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IP Subnet Broadcast not being forwarded by IP-Helper & IP forward-protocol.

Hi,

 

I've got an interesting problem I'm trying to fix. We have some software which needs to contact a remote license server, it attempts to do this by sending a subnet broadcast to UDP-1947. On the 4500 VSS switch we have the following things configured.

 

ip forward-protocol udp 1947

!

interface vlan 2

 IP address 10.5.4.1 255.255.255.0

 ip helper address 10.10.10.10 

!

 

When Vlan2 broadcast address is the default 255.255.255.255 the client PC packet to 10.5.4.255 on UDP-1947 is not forwarded.

 

If I manually configure the broadcast address of the SVI Vlan 2 to be 10.5.4.255 then the packet is forward to the license server which then replies and the software if operational. Therefore it is clear that the packets destined to the subnet broadcast address are not being forwarded with the default broadcast address on Vlan2.

 

The unicast routing is all working fine running OSPF but it is passive on Vlan2.

The PC and the license server can ping each other.

The other helpers for remote DHCP servers etc all work as they use the 255.255.255.255 dst.addr

 

Any ideas ?

 

Regards Jon