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
on page 312.
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
18121
813
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.
Chapter 15 User Datagram Protocol
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