage traffic at Layer 3. Routers can be configured statically or dynamically. Dynamically configured routers can adjust to changing network conditions.

Discussion Questions What is the difference between a broadcast and a multicast frame? Which switching method provides the best error correction? What is the difference between a switch and a router? Why is hop count not always the best way of determining route? What type of switching allows traffic to be prioritized on the basis of protocol?