Routers are internetworking devices that operate at OSI Layer 3, the network
layer. Routers tie together, or interconnect, network segments or entire
networks. They pass data packets between networks based on Layer 3 information.
Routers make logical decisions regarding the best path for the delivery
of data on an Internetwork based on Layer 3 information. Then routers direct
packets to the appropriate output port and segment. Therefore, routing is
sometimes referred to as Layer 3 switching. Routers come in a variety of sizes
and form factors, but have common physical characteristics, such as LAN/WAN
interfaces to provide connectivity between networks.
If IP or other
services can be accessed on the LAN, but Internet access is not available, the
router may be the failure point. Other connectivity issues, such as reaching
other VLANs, can be attributed to a router. In many cases, the router is
configured with access control lists (ACLs) to prevent unauthorized access. In
a very secure network, adding new devices requires planning and coordination.
Always consult the LAN/WAN administrator when connecting new devices to the
LAN.
Routers provide numerous integrated commands to assist in
monitoring and troubleshooting the internetwork, as shown in Figure
. If no
configuration problems exist on the router, other possible problems include
cabling problems at the router or service provider outages.
Using
show Commands
The show
commands, shown in Figure
, are
powerful monitoring and troubleshooting tools. Use the show
commands to perform a variety of functions, including the following:
- Monitor router behavior during initial installation.
- Monitor normal network operation.
- Isolate problem interfaces, nodes, media, or applications.
- Determine when a network is congested.
- Determine the status of servers, clients, or other neighbors.