At first glance at the diagram above, it may seem like Router R1 is not needed for the computers to communicate. However, the network is segmented into VLANs.
You can learn what a VLAN is and how it works in this other post [Read It].
Switch configuration:
interface GigabitEthernet0/1 switchport mode trunk interface FastEthernet0/1 switchport access vlan 10 switchport mode access interface FastEthernet0/7 switchport access vlan 20 switchport mode access interface FastEthernet0/13 switchport access vlan 30 switchport mode access interface FastEthernet0/19 switchport access vlan 40 switchport mode access
Router configuration:
int g0/0 no shut int g0/0.10 encapsulation dot1q 10 ip add 172.17.7.1 255.255.255.192 int g0/0.20 encapsulation dot1q 20 ip add 172.17.7.65 255.255.255.192 int g0/0.30 encapsulation dot1q 30 ip add 172.17.7.129 255.255.255.192 int g0/0.40 encapsulation dot1q 40 ip add 172.17.7.193 255.255.255.192 service dhcp ip dhcp pool POOL10 network 172.17.7.0 255.255.255.192 default-router 172.17.7.1 dns-server 172.17.7.1 ip dhcp excluded-address 172.17.7.1 ip dhcp pool POOL20 network 172.17.7.64 255.255.255.192 default-router 172.17.7.65 dns-server 172.17.7.65 ip dhcp excluded-address 172.17.7.65 ip dhcp pool POOL30 network 172.17.7.128 255.255.255.192 default-router 172.17.7.129 dns-server 172.17.7.129 ip dhcp excluded-address 172.17.7.129 ip dhcp pool POOL40 network 172.17.7.192 255.255.255.192 default-router 172.17.7.193 dns-server 172.17.7.193 ip dhcp excluded-address 172.17.7.193
Useful commands for this exercise:
show ip dhcp binding show arp interface range f0/1-24 encapsulation dot1Q 1 native
Note: the last command tells the router which VLAN a frame belongs to when it arrives without dot1Q encapsulation. Replace 1 with the desired Native (default) VLAN.
Bonus: if a router sits between the client and the DHCP server, it needs to be configured as a DHCP Relay:
interface g0/0 ip helper-address 10.0.0.1
Replace 10.0.0.1 with the IP address of the DHCP server.
The ‘ip helper-address’ command does not only relay DHCP, but also other services:
- Time (port 37)
- TACACS (port 49)
- DNS (port 53)
- BOOTP/DHCP Server (port 67)
- BOOTP/DHCP Client (port 68)
- TFTP (port 69)
- NetBIOS name service (port 137)
- NetBIOS datagram service (port 138)
You can control what gets forwarded (relayed) by using:
ip forward-protocol udp 37 no ip forward-protocol udp 53 ip directed-broadcast