OmniSwitch OS6860/OS6900/OS10K Troubleshooting Guide Part No.032996-00 Rev.A
AOS Release 7.X and 8.X January 2015
Alcatel-Lucent Page 57 of 148
DHCP Relay Agent Service packet flow
1. Device sends out DHCP Discovery packet
2. DHCP Discovery packet is trapped to CPU by the FFP
3. Packet Driver sends the packet to IPNI
4. IPNI sends the packet to UDP Relay CMM based on the UDP list
5. UDP Relay CMM received this packet from the socket opened
6. UDP Relay CMM looks up the VRF that VLAN 30 belongs to
7. UDP Relay CMM looks up the next hop IP address form its configuration
8. For Standard Mode, it will be all the next hop IP addresses
9. For Per VLAN Mode, it will look up if there is any next hop IP address configured for VLAN 30; f there is
none, the packet is discarded
10. Packet is sent to the Native Linux IP stack to be forwarded to the IP destination (the next hop IP address)
11. DHCP server reply with a DHCP Offer packet
12. This DHCP Offer packet is sent to ip interface 10.30.0.254
13. DHCP Offer packet is sent to the IP NI since the DHCP Server is to reply to the ip interface of the DHCP
Relay Agent
14. UDP Relay CMM looks up the network port that based on the destination MAC address of the DHCP Offer
packet
15. UDP Relay CMM looks up the NI that the network port resides
16. UDP Relay CMM sends the DHCP Offer packet to the IP NI where the network port is
17. IP NI sends the frame to Packet Driver where the packet is sent out into the network
18. All upstream DHCP packets from the device to the DHCP server follow the DHCP Discovery packet
19. All downstream DHCP packets from the DHCP server to the device follow the DHCP Offer packet
Generic UDP Relay Service packet flow
1. Device sends out a broadcast UDP frame with destination port 1122
2. Packet is trapped to CPU based on the destination UDP port
3. Packet is sent to IPNI from Packet Driver
4. IPNI sends the packet to UDP Relay CMM
5. UDP Relay CMM looks up the VRF of VLAN 30
6. UDP Relay CMM looks up the configuration for the UDP port 1122 and determined that this packet is to be
forwarded to VLAN 40, 50 and 60
7. UDP Relay CMM will send the packet to IP NI and then to the Packet Driver 3 times (one for VLAN 40,
one for VLAN 50 and one for VLAN 60)
8. Packet Driver is able to flood the packet to all the ports that belongs to VLAN 40, VLAN 50 and VLAN 60
DHCP Snooping Binding Database
By default, once DHCP Snooping is enabled at either the switch-level or the VLAN-level, the DHCP Snooping
Binding Database capability will be enabled. The DHCP Snooping Binding table is indexed by the physical port and
the client’s MAC address. It contains the following data:
Client’s MAC Address;
Client’s IP Address assigned by the DHCP Server;
The physical port where the DHCP request is coming from;
The VLAN Id where the DHCP request packet is coming from;
The lease time of the IP Address;
The type/nature of how the binding entry is populated, either static or dynamic.
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