Users Guide

UDP Helper with Configured Broadcast Addresses
Incoming packets with a destination IP address matching the configured broadcast address of any interface are forwarded to
the matching interfaces.
In the following illustration, Packet 1 has a destination IP address that matches the configured broadcast address of VLAN 100
and 101. If you enabled UDP helper and the UDP port number matches, the packet is flooded on both VLANs with an unchanged
destination address.
Packet 2 is sent from a host on VLAN 101. It has broadcast MAC address and a destination IP address that matches the
configured broadcast address on VLAN 101. In this case, Packet 2 is flooded on VLAN 101 with the destination address
unchanged because the forwarding process is Layer 2. If you enabled UDP helper, the packet is flooded on VLAN 100 as well.
Figure 54. UDP Helper with Configured Broadcast Addresses
UDP Helper with No Configured Broadcast Addresses
The following describes UDP helper with no broadcast addresses configured.
● If the incoming packet has a broadcast destination IP address, the unaltered packet is routed to all Layer 3 interfaces.
● If the Incoming packet has a destination IP address that matches the subnet broadcast address of any interface, the
unaltered packet is routed to the matching interfaces.
Troubleshooting UDP Helper
To display debugging information for troubleshooting, use the debug ip udp-helper command.
Example of the debug ip udp-helper Command
DellEMC(conf)# debug ip udp-helper
01:20:22: Pkt rcvd on Tf 5/1 with IP DA (0xffffffff) will be sent on Tf 5/2 Tf 5/3 Vlan 3
01:44:54: Pkt rcvd on Tf 7/1 is handed over for DHCP processing.
When using the IP helper and UDP helper on the same interface, use the debug ip dhcp command.
Example Output from the debug ip dhcp Command
Packet 0.0.0.0:68 -> 255.255.255.255:67 TTL 128
2017-08-05 11:59:35 %RELAY-I-PACKET, BOOTP REQUEST (Unicast) received at interface
172.21.50.193 BOOTP Request, XID = 0x9265f901, secs = 0 hwaddr = 00:02:2D:8D:46:DC,
giaddr = 0.0.0.0, hops = 2
2017-08-05 11:59:35 %RELAY-I-BOOTREQUEST, Forwarded BOOTREQUEST for 00:02:2D:8D:46:DC
420
IPv4 Routing