The core switch has DHCP

A core switch can act as a DHCP server to assign IP addresses to clients across multiple VLANs, with proper pool, VLAN, and relay configurations.

Enabling DHCP on a Core Switch

To configure a Cisco core switch as a DHCP server, first ensure that IP routing is enabled and the DHCP service is active:

Switch(config)# ip routingSwitch(config)# service dhcp

This allows the switch to route between VLANs and provide DHCP services to connected clients .

Configuring DHCP Pools

For each VLAN, create a DHCP pool specifying the network, default gateway, and DNS servers:

Switch(config)# ip dhcp excluded-address 192.168.10.1 192.168.10.10Switch(config)# ip dhcp pool VLAN10Switch(dhcp-config)# network 192.168.10.0 255.255.255.0Switch(dhcp-config)# default-router 192.168.10.1Switch(dhcp-config)# dns-server 8.8.8.8 8.8.4.4

Excluded addresses prevent conflicts with static IPs, such as the VLAN interface IP .

VLAN Interface Configuration

Assign an IP address to the VLAN interface (SVI) on the core switch:

Switch(config)# interface Vlan10Switch(config-if)# ip address 192.168.10.1 255.255.255.0Switch(config-if)# no shutdown

Ensure the VLAN exists and is active on the switch .

Handling Multiple VLANs

If clients are on different VLANs, DHCP requests may not reach the server due to broadcast boundaries. You can either:

• Configure the DHCP service on the core switch for each VLAN, or
• Use a DHCP relay agent on Layer 3 interfaces to forward requests to a central DHCP server:

Switch(config)# interface Vlan20Switch(config-if)# ip helper-address 192.168.10.1

This ensures clients in other VLANs receive IP addresses from the DHCP server .

DHCP Snooping for Security

Enable DHCP snooping to prevent rogue DHCP servers:

Switch(config)# ip dhcp snoopingSwitch(config)# ip dhcp snooping vlan 10,20Switch(config)# interface GigabitEthernet0/1Switch(config-if)# ip dhcp snooping trust

Trusted interfaces connect to legitimate DHCP servers, while untrusted interfaces are monitored to block unauthorized DHCP messages .

Verification

Check DHCP bindings and pool status:

Switch# show ip dhcp bindingSwitch# show running-config | include dhcp

This confirms which IPs are leased and ensures the DHCP configuration is active .

Best Practices

• Exclude SVI IP addresses and static devices from DHCP pools.
• Use spanning-tree portfast on access ports to speed up client connectivity.
• Monitor DHCP lease usage to prevent exhaustion.
• For IP phones, include options like Option 150 for TFTP server addresses . By following these steps, a core switch can efficiently manage DHCP for multiple VLANs while maintaining network security and reliability.