This article describes a scenario related to DHCP client not getting IP address. DHCP server is reachable, but DHCP discover packet gets dropped on the transit device. 

Topology:-

Model: QFX10002-36q

Version 18.2X75-D54.2

• When a packet capture is taken on ingress and egress interface of switch1 and switch2, we may find that packet is egressing from switch2 interface.
• DHCP discover packet is received on switch3 interface however its not egressed.
• We have used the same DHCP configuration for all the switch.

forwarding-options {

  dhcp-relay {

    forward-snooped-clients configured-interfaces;

    server-group {

       test1 {

        192.168.10.1;

        192.168.10.2;

      }

    }

    active-server-group test1;

    group test1 {

      active-server-group test1;

      overrides {

        bootp-support;

      }

      interface irb.100;

    }

  }
 

• We connect DHCP server at interface et-0/0/7 on switch3.
• Switch1 and Switch2 working in L2 domain so they have transit the DHCP discover packets. but its dropped on switch3.

Scenario1 on Switch3 :-

• With below configuration. DHCP packet is receiving on ingress interface and not egressed.

forwarding-options {

  dhcp-relay {

    forward-snooped-clients configured-interfaces;

    server-group {

      test1 {

        192.168.10.1;

        192.168.10.2;

      }

    }

    active-server-group test1;

    group test1 {

      active-server-group test1;

      overrides {

        bootp-support;

      }

      interface irb.100;

    }

  }

• We have applied the monitor capture and we can see the ingress traffic but not egressed.

Note:-  By default DHCP snooping is enabled when server or relay is configured.   Monitor traffic  capture will work in presence  of DHCP snooping, Else switch will treat DHCP packet as data plane traffic.

Scenario 2 on switch3 :-

• We removed all the DHCP configuration and we have observed DHCP is working. 

Scenario 3 on switch3:-

• We have enabled the “set forwarding-options dhcp-relay no-snoop ” configuration with existing configuration. Then DHCP is also working.

DHCP would work in absence of DHCP snooping on Switch3.