Tuesday, January 31, 2006


Troubleshooting Cisco IOS DHCP Relay. (Question #6)

A Cisco IOS router has been configured to act as a DHCP relay bewteen a DHCP client and a DHCP server. Assume that the server and client are configured properly. Given the following configuration on the DHCP relay router, can you determine why the client is unable to obtain an IP address from the DHCP server?

DHCP-RELAY#sh runn
Building configuration...

Current configuration : 660 bytes
hostname DHCP-RELAY
interface Ethernet0
ip address
ip access-group 101 in
ip helper-address
no keepalive
interface Ethernet1
ip address
no keepalive
access-list 101 deny udp any any


This is a good site, I hope you keep more of these. I've been answering the easy ones.

You're access-list 101 will deny any UDP packets coming into the E0 interface. This will drop any UDP packets involved in the later stages of DHCP negotiations.
Thanks for your kind words regarding this blog!

Your answer is correct. The only clarification I would make is that UDP is used by DHCP from the get-go and not just in the later stages and so all DHCP packets are dropped; not just the later ones.
Actually, one quick thing to mention though.. the ACL has an implicit deny! So actually ALL traffic (not just UDP) is blocked! So DHCP or not, nothing would make it through.
Ryan, that's a good point. I should have put an explicit statement to allow all IP packets after the deny for the UDP packets for that access list to be practical. Thanks for catching that!
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