This chapter discusses devices that are used to connect one network to another. The topics of this chapter are:
To help you understand this chapter consider the following definitions:
The chapter concerns the different connection devices used in both networking and internetworking. It is important to know which kind of job a given device does. A previous text stated that "network connectivity devices connect individual devices to a single network", and that "internetwork connectivity devices connect multiple independent networks together to provide access to remote resources". Those are valuable statements that you should use to sort out devices. Most devices fall into one category or the other, but we will discuss a couple of devices that can be used in both.
Modems, for example, can be used to dial in to networks, instead of using a standard connection. This is most common for mobile or remote users. (Remote users dial in to use one network, mobile users dial in to use parts of several networks. This distinction is made by several texts.) This makes a modem a network connection device, not an internetworking device.
Each kind of medium used in networking has a maximum segment length. Multiple segments can be joined together, creating a larger network than a single segment can support. These connectivity devices can do this:
To connect to specific devices, some kind of addressing must be used. Devices that work on the Data Link layer of the OSI model use the MAC address of a device, which is typically set permanently on the device's NIC. This kind on addressing is used inside networks.
A repeater is discussed on page 6-4. It can be an amplifier, which will amplify static as well as signal, or a signal regenerating repeater, which repeats only the signal. Note that rules exist that limit the number of repeaters allowed on a network. A repeater is used to link segments of the same network. It is a networking device, not an internetworking device.
Repeaters are placed at the ends of LAN segments, joining two pieces that would be too long together to be one segment. There are differences between repeaters on Ethernets and token rings:
Bridges connect network segments together and act as filters, to minimize traffic. Without such filters, all traffic on the net would go to all stations on the net, on all segments. Bridges connect two LAN segments for convenience, and filter traffic so that not all signals have to appear on both segments. Since bridges use hardware addresses to make their decisions, bridges are considered Data-Link layer devices. (They depend on Physical layer processes, too, but their work happens at the Data-Link layer.)
In general, a bridge would prevent traffic on segment A from reaching segment B, unless it was meant for a station on segment B. (In that case, all stations on both segments would get the traffic.) Bridges work by learning the MAC addresses of devices on each side of them. Traffic that begins on one side, meant for another device on the same side, is prevented from crossing the bridge. This improves performance on baseband networks. Normally, only one signal can be sent at a time. The bridge allows for simultaneous signals on each side of it, provided that those signals do not need to cross over to the other segment.
You should be aware of the 80/20 Rule. Bridges are most useful if they follow this rule. You should place bridges and nodes so that 80% of the traffic on a bridged segment stays on that segment (is local to that segment), and only 20% of the traffic should have to cross the bridge. If this rule is violated, the bridge is in the wrong place, a node is on the wrong segment, or perhaps you need a repeater instead of a bridge.
Bridges can also serve as translating devices. A translating bridge links two segments that do not use the same protocols and translates between them.
A tunneling or encapsulating bridge wraps an electronic envelope around packets to make them acceptable to a dissimilar segment. To use the example in your text, a packet may start on a token ring segment (SegA), be wrapped by a bridge (B1) in an Ethernet wrapper to pass across an Ethernet segment (SegB), then be received by a second bridge (B2) that removes the wrapper and passes the packet to another token ring segment (SegC). The drawback to this approach is that the packets are not usable on the intermediate segments, so sending signals directly from a device on SegA to a device on SegB cannot be done this way.
The intelligent part of the bridge, the part that decides whether to forward a frame to a specific segment, is called the Relay Entity. In order for a frame to be forwarded, a list of requirements must be met:
Transparent bridges store the incoming frames, check the above criteria, check the CRC to repair data errors, and forward frames that need forwarding. Because the frame is stored and processed this way, the bridges are operating in a store-and-forward mode. There is an inevitable delay while the checks are made, referred to as the latency of the bridge. The larger the frames, the longer the latency.
The list of devices and their MAC addresses that a bridge builds is called the filtering database. A filtering database starts out empty each time a bridge is restarted, and information is filled in as frames are received from various stations.
Now for the problem: a bridging loop. First, you need to know that redundant bridges can be put between segments, in case one breaks. A bridging loop can be created when packets pass endlessly from one segment to the next across the two bridges. It can also happen if the bridges generate a broadcast storm of new packets. An example: Assume two segments are connected by two bridges. A frame is generated on Segment A from workstation W1, and hits both bridges. Both bridges copy the frame, learn that W1 is on Segment A, and both forward the frame to Segment B. However, each bridge will receive the copy that the other bridge forwards to Segment B. This will cause the bridges to update their databases to show workstation W1 as being on Segment B, and they will forward each these frames back to Segment A. Then the process repeats, again and again and again. This is not good.
To avoid the bridging loop problem, IEEE (Institute of Electrical and Electronics Engineers) standard 802.1d gives us the spanning tree protocol. This says that in each redundant pair of bridges, one is the designated bridge, and the other is the backup bridge. Bridges communicate with bridge protocol data units (BPDUs) to determine which is the designated bridge, and when the backup bridge must take over. The network should be diagrammed like a tree. One bridge is chosen to be the root bridge, which sends configuration messages to designated bridges. The root bridge is chosen by its bridge ID, which is an eight byte (16 hex digits) number composed of two bytes assigned by the administrator and all six bytes from the MAC address of the port adapter (NIC). The bridge in the tree with the lowest bridge ID number is the root bridge. Bridges elect a root bridge by sending packets to all ports proclaiming themselves to be the root bridge. If packets are received from bridges with lower IDs, each bridge will acknowledge by changing its opinion, and sending packets that identify the new candidate as the root bridge. (Note that these packets contain the address for the bridge sending the packets in one field, and the address of the bridge it believes to be the root bridge in another field.) A root bridge will continue to send BPDUs every two seconds, by the IEEE standard.
The terms hub and concentrator are often used for a devices
used to collect connections at one point. Hubs can be described by their
Interconnectivity hardware involves connecting one or more networks together. Three classes of such devices are:
Routers pass signals from one network to another. Routers use software addresses instead of hardware addresses. This makes them independent of protocols used at lower layers. Almost. Example: a transmission is sent from a station on network 1 to a station on network 50. It could pass along any number of routes. What happens is like this
Obviously, this system would fail if routers did not have the ability to learn what routers can reach what networks. Passing a packet from one router to another is called a hop. Routers keep tables of router names, networks those routers can connect to, and how many hops away a network is through a given router. Some routers also track a cost value, which can be based on line speed. Route tables are usually constructed by using a route discovery protocol.
Routing protocols can be static or dynamic. A static protocol would be one in which the administrator defines the routing table. A dynamic protocol could be one of two types:
The count-to-infinity problem exists only in Distance Vector routing. It works like this:
To combat the count-to-infinity problem, three methods are used:
Load balancing is a desireable end. If only the best routes were ever used, then some routes would go unused, which is inefficient. In fact, the best route would become congested, and a lesser route might be preferable. Load balancing attempts to use all routes to better advantage.
Some protocols are described as nonroutable (unroutable). This means that the protocols either do not use dynamic routing, or they have no Network Layer.
Nonroutable protocols can be bridged or encapsulated (tunneled). Brouters and switches can bridge nonrouting protocols.
A comparison of the features of bridges, switches and routers give you reasons for choosing each piece of equipment: