DHCP Client IP Address Loss on Stack Failover

  • 0
  • 1
  • Question
  • Updated 3 years ago
  • Answered
I have a configuration where I have a set of two stacked X440-48t switches (4 physical switches, 2 sets of 2-stacks). SW1 and SW2

I have a DHCP server connected to SW1 port 1:1

SW1 port 1:48 <-> SW2 port 1:48
SW1 port 2:48 <-> SW2 port 2:48 

Here's the relevant configuration:

Firmware: 15.6.1.4 on both switches
stpd disabled on both switches

SW1:
enable mac-lockdown-timeout ports 1:48
configure mac-lockdown-timeout ports 1:48 aging-time 60
enable sharing 1:48 grouping 1:48, 2:48 algorithm address-based L2
enable dhcp vlan Default

SW2:
enable mac-lockdown-timeout ports 1:48
configure mac-lockdown-timeout ports 1:48 aging-time 60
enable sharing 1:48 grouping 1:48, 2:48 algorithm address-based L2
enable dhcp vlan Default

So the problem happens when I kill the power to the stacking master on SW2 - when I kill the power, the DHCP server can no longer reach SW2. I have a console open to both switches for SW2, I can issue a 'disable dhcp vlan default' and 'enable dhcp vlan default' on the new master switch (the one that remained powered on) and I can then reach the switch again.

I'm wondering if there's a way to automate this without having to console into the failed over switch and forcing a new DHCP request from the master switch.

Any help is appreciated! 

Thanks.
Photo of Karl Witthuhn

Karl Witthuhn

  • 120 Points 100 badge 2x thumb

Posted 3 years ago

  • 0
  • 1
Photo of Patrick Voss

Patrick Voss, Alum

  • 11,594 Points 10k badge 2x thumb
Does the same behavior happen when you simply unplug or disable port 1:48 on SW2? are you continuously pinging the DHCP address and it goes completely down or are you simply trying to connect to the address from the other switch, using telnet?
(Edited)
Photo of Karl Witthuhn

Karl Witthuhn

  • 120 Points 100 badge 2x thumb
Yeah - both SW1 and SW2 are a pair of stacked X440 switches (2 X440's in each stack)

The DHCP server is connected to SW1 port 1:1 

There's a static port-channel between SW1 <-> SW2 (ports 1:48 & 2:48 on both switches) 

Both switches have a DHCP entry on the DHCP server using the stack mac-address (02:04:XX:XX:XX:XX instead of the individual switch mac-address of 00:04:XX:XX:XX:XX) because when the switches are stacked and DHCP is enabled, it will use the stacked mac-address instead of individual mac-address. 

I can have a continuous ping from the DHCP server -> SW2 and as soon as I kill the master switch on SW2 (ports 1:X), the ping is lost to SW2 and will never return unless I console into SW2 (ports 2:X) and disable -> enable DHCP on vlan default.

I'm trying to connect to SW2 via telnet from the DHCP server after a partial stack failure, so I need the IP address to remain reachable without console access if at all possible.

I'm wondering if there's a special script or method to automatically run a series of commands upon a stack failure. That way I could script it out so that if a switch became a master switch in a stack, it could run 'disable dhcp vlan default' and then 'enable dhcp vlan default' since that seems to fix my problem via console.
Photo of Patrick Voss

Patrick Voss, Alum

  • 11,594 Points 10k badge 2x thumb
Hi Karl,

I don't believe this is normal behavior.  I would reach out to GTAC and open a case. If the proper entitlement is added to this device we may be able to attempt a replication and look for a proper fix.