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WHy DHCP relay configs on Switchs that do not route packets?

 
fborup
Occasional Visitor

WHy DHCP relay configs on Switchs that do not route packets?

I have a DHCP relay configuration - Switchs in Core and Edge topology and i´m trying to figure out what the previous sysadmin did:


I have 8 * HP E2620-48-PoEP Switch(J9627A) as EDGE Switchs, doing only Layer2
I have 2 * HP A5500-48G EI, stacked, as CORE Switchs, also doing Laye3 routing

I Have several VLANs, and a Windows DHCP Server configured to lease IP for all VLANs and is working perfectly

But recently, i found something weird in the configs

1) I found ip helper configs pointing to my DHCP Server on all EDGE switches (that ones that do not route, the "CORE" switchs do the routing/layer 3 processing), so i think maybe it´s an error from the past, am I right?

2) The config is on the SAME VLAN the DHCP Server is running, so, again, looks like a ghost from the past or the crazyness of a sysadmin, am I right?


A sample:

=======================================
vlan 800
name "LEGACY"
untagged 9,46
tagged 38,Trk1
no ip address
ip helper-address 1.2.3.4
exit
no dhcp config-file-update
=======================================

 

In the "CORE" Switch, i have the config, i´m supposed to be right:

=======================================
dhcp relay server-group 0 ip 1.2.3.4

interface Vlan-interface107
ip address 10.121.131.1 255.255.255.0
dhcp select relay
dhcp relay server-select 0
=======================================


But even in the "CORE" Switch, again, another wierd stuff (at least to me): why does SOME VLANs have the udp-helper configurarion? The DHCP is also a DNS/NTP Server, but i can´t understand why is that, the documentation is crystal clear about not using udp helper for DCHP purposes, so i have no idea on why is that


====================================================
interface Vlan-interface700
ip address 10.121.62.1 255.255.255.0
dhcp select relay
dhcp relay server-select 0
udp-helper server 1.2.3.4
====================================================

 

 

 

1 REPLY 1
Michael Patmon
Trusted Contributor

Re: WHy DHCP relay configs on Switchs that do not route packets?

Hello.  Basically you are correct on all your points.  To your questions:

1.  You can still do DHCP relay on switches that do not route, it's just more common to do it at the router.  In the sample "vlan 800" config it would not work though as there is no IP address to relay from.

2.  Yes, if there is a local DHCP server on the VLAN there there is no need to relay.

 

UDP helper is basically the same function as DHCP relay but is used to forward some other UDP application.  No sure in the example config what UDP port they wanted to forward.