Learn Computer Networking PNETLab - Infrastructure Services (DHCP Server, DHCP Relay & NTP/Syslog)

#Introduction

A network that only contains routing and switching is not yet complete. User devices need automatic IP addresses, all routers' clocks must be consistent, and logs must be gathered in one place. All of that is the infrastructure services discussed in episode 18.

We will configure a Cisco router as a DHCP server, build a DHCP relay between VLANs with ip helper-address, then close with NTP time synchronization and centralized log collection through syslog. These services are what make a network feel "alive" to its users.

#Configuring a Cisco Router as a DHCP Server

#Creating a Pool and Excluded Addresses

A Cisco router can hand out IP addresses automatically to entire VLANs. Configuration on R1 for VLAN 10:

configure terminal
ip dhcp excluded-address 192.168.10.1 192.168.10.10
ip dhcp pool LAN_POOL
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 8.8.8.8
exit

ip dhcp excluded-address excludes the range used by the server and gateway, while ip dhcp pool LAN_POOL defines the pool with network, default-router, and dns-server. Hosts set to DHCP automatically receive the IP, gateway, and DNS from this pool.

#Verifying DHCP Leases

After a PC in VLAN 10 is set to DHCP, check the address assignment:

R1# show ip dhcp binding
IP address     Hardware address      Lease expiration  Type
192.168.10.11  0050.7966.6800        08:12:35 UTC      Automatic

show ip dhcp binding displays the addresses already handed out along with the client MAC address and lease expiration time.

#DHCP Relay Agent (ip helper-address)

#Forwarding Broadcast Requests Between VLANs

DHCP discovery is a broadcast. Broadcasts never cross a router, so clients in another VLAN cannot reach a centralized DHCP server. The solution: make the router interface a DHCP relay that converts the broadcast into a unicast toward the server.

On R2, interface g0/0 faces VLAN 20 and the DHCP server is at 192.168.10.1:

configure terminal
interface g0/0
 ip helper-address 192.168.10.1
exit

ip helper-address 192.168.10.1 forwards DHCP requests from VLAN 20 to the central server. A single centralized DHCP server architecture now serves many VLANs — a very common pattern in enterprise networks.

#Network Time with NTP

#Synchronizing the Time of All Devices

Logs with inconsistent timestamps cannot be analyzed. NTP (Network Time Protocol) synchronizes the clocks of all devices to a single source. Make the router an NTP client toward a time server:

configure terminal
ntp server 192.168.10.100
ntp update-calendar
exit

ntp server 192.168.10.100 points to the NTP server, and ntp update-calendar saves the synchronized time to the hardware clock. Verify the synchronization:

R1# show ntp status

show ntp status displays the synchronization state and the time source. When all routers use the same NTP, timestamps across the entire lab become comparable.

#Centralized System Logging

#Sending Logs to a Syslog Server

Router logs that only exist locally will be lost at reboot. Point the logs at a centralized syslog server so all events are gathered in one place:

configure terminal
logging host 192.168.10.100
logging trap informational
service timestamps log datetime msec
exit

logging host 192.168.10.100 directs the logs to the syslog server, logging trap informational sets the level of events sent, and service timestamps log datetime msec adds precise timestamps to every log line. With NTP and syslog running together, the entire history of network events can be sorted and analyzed.

#Testing All Services in PNETLab

Build one lab containing R1 (DHCP server, NTP, syslog), R2 (relay), and two PCs in different VLANs. Set the PCs to DHCP: both must receive IPs from the central pool. Check show ip dhcp binding, then generate a log (for example by shutting down an interface) and observe the new log lines appear on the syslog server. The three services running together indicate your lab infrastructure is healthy.

#Closing

Key takeaways:

• ip dhcp pool defines the network, gateway, and DNS for clients.
• ip dhcp excluded-address reserves a range for servers and gateways.
• ip helper-address forwards DHCP broadcasts between VLANs to a central server.
• ntp server synchronizes time; show ntp status verifies it.
• logging host gathers logs to a centralized syslog server.
• NTP and syslog together produce consistent, analyzable logs.

In the next episode, episode 19, we secure and monitor the network: Network Management, SNMP & AAA — the SNMP manager and agent concepts, SNMPv2c community configuration and the advantages of SNMPv3, an introduction to AAA with TACACS+ and RADIUS, and hardening Cisco devices by disabling unused services.