Phones on some subnets can’t connect to PBX

There are several possible causes, all network configuration related. However, the most common cause has to do with the default route on your PBX causing an asymmetric network path.

In other words, packets come in ethernet1 and (try to) go out ethernet2. For security reasons some Linux versions prohibit this, and some routers/gateways can’t handle this. The first thing to do is ensure that your Linux configuration allows this type of asymmetrical path. Type the following from a Bash prompt:



echo “0” > /proc/sys/net/ipv4/conf/eth0/rp_filter
echo “0” > /proc/sys/net/ipv4/conf/eth1/rp_filter

and replace eth0 and eth1 with the names of your network interfaces. This should take effect immediately (no restart required).

For more details of what the above commands do (and what problem they address) please visit https://access.redhat.com/solutions/53031 . Note that allowing asymmetric routes is sometimes the best solution, but other times it’s best to adjust your routing tables to cause symmetry in packet flow.