Reference Guide
IPv4 Routing | 477
In the following illustration, Packet 1 has a destination IP address that matches the configured broadcast
address of VLAN 100 and 101. If UDP helper is enabled 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 UDP helper is
enabled, the packet is flooded on VLAN 100 as well.
Figure 24-15. UDP Helper with Configured Broadcast Addresses
UDP Helper with No Configured Broadcast Addresses
• If the incoming packet has a broadcast destination IP address, then 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, then the unaltered packet is routed to the matching interfaces.
Troubleshooting UDP Helper
Display debugging information using the command debug ip udp-helper, as shown in the example below.
Figure 24-16. Debugging UDP Broadcast
Use the command debug ip dhcp when using the IP helper and UDP helper on the same interface, as shown
in the following example.
P
acket 1
D
estination Address:
1
.1.255.255
1/1
Ingress interface
IP Address: 2.1.1.1/24
UDP helper enabled
1/2
1/3
VLAN 100
IP address: 1.1.0.1/24
Subnet broadcast address: 1.1.0.255
Configured broadcast address: 1.1.255.2
55
Hosts on VLAN 100: 1.1.0.2, 1.1.0.3, 1.1.0.4
VLAN 101
IP address: 1.11.1/24
Subnet broadcast address: 1.1.1.255
Configured broadcast address: 1.1.255.255
Hosts on VLAN 100: 1.1.1.2, 1.1.1.3, 1.1.1.4
fnC0048m
p
Packet 2
Switched Packet
Destination Address:
1.1.255.255
Dell(conf)# debug ip udp-helper
01:20:22: Pkt rcvd on Te 5/0 with IP DA (0xffffffff) will be sent on Te 5/1 Te 5/2 Vlan 3
01:44:54: Pkt rcvd on Te 7/0 is handed over for DHCP processing.