Switching Technologies
17
Bridged networks break up collision domains, but the network is still one
large broadcast domain. Similarly, layer 2 switches (bridges) cannot break
up broadcast domains, which can cause performance issues and limits the
size of your network. Broadcast and multicasts, along with the slow conver-
gence of spanning tree, can cause major problems as the network grows.
Because of these problems, layer 2 switches cannot completely replace rout-
ers in the internetwork.
Routing
We want to explain how routing works and how routers work in an inter-
network before discussing layer 3 switching in the next section. Routers and
layer 3 switches are similar in concept but not design. In this section, we'll
discuss routers and what they provide in an internetwork today.
Routers break up collision domains like bridges do. In addition, routers
also break up broadcast/multicast domains.
The benefits of routing include:
Break up of broadcast domains
Multicast control
Optimal path determination
Traffic management
Logical (layer 3) addressing
Security
Routers provide optimal path determination because the router examines
each and every packet that enters an interface and improves network seg-
mentation by forwarding data packets to only a known destination network.
Routers are not interested in hosts, only networks. If a router does not know
about a remote network to which a packet is destined, it will just drop the
packet and not forward it. Because of this packet examination, traffic man-
agement is obtained.
The Network layer of the OSI model defines a virtual--or logical--net-
work address. Hosts and routers use these addresses to send information
from host to host within an internetwork. Every network interface must
have a logical address, typically an IP address.
Security can be obtained by a router reading the packet header informa-
tion and reading filters defined by the network administrator (access lists).
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