Poll a router and a switch with SNMP gets, a getnext and a set

Steps

1. Drag each device from the Topology onto the workspace and set its display name (click the device, Config tab, Display Name) to the name in the Devices table. Cable the ports the Links table lists: Packet Tracer's automatic connection type picks the right cable, or use a copper cross-over between two routers or between a router and a PC or server. If a router asks whether to enter the initial configuration dialog, answer no.
2. Address R1 and give S1 a management address on VLAN 1. Give PC1 its address and gateway (Desktop > IP Configuration).

Router> enable
Router# configure terminal
Router(config)# hostname R1
R1(config)# interface g0/0/1
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# end

Switch> enable
Switch# configure terminal
Switch(config)# hostname S1
S1(config)# interface vlan 1
S1(config-if)# ip address 192.168.10.5 255.255.255.0
S1(config-if)# no shutdown
S1(config-if)# exit
S1(config)# ip default-gateway 192.168.10.1
S1(config)# end
C:\> ping 192.168.10.5

3. Make both devices agents. R1 gets a read-only community and, for now, a read-write one so you can see a set work; S1 gets read-only only. These strings are lab-only:

R1# configure terminal
R1(config)# snmp-server community lab-only-ro RO
R1(config)# snmp-server community lab-only-rw RW
R1(config)# end
S1# configure terminal
S1(config)# snmp-server community lab-only-ro RO
S1(config)# end

4. Break and fix: the wrong community. On PC1 open Desktop > MIB Browser and click Advanced. Set Address 192.168.10.1, Port 161, Read Community public, SNMP Version v2c, and click OK. In the MIB tree open .iso.org.dod.internet.mgmt.mib-2.system, select sysName, choose Get as the operation and click GO. No value comes back: R1 silently ignores a request with a community it does not know, so to the manager it looks exactly like a device that is down. Open Advanced again, set Read Community to lab-only-ro, and repeat the get. The result table now shows sysName with the value R1.
5. Get sysUpTime the same way. The value is in timeticks, hundredths of a second. Convert it on paper as lesson 27 does: divide by 100 for seconds, then take out days (86,400 seconds), hours (3,600) and minutes (60). Get sysUpTime again a minute later and confirm it grew by about 6,000 ticks.
6. Walk one step with getnext. Select sysDescr, choose Get Next and click GO. The agent answers with the object after sysDescr in the tree, not sysDescr itself. Repeat Get Next from each answer to walk through the system group: that is how a manager reads a table it does not know the size of.
7. Poll the switch too. In Advanced set Address 192.168.10.5 with Read Community lab-only-ro and get sysName. The value is S1. A manager polls every device this way to build its inventory.
8. Write with a set. Point the browser back at 192.168.10.1. Set Write Community to lab-only-ro, select sysLocation, choose Set, enter the value lab-bench-2 as an OctetString and click GO: the set is refused, because a read-only community cannot write. Change Write Community to lab-only-rw and repeat the set, then get sysLocation to read back lab-bench-2.
9. See the clear text. Switch Packet Tracer to Simulation mode, set the event list filter to SNMP only, and repeat the sysName get. Click the SNMP packet and open its details: it is UDP to destination port 161, and with SNMPv2c nothing in it is encrypted, including the community string. Return to Realtime mode.
10. Apply lesson 27's good practice: no read-write community unless a tool needs one.

R1# configure terminal
R1(config)# no snmp-server community lab-only-rw
R1(config)# end

Repeat the set with Write Community lab-only-rw. It is now refused as well.