Routing

Routing
We want to explain how routing works and how routers work in an internetwork before discussing
layer 3 switching next. 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 as bridges do. In addition, routers also break up broadcast/
multicast domains.
The benefits of routing include:
 Breakup 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 segmentation 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 management
is obtained.
The Network layer of the OSI model defines a virtual—or logical—network 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 information and reading
filters defined by the network administrator (access control lists).