Ethernet Switches & Routers

Brocade FastIron - Ip Helper (DHCP relay)

by gfcm on ‎04-29-2011 12:09 AM (1,413 Views)

Background information

Traditional DHCP requests from a client must find a DHCP server using a Layer 2 broadcast (thus in the same subnet). So, if  the router is not able to function as a proxy for the broadcasts, it would be necessary to put a DHCP server on every network segment where such service is needed. Thanks to the ip-helper command, when a client sends a DHCP request packet to a broadcast address, the router replaces the source address with its own IP address for the interface that received the request. And it replaces the destination with the address specified in the ip-helper command. Thus, as soon as a pool is configured for the given subnet, the DHCP server can retrieve an address for a host that is not in the same subnet.

Equipment used

  • FastIron FCX that runs FCXR07100a.bin (Router)
  • FastIron FCX that runs FCXR07100a.bin (Router that acts as a DHCP Server)

Network Diagram

ip_helper.jpg

Configuration

  • DHCP Server

!

ip dhcp-server enable

!

ip dhcp-server pool vlan4

  dhcp-default-router 192.168.4.1

  excluded-address 192.168.4.1 192.168.4.100

  lease 1 0 0

  network 192.168.4.0 255.255.255.0

  deploy

!

ip route 0.0.0.0 0.0.0.0 192.168.5.2

!

interface ethernet 1/1/5

  ip address 192.168.5.1 255.255.255.252

!


  • Router

!

vlan 4 by port

  untagged ethe 1/1/4

  router-interface ve 4

!

interface ethernet 1/1/5

  ip address 192.168.5.2 255.255.255.252

!

interface ve 4

  ip address 192.168.4.1 255.255.255.0

  ip helper-address 1 192.168.5.1

!

Comments
by paul.nicolette
on ‎05-10-2011 11:01 AM

If you also include the global configuration command

   ip helper-use-responder-ip

then the DHCP client will know the address of the actual DHCP server.  Otherwise the client thinks its DHCP info came from the switch, which really was only the messenger.


by stephen.wong
on ‎05-18-2011 09:17 PM

Hi Paul,

I would like to ask one question.

Based on the above configuration, The FastIron CX role is as DHCP server to assign the IP address for the client.  It means that we do not need to setup a Microsoft DHCP server.  Am I right?

Stephen

by gfcm
on ‎05-19-2011 12:21 AM

Hi Stephen,

You're right, in the configuration above, the router acts as a DHCP server and assigns IP addresses for vlan4 clients. Thus no need to setup a Microsoft or any other DHCP server.

by paul.nicolette
on ‎06-23-2011 01:10 PM

Stephen,

gfcm is indeed correct that the FCX is acting as the DHCP server in the above configuration, so the ip helper-use-responder-ip command is not needed.  But our environment eventually required a Microsoft (or Cisco) DHCP server anyway, and that's when we used the command.

When I tried serving DHCP from our FCX stack with 7.2.0.2a routing code, problems appeared which support said won't be fixed until the "d" release next March.  Our issues included high CPU killing routing on the FCX, non-renewal of DHCP leases by common devices including Linksys and Apple WiFi APs, and non-issuance of new IPs when Windows clients move between DHCP subnets.  This being my first experience with Brocade, I was surprised that DHCP would ship with less-than-robust testing, and that the fixes are promised for March 2012 when the DHCP code was first released in June 2010.

Paul