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DHCP gremlins on 3850

Level 1
Level 1

Upgraded from a 3560G poe switch as my L3 switch (routing w/EIGRP) to a 3850. DHCP has stopped working on the LANs.

Validated dhcp helper configs are still there. EIGRP config looks good.

If I set a static IP on a PC on the LAN, it routes everywhere just fine.

Some phones that were rebooted got an IP and registered to CUCM just fine; others just sat there spinning, trying to get an IP (vlan 143).

None of the PCs would get a DHCP IP (vlan 43).

I added in another 3560G, added the VLAN L3s and EIGRP routing to it, trunked it to the 3850, and then turned off the VLAN L3s and EIGRP routing on the 3850. Immediately, all phones got an IP, registered, and PCs could grab a DHCP IP again. I'm confused as heck!!

3850 config attached

3 Replies 3

Hall of Fame
Hall of Fame

You may be hitting with Bug i guess. try upgrade to 16.12.7 and see of that fix the issue ?


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DHCP Server need to know the return traffic to interface of DHCP relay, 
this done by EIGRP but since you not run EIGRP or EIGRP not advertise the DHCP Server can not know return path and DHCP failed.
so check the show ip route in new SW and check the return path 

Are you performing dhcp snooping on the access-layer?, its possible you may need to add the following to the new L3 switch as its performing the relay function - try it anyway and test.

ip dhcp relay information trust-all

Also make sure the new L3 switch can reach the dhcp server, ping the relay addresses sourcing and make sure you get a reply.

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