3850 Switch with DHCP odd behavior

[ken.montgomery]

We have begun using the new generation of 3850 switches, specifically WS-C3850-24P-P switches.  We use the switch to supply DHCP for the VLANs. 

We are running software version:

cat3k_caa-universalk9, version 03.03.00SE

So, we built the pool according to these guidelines:

ip dhcp excluded-address 10.96.81.200 10.96.81.209

ip dhcp excluded-address 10.96.81.220 10.96.81.254

ip dhcp excluded-address 10.96.81.1 10.96.81.12

ip dhcp excluded-address 10.96.81.30 10.96.81.50

!

!

ip dhcp pool LEGACY-DATA

network 10.96.81.0 255.255.255.0

domain-name eng-prod.com

dns-server 10.1.1.10 10.96.200.11

default-router 10.96.81.4

So, here is the kicker... users are not getting addresses.  The dhcp server doesn't respond BUT... it responds to a certain point.  Immediately after restarting the DHCP service on teh switch, this is what the pool statistics show:

Pool LEGACY-DATA :

Utilization mark (high/low)    : 100 / 0

Subnet size (first/next)       : 0 / 0

Total addresses                : 254

Leased addresses               : 0

Excluded addresses             : 148

Pending event                  : none

1 subnet is currently in the pool :

Current index        IP address range                    Leased/Excluded/Total

10.96.81.13          10.96.81.1       - 10.96.81.254      0     / 148   / 254

According to my math, there are 54 exclusions.  The pool shows 148 before any leases handed out, and as each lease is handed out, the excluded rises.  I know the older version of this new Cat-os had DHCP issues... does this version as well?

Any suggestions?  Our pool appears to run out of leases iwth about 100 leased, showing 106 leased, 148 excluded, for a total of 254... which sounds right, but there are only 54 excluded.

Help?

[Reza Sharifi]

Not that this will resolve the issue you have, but not sure how you came up with 54 exclusions.

10.96.81.200 10.96.81.209   10 addresses

10.96.81.220 10.96.81.254   35 addresses

10.96.81.1 10.96.81.12        12 addresses

10.96.81.30 10.96.81.50       21 addresses

78 addresses?

[ken.montgomery]

Bad math I suppose... was doing too many things at once to add properly

[paul driver]

Hello

Try to decrease you lease time from defat of 1 day  and stop and stop and start your dhcp server

res

Paul

Please don't forget to rate any posts that have been helpful.

Thanks.

[ken.montgomery]

Hi Pdriver,

Well, I appreciate the suggestion, but that doesn't work.

We have decreased to 1 day (the default) and the problem recurs.  DHCP settings don't go less than that on the switch (to my knowledge)  The only way to fix it is to reboot the switch or stop and start the dhcp service, but the problem recurs.  It takes about 48 horus before addresses are exhausted, then it won't hand any more out, even though a number of them are expired.

Thanks again for the suggestion,

Ken

[umesh.chowdhary]

Hi,

Try to run below command.

'no ip dhcp confli logging" on the switch 

Thanks

[networks7]

Hi All,

I also have same DHCP issue with cisco 3850 48 TDL switches, we have Per VLAN basis DHCP Pool and some Ip address have bind with users device Mac-address. The DHCP issue is so frustrating and irritating, when it occur i have to manually clear ip dhcp bindings and then i have to manually restart the DHCP process by deleting that pool and recreating.

Please guide me over this issue.

I have attached the switch configuration.

[paul driver]

Helllo

Curious to what was he lease time set before you decreased.it to a day?

FYI - Dhcp Lease time defaults
to 1day but this can be changed

Below config changes.lease.times.to 8 hours

ip dhcp pool LEGACY-DATA
Lease 0 8 0

No.service dhcp
Service dhcp

Res
Paul

Sent from Cisco Technical Support Android App

[ken.montgomery]

Thanks for the clarification on shorter leases, but it still doesn't solve the problem.

[ken.montgomery]

Also, lease time was 2 days before being reduced to default of one day.

[paul driver]

Hello

Also FYI -The dhcp clients will.renegotiate its lease around half its.specified lease time

given.the.little amount.of users in this scope.the.lease could go even lower if you desired!

I assume this is the only dhcp servet running correct - no chance.of Rouge server amywhere?

Res
Paul


Sent from Cisco Technical Support Android App

[ken.montgomery]

Again, that shortens the lease, but doesn't solve the problem.

When my office is filled with staff, there still won't be enough addresses, as the system isn't handing out the full range.  This is still a problem.

[Leo Laohoo]

Ken,

What VLAN?  Can you post the VLAN details including the entries in the VLAN database?

Best Regards/Leo

[ken.montgomery]

Why would the VLAN number make a difference?

And no, no need to post them as they don't affect this.  The requests are coming in on the right VLAN and assignment of IP addresses is happening on the right vlan.

[Leo Laohoo]

Why would the VLAN number make a difference?

1.  If your VLAN has an IP address out of scope;

2.  Favorite mistake:  Your VLAN is in "shutdown"