DHCPv4 Option 43 for Fast Offline Migration

[thisisdisign]

Hi everyone,

I have a question about DHCPv4 Option 43 for Fast Offline Migration.

When I use DHCP Option 43 for the purpose of Fast Offline Migration for Wi-Fi 7 APs, does fast offline migration(f3xxx) work for non Wi-Fi 7 APs?
Should I split the DHCP scope for non-Wi-Fi 7 APs?

[Saikat Nandy · Accepted solution]

Fast offline migration is only for WIFI7 APs which means f3xx will work. For existing APs you will still have to use f1xx.

[Rich R]

Not entirely true @Saikat Nandy
@thisisdisign I've tested it (in this case on an 1832 because if it works on that old AP then it should work on all newer models too).
The older AP still works fine with the Fast Offline Migration option 43 configured.
It is still able to get the WLC IP address from DHCP correctly.
As I expected it is the software version that matters, not the AP model. 
As long as the AP has the 17.15.3 software installed it will understand the new option 43 format.
However (not surprisingly) when the AP is running 8.10.196.0 software it is not able to discover the controller from the new format option 43.

So the caveat is that the new option 43 will work for all APs running 17.15.3 and later software (regardless of whether they're WiFi 7 or not) but if you expect to deploy APs which have not been staged with the latest software pre-installed then you might need to operate separate DHCP scopes for the older APs to allow them to discover the WLC initially.  Once they have the new software installed you could move them all into the same VLAN/DHCP scope.  Alternatively you could also use DNS or helper address (to forward the broadcast joins to the WLC) to get those older APs joined and downloaded and then remove the DNS or helper once they're online.  That would avoid the need for a second DHCP scope.

[Saikat Nandy · Accepted solution]

 Thanks @Rich R for sharing your thoughts. I truly believe I was not clear enough with my response before and you are right. So let me try to put my previous answer in a different way.

# Fast Offline Migration uses new DHCP/DNS options and will bypass the 8-minute wait timer. Without FOM, CW917x APs will continue to look for cloud for 8 mins.
# This 8 mins wait timer has been introduced in 917x series AP and to run a 917x series AP you need 17.15.2 and above - that's also true.
# If you are not having any 917x series AP in the network, then you should not run f3xx (as you can get the job done using f1xx simply). f3xx is ‘only’ meant to reduce the 8 mins wait time for 917x series AP - nothing else.

So in a brownfield deployment, lets say 9136 and 9176 are in place and if you already have f1xx configured, better continue using that - if and only if you dont want to wait for 15mins for 917x to come up and in a day zero setup with 917x, I don’t think 15mins wait time is a show-stopper from any angle.

[thisisdisign]

Thanks @Saikat Nandy @Rich R

So that means Fast offline migration (f3xxx) doesn't work except for 917x?
If I choose f1xx for 917x, the long waiting time is usually only applicable for Day-0, right?

[Saikat Nandy]

Correct. The FOM concept is not even applicable for any other APs except 917x.
If you use f1xx even with 917x, the wait time will be around 15 mins (first 8mins wait & next 7mins to move to catalyst+dhcp+discovery.......) and that is only for day 0.

[thisisdisign]

@Saikat Nandy,

Thanks, got it.