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S5600 switch configured dhcp-snooping function caused S9300 connecting AP cannot get IP normally

Publication Date:  2019-07-23 Views:  138 Downloads:  0

Issue Description

      S9300(2/1/1)---(1/1/1)S5600(1/1/2)------ OLT-----ONU------AP(3个)
     New bulit 3 AP MAC address can be learned on S5600 switch,but it cannot be learned on S9300 switch;3 new adding AP cannot get IP address from DHCP Server but S5600 switch connecting other device (static user) would work normally.

Alarm Information


Handling Process

1、Check S9300 device and no abnormity, other mac address learning is normal, middle link is normal too.
2、statisic analysis on S5600 upstream interface,the user packet flow did not reached upstream interface, it can judge that S5600 did not forward the packet, check the device and no software or handware abnormal information, check S5600 configuration script,the client opened dhcp-snooping,the upsteam interface(connecting DHCP Server)did not configure dhcp-snooping trust,the reason has be found,when useer DHCP packed reached S5600,if the switch configured dhcp-snooping function, but the upstram interface did not configured dhcp-snooping trust,user DHCP packet send to S5600 CPU,it cannot forward normally,it caused upstream S9300 device cannot learn user MAC address,new adding AP cannot get ip address from DHCP Server normally.
3、Close dhcp-snooping function on S5600 switch, S9300 learn MAC address normally, AP can get IP address normally.

Root Cause

It caused by the following reason according to failure:
1、User packet flow did not reached S9300 switch,it maybe that S56 did not forward or middle link lost;
2、Packet has reached to S9300 switch,S9300 switch abnormal caused learning mac or handling abnormal.


Compare H3C low-end switch(instance:S5600/S3552/S3500-EA/S3900),it suggested that dhcp-snooping trust add to upstream interface after open dhcp-snooping function,nor it will cause DHCP packet cannot be frowarded notmally.