NETWORKING

  • 11/23/2015
    7:30 AM
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Cisco IP Helper Address Complications

In this video, Tony Fortunato shows how using the ip helper address command can overwhelm a DHCP server and cause network performance issues.

Network technicians have used the Cisco ip helper-address command to provide a way for DHCP broadcast packets to traverse routers. Other vendors use dhcp-proxy, dhcp-relay and other similar solutions, but the big difference is that ip-helper forwards bootp/dhcp broadcasts as well as other protocols such as time, NetBIOS, ND, and DNS.

In this video, I demonstrate a common configuration problem I run into with the ip helper-address command. 

I caution my clients to be careful when using ip helper commands since you might overwhelm your DHCP server with other protocols to the point where it cannot reliably respond to DHCP requests.

In the video, I also discuss how I use the IP identifier to correlate multiple trace files and how you can refer to the ICMP payload to see the originating packet that caused the ICMP error.

Just a note of caution: In certain cases, you might not see the ICMP Destination Unreachable packets if ICMP is filtered out by routers or firewalls, or if you have a firewall enabled on the capturing device.

If you find that ip-helper address configuration commands are causing you issues, look into alternative commands like ip forward-protocol udp or other configurations that use dhcp relay commands.


Comments

Local DHCP Pool

Thanks for posting this informative blog, one of the limitation i observed, we cannot configure local DHCP pool for  switch with ip helper address, i.e. I want to send dhcp request information from my clients to my analyser server.

Re: Local DHCP Pool
To capture dhcp packets either I capture from that client or span the clients port
Re: Local DHCP Pool

Thank's @thetechfirm, but i am not sure if it can be done on same switch using ip helper address and and local dhcp pool i.e. in case of dhcp connected locally and analyser being at remote.

Re: Local DHCP Pool

I understand now, yes that one is definitely a lot trickier.