SVI and DHCP issues on trunked switches

config t · January 26, 2015, 05:04:38 AM

Just wondering if anyone else has come across this..

I have two switches trunked together, the data VLAN is the same on both, but they are in different subnets.

Example:
Switch1:

interface vlan 128
ip address 10.10.10.1 255.255.255.0
ip helper-address 10.10.0.1

Switch 2:

interface vlan 128
ip address 22.22.22.1 255.255.255.0
ip helper-address 10.10.0.1

Everything is routing properly, but even with a static reservation in DHCP and a static VLAN reservation on the access port I have devices pulling, at random, from either SVI. There is no DHCP service running at the switch or router level, it's pointing to a server.

I don't get it, and better yet, how do I make it stop?

javentre · January 26, 2015, 06:12:19 AM

Quote from: config t on January 26, 2015, 05:04:38 AMI have two switches trunked together, the data VLAN is the same on both, but they are in different subnets.

If the VLAN number is the same, but they have 2 different IP subnets, and they're interconnected with a trunk - then you have 2 subnets in 1 VLAN.

Quote from: config t on January 26, 2015, 05:04:38 AMeven with a static reservation in DHCP and a static VLAN reservation on the access port I have devices pulling, at random, from either SVI. There is no DHCP service running at the switch or router level, it's pointing to a server.

I don't get it, and better yet, how do I make it stop?

It's probably using the first DHCP answer that it gets back. This behavior would be expected with your configuration.

icecream-guy · January 26, 2015, 07:33:57 AM

you should renumber one of the VLAN's to fix the issue.

config t · January 28, 2015, 08:21:43 PM

Thank you for the replies.

I wasn't aware it would behave that way, good to know. Some of the issues I was having in a couple other buildings now makes sense as well.

wintermute000 · January 28, 2015, 11:40:34 PM

There's actually nothing fundamentally technically broken re: running multiple subnets in the same VLAN, its just messy and confusing

Juniper lets you configure as many IPv4 addresses as you want on an interface

And with IPv6 you can have as many as you want as well, its part of the basic design.....

sgtcasey · January 29, 2015, 05:04:49 PM

Quote from: wintermute000 on January 28, 2015, 11:40:34 PM
Juniper lets you configure as many IPv4 addresses as you want on an interface

ip address <ip address> <mask> secondary

This does the same thing for a Cisco VLAN interface. I've seen this way too many times in poorly implemented networks.

wintermute000 · January 29, 2015, 07:45:21 PM

I know, plus love how it doesn't show in show ip interface brief

config t · January 31, 2015, 06:07:15 AM

Quote from: wintermute000 on January 29, 2015, 07:45:21 PM
I know, plus love how it doesn't show in show ip interface brief

Noted.