UDP broadcast forwarding

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.)

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.

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:

  • Forwarded to a specific host if a unicast server address is configured for that port number.

  • Broadcast on the appropriate destination subnet if a subnet address is configured for that port number.

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 the following table.

Table 33: Example of a UDP packet-forwarding environment

Interface

IP address

Subnet mMask

Forwarding address

UDP port

Notes

VLAN 1

15.75.10.1

255.255.255.0

15.75.11.43 15.75.11.25515.75.12.255

1188 18121813

Unicast address for forwarding inbound UDP packets with UDP port 1188 to a specific device on VLAN 2. Broadcast address for forwarding inbound UDP packets with UDP port 1812 to any device in the 15.75.11.0 network.Broadcast address for forwarding inbound UDP packets with UDP port 1813 to any device in the 15.75.12.0 network.

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.

NOTE:

If an IP server or subnet entry is invalid, a switch will not try to forward UDP packets to the configured device or subnet address.