The Network Time Protocol (NTP) synchronizes timekeeping among a set of
distributed time servers and clients. This synchronization allows events to be
correlated when system logs are created and other time-specific events occur.
For timestamps to be of use, it is a good idea for all the routers and switches
in the network to derive time from a common network time source.
Configuring time services on routers requires exec and configuration
commands.
To configure the
time zone properties on the router, the configuration commands clock
timezone and clock summer-time are used. The
commands ntp server ip-addr and ntp source
interface define the NTP server(s) and the source IP address of
the NTP requests.
The internal clock of the router is set using the EXEC
command clock set. To view NTP peer status information, use
the show ntp associations and show ntp
status commands.
SNMP
The network management software used on TCP/IP networks is
based on SNMP. SNMP is a client/server protocol. In SNMP terminology, it is
described as a manager/agent protocol. The agent (the server) runs on the
device being managed, which is called the Managed Network Entity. The agent
monitors the status of the device and reports that status to the manager.
The manager (the client) runs on the Network Management Station (NMS). The
NMS collects information from all of the different devices that are being
managed, consolidates it, and presents it to the network administrator. This
design places all of the data manipulation tools and most of the human
interaction on the NMS. Concentrating the bulk of the work on the manager means
that the agent software is small and easy to implement. This is why most TCP/IP
network equipment comes with an SNMP management agent.
The NMS
periodically requests the status of each managed device (GetRequest) and each
agent responds with the status of its device (GetResponse). Making periodic
requests is called polling. Polling reduces the burden on the agent because the
NMS decides when polls are needed, and the agent simply responds. Polling also
reduces the burden on the network because the polls originate from a single
system at a predictable rate. The shortcoming of polling is that it does not
allow for real-time updates. If a problem occurs on a managed device, the
manager does not find out until the agent is polled. To handle this, SNMP uses
a modified polling system called trap-directed polling.
A trap is an
interrupt signaled by a predefined event. When a trap event occurs, the SNMP
agent does not wait for the manager to poll. Instead it immediately sends
information to the manager. Traps allow the agent to inform the manager of
unusual events while allowing the manager to maintain control of polling. SNMP
traps are sent on UDP port 162. The manager sends polls on port 161 and listens
for traps on port 162.
The commands in Figure
display
information about network management applications. A troubleshooter uses the
information from these commands to isolate problems at the application layer
that are related to the SNMP and NTP protocols.
Figure
lists commands
which make configuration changes that troubleshooters can use to correct
problems with network management protocols at the application layer.