DHCP Discovery packet loss

[artem.matsyplyuk77]

Experiencing issue with packet loss for broadcast DHCP discovery traffic between two switches over port-channel. 

When performing wireshark analysis i see multiple DHCP discovery packets being sent out from port and i see those discovery packet hitting other ports on a switch but when i am performing DHCP packet debug on a switch with DHCP pool i am not seeing any entries for those discoveries. 

Switch with DHCP pool is 9300x 10gig ports

Clients connect from 3850 switch stack with 1gig ports over port-channel 

During low traffic utilization DHCP discovery packets cross port-channel without any issues but when there is increased utilization DHCP discovery timing out. 

The only thing i noticed is output drops on port-channel interface. 

Any suggestion? 

[MHM Cisco World]

try change hashing algorithm for port-channel.

[Reza Sharifi]

Can you post the PO config from both switches?

Also, can you check if you are using the same load-balancing method, e.g source-mac or source-des-mac on both sides of the connection?

HTH

[Georg Pauwen]

Hello,

on the 3850, the 'traditional' way to get rid of output drops was to configure something like the below:

qos queue-softmax-multiplier 1200

[artem.matsyplyuk77]

was working with Cisco tec it looks like control plane policing issue. 
Just in case someone runs into the same issue. 

------------------ show platform hardware fed switch active qos queue stats internal cpu policer ------------------

<example>

                         CPU Queue Statistics

============================================================================================

                                              (default) (set)     Queue        Queue

QId PlcIdx  Queue Name                Enabled   Rate     Rate      Drop(Bytes)  Drop(Frames)

--------------------------------------------------------------------------------------------

6    0      ICMP Redirect               Yes     600       600      349103597224  987298676

12   0      BROADCAST                   Yes     600       600      102580649    325870

Class-map: system-cpp-police-data (match-any)
0 packets, 0 bytes
5 minute offered rate 0000 bps, drop rate 0000 bps
Match: none
police:
rate 600 pps, burst 146 packets
conformed 8803604872322 bytes; actions:
transmit
exceeded 105795709285435 bytes; actions:
drop

[artem.matsyplyuk77]

Solution is to disable un-necessary broadcasts, disable ICMP redirects on layer 3 interfaces. Also identified application on a network that is generating ICMP time-to-live exceeded broadcast messages which is in the same control plane queue as DHCP broadcasts.