tutorials: FAQ3

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Ethernet Frequently Asked Questions:

Hardware Basics

What is a transceiver?
What exactly does a repeater do?
What is a "hub"?
What exactly does a bridge do?
What does a "learning bridge" do?
What is a remote bridge?
Is there a maximum number of bridges allowed on a network?
What exactly does a router do?
So should I use a router or a bridge?
Are there problems mixing bridging & routing?

Q: What is a transceiver?

A: A transceiver allows a station to transmit and receive to/from the common medium. In addition, Ethernet transceivers detect collisions on the medium and provide electrical isolation between stations. 10Base2 and 10Base5 transceivers attach directly to the common bus media, though the former usually use an internal transceiver built-onto the controller circuitry with a "T" connector to access the cable, while the latter use a separate, external transceiver and an AUI (or transceiver) cable to connect to the controller. 10BaseF, 10BaseT and FOIRL also usually use internal transceivers. Having said that, there also external transceivers for 10Base2, 10BaseF, 10BaseT and FOIRL that can connect externally to the controller’s AUI port, either directly or via an AUI cable.

Q: What exactly does a repeater do?

A: A repeater acts on a purely electrical level to connect to segments. All it does is amplify and reshape (and, depending on the type, possibly retime) the analog waveform to extend network segment distances. It does not know anything about addresses or forwarding, thus it cannot be used to reduce traffic as a bridge can in the example above.

Q: What is a "hub"?

A: A hub is a common wiring point for star-topology networks, and is a common synonym for concentrator 10BaseT Ethernet and 10BaseF Ethernet and many proprietary network topologies use hubs to connect multiple cable runs in a star-wired network topology into a single network hub. Hubs have multiple ports to attach the different cable runs. Some hubs (such as 10BaseT ) include electronics to regenerate and retime the signal between each hub port.

Q: What exactly does a bridge do?

A: A bridge will connect to distinct segments (usually referring to a physical length of wire) and transmit traffic between them. This allows you to extend the maximum size of the network while still not breaking the maximum wire length, attached device count, or number of repeaters for a network segment.

Q: What does a "learning bridge" do?

A: A learning bridge monitors MAC (OSI layer 2) addresses on both sides of its connection and attempts to learn which addresses are on which side. It can then decide when it receives a packet whether it should cross the bridge or stay local (some packets may not need to cross the bridge because the source and destination addresses are both on one side). If the bridge receives a packet that it doesn’t know the addresses of, it will forward it by default.

Q: What is a remote bridge?

A: A bridge as described above that has an Ethernet interface on one side and a serial interface on the other. It would connect to a similar device on the other side of the serial line. Most commonly used in WAN links where it is impossible or impractical to install network cables. A high-speed modem (or T1 DSU/CSU’s, X.25 PAD’s, etc) and intervening telephone lines or public data network would be used to connect the two remote bridges together.

Q: Is there a maximum number of bridges allowed on a network?

A: Per IEEE 802.1 (d), the maximum number of concatenated brides in a bridged LAN is 7. This number is rather arbitrary, however, and is based on simulations of application performance with expected bridge delays. In addition, the number assumes that all bridges are LOCAL (no remote WAN connections), and that the default Hold Time of 1 second is in place (this is the time after which a bridge will discard a frame it is holding). This prevents extra-late frame delivery. (i.e., a frame should never be delivered more than ~7 seconds after is it sent). I personally find this to be much too long an allowance. My "rule of thumb" for bridged LANs is to limit the number of hops to 4, with not more than one of these being a WAN linked remote bridge.

Q: What exactly does a router do?

A: Routers work much like bridges, but they pay attention to the upper network layer protocols (OSI layer 3) rather than physical layer (OSI layer 1) protocols. A router will decide whether to forward a packet by looking at the protocol level addresses (for instance, TCP/IP addresses) rather than the MAC address. Because routers work at layer 3 of the OSI stack, it is possible for them to transfer packets between different media types (i.e., leased lines, Ethernet, token ring, X.25, Frame Relay and FDDI). Many routers can also function as bridges.

Q: So should I use a router or a bridge?

A: There is no absolute answer to this. Your network layout, type and amount of hosts and traffic, and other issues (both technical and non-technical) must be considered. Routing would always be preferable to bridging except that routers are slower and usually more expensive (due to the amount of processing required to look inside the physical packet and determine which interface that packet needs to get sent out), and that many applications use non-routable protocols (i.e., NetBIOS, DEC LAT, etc.). Rules of thumb: Bridges are usually good choices for small networks with few, if any, slow redundant links between destinations. Further, bridges may be your only choice for certain protocols, unless you have the means to encapsulate (tunnel) the unroutable protocol inside a routable protocol. Routers are usually much better choices for larger networks; particularly where you want to have a relatively clean WAN backbone. Routers are better at protecting against protocol errors (such as broadcast storms) and bandwidth utilization. Since routers look deeper inside the data packet, they can also make forwarding decisions based on the upper-layer protocols. Occasionally, a combination of the two devices are the best way to go. Bridges can be used to segment small networks that are geographically close to each other, between each other and the router to the rest of the WAN.

Q: Are there problems mixing bridging & routing?

A: Only if you plan on having bridged links in parallel with routed links. You need to be very careful about running bridges providing links in parallel to a router. Bridges may forward broadcast requests, which will confuse the router there are, lots of protocols you may not think of filtering (e.g. ARP, Apple ARP over 802.3 etc. etc.). Also, DECnet routers have the same MAC address on all ports. This will probably cause the bridge to think it is seeing an Ethernet loop.