18.3. Using PTP with Multiple Interfaces

When using PTP with multiple interfaces in different networks, it is necessary to change the reverse path forwarding mode to loose mode. Red Hat Enterprise Linux 7 defaults to using Strict Reverse Path Forwarding following the Strict Reverse Path recommendation from RFC 3704, Ingress Filtering for Multihomed Networks. See the Reverse Path Forwarding section in the Red Hat Enterprise Linux 7 Security Guide for more details.
The sysctl utility is used to read and write values to the kernel. Changes to a running system can be made using sysctl commands directly on the command line and permanent changes can be made by adding lines to the /etc/sysctl.conf file.
  • To change to loose mode filtering globally, enter the following commands as root:
    ~]# sysctl -w net.ipv4.conf.default.rp_filter=2
    sysctl -w net.ipv4.conf.all.rp_filter=2
  • To change the reverse path filtering mode per network interface, use the net.ipv4.interface.rp_filter command on all PTP interfaces. For example, for an interface with device name em1:
    ~]# sysctl -w net.ipv4.conf.em1.rp_filter=2
To make these settings persistent across reboots, modify the /etc/sysctl.conf file. For example, to change the mode for all interfaces, open the /etc/sysctl.conf file with an editor running as the root user and add a line as follows:
net.ipv4.conf.all.rp_filter=2
To change only certain interfaces, enter multiple lines in the following format:
net.ipv4.conf.interface.rp_filter=2