Phones on some subnets can't connect to PBX

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CRM User
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Phones on some subnets can't connect to PBX

Post by CRM User » Mon Jun 26, 2017 12:07 am

I have a multihomed PBX cluster. On each node one NIC is on a management subnet and another NIC on a voice subnet.

Phones on the Voice subnet have no problem reaching the PBX. But phones on other subnets cannot reach the PBX. Why?
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Re: Phones on some subnets can't connect to PBX

Post by Telium Support » Mon Jun 26, 2017 12:13 am

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:

Code: Select all

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 . 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.
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