17.2. Network partitioning hardware
Network partitioning involves dividing a
single backbone into multiple
segments, joined by some piece of hardware that forwards packets.
There are multiple types of these devices: repeaters, hubs, bridges,
switches, routers, and gateways. These terms are sometimes used
interchangeably although each device has a specific set of policies
regarding packet forwarding, protocol filtering, and transparency on
the network:
- Repeaters
- A repeater joins two segments at the
physical layer. It is a purely electrical connection, providing
signal amplification and pulse "clean up" functions
without regard for the semantics of the signals. Repeaters are
primarily used to exceed the single-cable length limitation in
networks based on bus topologies, such as 10Base5 and 10Base2. There
is a maximum to the number of repeaters that can exist between any
two nodes on the same network, keeping the minimum end-to-end transit
time for a packet well within the Ethernet specified maximum
time-to-live. Because repeaters do not look at the contents of
packets (or packet fragments), they pass collisions on one segment
through to the other, making them of little use to relieve network
congestion.
- Hubs
- A hub joins multiple hosts by acting as a
wiring
concentrator in networks based on star topologies, such as 10BaseT. A
hub has the same function as a repeater, although in a different kind
of network topology. Each computer is connected, typically over
copper, to the hub, which is usually located in a wiring closet. The
hub is purely a repeater: it regenerates the signal from one set of
wires to the others, but does not process or manage the signal in any
way. All traffic is forwarded to all machines connected to the hub.
- Bridges
- Bridges function
at
the data
link layer, and perform selective forwarding of packets based on
their destination MAC addresses. Some delay is introduced into the
network by the bridge, as it must receive entire packets and decipher
their MAC-layer headers. Broadcast packets are always passed through,
although some bridge hardware can be configured to forward only ARP
broadcasts and to suppress IP broadcasts such as those emanating from
ypbind.
Intelligent or learning bridges glean the MAC addresses of machines
through observation of traffic on each interface. "Dumb"
bridges must be loaded with the Ethernet addresses of machines on
each network and impose an administrative burden each time the
network topology is modified. With either type of bridge, each new
segment is likely to be less heavily loaded than the original
network, provided that the most popular inter-host virtual circuits
do not run through the bridge.
- Switches
- You can think of a switch as an intelligent
hub having the functionality of a bridge. The switch also functions
at the data link layer, and performs selective forwarding of packets
based on their destination MAC address. The switch forwards packets
only to the intended port of the intended recipient. The switch
"learns" the location of the various MAC addresses by
observing the traffic on each port. When a switch port receives data
packets, it forwards those packets only to the appropriate port for
the intended recipient. A hub would instead forward the packet to all
other ports on the hub, leaving it to the host connected to the port
to determine its interest in the packet. Because the switch only
forwards the packet to its destination, it helps reduce competition
for bandwidth between the hosts connected to each port.
- Routers
- Repeaters, hubs, bridges, and switches
divide the network into multiple distinct physical pieces, but the
collection of backbones is still a
single
logical network. That is, the IP network number
of all hosts on all segments will be the same. It is often necessary
to divide a network logically into multiple IP networks, either due
to physical constraints (i.e., two offices that are separated by
several miles) or because a single IP network has run out of host
numbers for new machines.
Multiple IP networks are joined by routers that forward packets based
on their source and destination IP addresses rather than 48-bit
Ethernet addresses. One interface of the router is considered
"inside" the network, and the router forwards packets to
the "outside" interface. A router usually corrals
broadcast traffic to the inside network, although some can be
configured to forward broadcast packets to the "outside"
network. The networks joined by a router need not be of the same type
or physical media, and routers are commonly used to join local area
networks to point-to-point long-haul internetwork connections.
Routers can also help ensure that packets travel the most efficient
paths to their destination. If a link between two routers fails, the
sending router can determine an alternate route to keep traffic
moving. You can install a dedicated router, or install multiple
network interfaces in a host and allow it to route packets in
addition to its other duties. Appendix A, "IP Packet Routing" contains a
detailed description of how IP packets are forwarded and how routes
are defined to Unix systems.
- Gateways
- At the top-most level in the network
protocol
stack, a gateway performs forwarding functions at the application
level, and frequently must perform protocol conversion to forward the
traffic. A gateway need not be on more than one network; however,
gateways are most commonly used to join multiple networks with
different sets of native protocols, and to enforce tighter control
over access to and from each of the networks.
Replacing an Ethernet hub with a Fast Ethernet hub is
like
increasing the speed limit of a highway. Replacing a hub with a
switch is similar to adding new lanes to the highway. Replacing an
Ethernet hub with a Fast Ethernet switch
is the equivalent of both
improvements, although with a higher cost.
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