Some applications rely on client requests sent as limited IP broadcasts addressed to a UDP application port. If a server for the application receives such a broadcast, the server can reply to the client. Since typical router behavior, by default, does not allow broadcast forwarding, a client's UDP broadcast requests cannot reach a target server on a different subnet unless the router is configured to forward client UDP broadcasts to that server.
A switch with routing enabled includes optional per-VLAN UDP broadcast forwarding that allows up to 256 server and/or subnet entries on the switch (16 entries per-VLAN.) If an entry for a particular UDP port number is configured on a VLAN, and an inbound UDP broadcast packet with that port number is received on the VLAN, the switch routes the packet to the appropriate subnet. (Each entry can designate either a single device or a single subnet. The switch ignores any entry that designates multiple subnets.)
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NOTE: The number of UDP broadcast forwarding entries supported is affected by the number of IP helper addresses configured to support DHCP relay. See Operating notes for UDP broadcast forwarding. |
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A UDP forwarding entry includes the desired UDP port number and can be either an IP unicast address or an IP subnet broadcast address for the subnet the server is in. Thus, an incoming UDP packet carrying the configured port number will be:
A UDP forwarding entry for a particular UDP port number is always configured in a specific VLAN and applies only to client UDP broadcast requests received inbound on that VLAN. If the VLAN includes multiple subnets, the entry applies to client broadcasts with that port number from any subnet in the VLAN.
For example, VLAN 1 (15.75.10.1) is configured to forward inbound UDP packets as shown in Example of a UDP packet-forwarding environment.
Example of a UDP packet-forwarding environment
| Interface | IP address | Subnet mask | Forwarding address | UDP port | Notes |
|---|---|---|---|---|---|
| VLAN 1 | 15.75.10.1 | 255.255.255.0 |
15.75.11.43 |
1188 |
Unicast address for forwarding inbound UDP packets with UDP port 1188 to a specific device on VLAN 2. |
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15.75.11.255 |
1812 |
Broadcast address for forwarding inbound UDP packets with UDP port 1812 to any device in the 15.75.11.0 network. |
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15.75.12.255 |
1813 |
Broadcast address for forwarding inbound UDP packets with UDP port 1813 to any device in the 15.75.12.0 network. |
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| VLAN 2 | 15.75.11.1 | 255.255.255.0 | None | N/A |
Destination VLAN for UDP 1188 broadcasts from clients on VLAN 1. The device identified in the unicast forwarding address configured in VLAN 1 must be on this VLAN. Also the destination VLAN for UDP 1812 from clients on VLAN 1. |
| VLAN 3 | 15.75.12.1 | 255.255.255.0 | None | N/A |
Destination VLAN for UDP 1813 broadcasts from clients on VLAN 1. |
The subnet mask for a UDP forwarding address is the same as the mask applied to the subnet on which the inbound UDP broadcast packet is received. To forward inbound UDP broadcast packets as limited broadcasts to other subnets, use the broadcast address that covers the subnet you want to reach. For example, if VLAN 1 has an IP address of 15.75.10.1/24 (15.75.10.1 255.255.255.0), you can configure the following unicast and limited broadcast addresses for UDP packet forwarding to subnet 15.75.11.0:
To configure and enable UDP broadcast forwarding on the switch:
This command routes an inbound UDP broadcast packet received from a client on the VLAN to the unicast or broadcast address configured for the UDP port type.
Syntax:
Used in a VLAN context to configure or remove a server or broadcast address and its associated UDP port number. You can configure a maximum of 16
forward-protocol udpassignments in a given VLAN. The switch allows a total of 256forward-protocol udpassignments across all VLANs.You can configure UDP broadcast forwarding addresses regardless of whether UDP broadcast forwarding is globally enabled on the switch. However, the feature does not operate unless globally enabled.
<ip-address>This can be either of the following:
The unicast address of a destination server on another subnet. For example: 15.75.10.43.
The broadcast address of the subnet on which a destination server operates. For example, the following address directs broadcasts to All hosts in the 15.75.11.0 subnet: 15.75.11.255.
NOTE: The subnet mask for a forwarded UDP packet is the same as the subnet mask for the VLAN (or subnet on a multinetted VLAN) on which the UDP broadcast packet was received from a client.
<udp-port-#>Any UDP port number corresponding to a UDP application supported on a device at the specified unicast address or in the subnet at the specified broadcast address. For more information on UDP port numbers, see TCP/UDP port number ranges. <port-name>Allows use of common names for certain well-known UDP port numbers. You can type in the specific name instead of having to recall the corresponding number:
dnsDomain name service (53)
netbios-nsNetBIOS name service (137)
netbios-dgmNetBIOS datagram service (138)
radiusRemote authentication dial-in user service (1812)
radius-oldRemote authentication dial-in user service (1645)
ripRouting information protocol (520)
snmpSimple network management protocol (161)
snmp-trapSimple network management protocol (162)
tftpTrivial file transfer protocol (69)
timepTime protocol (37)
Example:
The following command configures the routing switch to forward UDP broadcasts from a client on VLAN 1 for a time protocol server:
Syntax:
Displays the current status of UDP broadcast forwarding and lists the UDP forwarding addresses configured on all static VLANS in the switch or on a specific VLAN.
Displaying global IP forward-protocol status and configuration shows the global display showing UDP broadcast forwarding status and configured forwarding addresses for inbound UDP broadcast traffic for all VLANs configured on the routing switch.
Displaying global IP forward-protocol status and configuration
HP Switch(config)# show ip forward-protocol
IP Forwarder Addresses
UDP Broadcast Forwarding: Disabled
VLAN: 1
IP Forward Addresses UDP Port
-------------------- --------
15.75.11.43 37
15.75.11.255 53
15.75.12.255 1813
VLAN: 2
IP Forward Addresses UDP Port
-------------------- --------
15.75.12.255 1812
Displaying IP forward-protocol status and per-VLAN configuration shows the display of UDP broadcast forwarding status and the configured forwarding addresses for inbound UDP broadcast traffic on VLAN 1.
The number of UDP broadcast entries and IP helper addresses combined can be up to 16 per VLAN, with an overall maximum of 2048 on the switch. (IP helper addresses are used with the switch's DHCP relay operation.)
For example, if VLAN 1 has 2 IP helper addresses configured, you can add up to 14 UDP forwarding entries in the same VLAN.
For more information, including a listing of UDP/TCP port numbers, go to the Internet Assigned Numbers Authority (IANA) website at: www.iana.org.
| Message | Meaning |
|---|---|
udp-bcast-forward: IP Routing support must be enabled first. |
Appears in the CLI if an attempt to enable UDP broadcast forwarding has been made without IP routing being enabled first. Enable IP routing, then enable UDP broadcast forwarding. |
UDP broadcast forwarder feature enabled |
UDP broadcast forwarding has been globally enabled on the router. Appears in the Event Log and, if configured, in SNMP traps. |
UDP broadcast forwarder feature disabled |
UDP broadcast forwarding has been globally disabled on the routing switch. This action does not prevent you from configuring UDP broadcast forwarding addresses, but does prevent UDP broadcast forwarding operation. Appears in the Event Log and, if configured, in SNMP traps. |
UDP broadcast forwarder must be disabled first. |
Appears in the CLI if you attempt to disable routing while UDP forwarding is enabled on the switch. |