When configuring VLANs on a layer 3 Cisco switch we often tend to segment our network by grouping devices in certain VLANs for security/departmental/geographical purposes.

Enabling inter-VLAN routing allows all the devices on different VLANs to communicate with each other.

If you’ve placed DHCP devices on a VLAN separate from the DHCP server you’ll notice that those devices will not receive an IP address.

For example, your DHCP server is on VLAN 2 and your users are on VLAN 3. To allow your users’ computers to acquire an IP address from the DHCP server on VLAN 2 you’ll have to configure the ip helper-address command on VLAN 3:

Enter VLAN 3 interface configuration

conf t
int vlan 3

configure the ip helper-address command to point to your DHCP server

ip helper-address 192.168.1.5
exit

When the DHCP devices on VLAN 3 send a broadcast over the network for DHCP, VLAN 3 will forward this request to 192.168.1.5. Be sure to replace 192.168.1.5 with your own DHCP server.

Related posts:

1. How To Configure a Router-on-a-Stick
2. 5 Easy Steps to Securing Your Cisco Switch or Router
3. Virtual LANs Notes

On random, rare, occasions one of our offices will drop off the face of the Earth. After troubleshooting with users in that office, you’ll find that there is always one symptom:

They can browse the web but cannot access any network resources in the company.

What will work 99% of the time is entering these two commands into your Cisco ASA firewall:

clear crypto isakmp sa
clear crypto ipsec sa

What these two lines of code will do is drop ALL site-to-site vpn connections and rebuild the tunnels.
It takes about a minute or so for everything to start working again but these two commands have saved me time and again.

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