14169000-IG, Edition 11.0
January 2018, Original document
33
Quality and Configuration of the Switches
The LAN based synchronization is highly dependent on the quality and configuration of the deploy-
ment network. The single most important property of the switches in the network is their ability to for-
ward multicast Ethernet packets with low jitter, i.e. close to a constant delay. The total forwarding
jitter added by switches on any path through the deployment network should be less than one micro-
second and preferably less than 100 nanoseconds.
Unfortunately, it is usually difficult to find the forwarding jitter specified for a given switch. Lab tests
indicates that enterprise level switches generally has adequately low forwarding jitter, whereas
SOHO and unmanaged switches often do not meet the requirements and thus must not be used.
When configuring the deployment network, multicast setup is critical for LAN synchronization to
work. Multicast is usually either blocked, forwarded as broadcast to all ports, forwarded according to
static configuration or forwarded to selected ports learned by IGMP snooping. The simplest option is
to forward as broadcast to all ports, but this might create unwanted traffic on unrelated network
parts. When using static configuration, the relevant multicast addresses listed earlier must be for-
warded to the ports forming the deployment network. Enabling IGMP snooping on the switches
allow them to automatically configure which ports the multicast packet should be forwarded to, min-
imizing the network load caused by the LAN synchronization. In order to keep the multicast con-
figuration updated, a IGMP querier must be present in the network ā this functionality can be enabled
in many enterprise class switches.
Traffic Priority:
All time critical PTPv2 packets sent by the LAN synchronization software is by default marked with
either an Expedited Forwarding (EF) (46/0x2e) priority for IPv4 and IPV6 packets or a Class of Ser-
vice value of 7 for VLAN encapsulated Ethernet packets. This is to allow the switches to give pref-
erence to the LAN synchronization packets.
Since the Expedited Forwarding priority on IP packets is shared with voice RTP packets, this is not
sufficient to ensure strict priority over all other traffic for the PTPv2 events packets.
There are two possible solutions to this:
l Give the highest priority to a custom IP priority and configure the server to apply this IP priority
to PTPv2 traffic.
l Give the highest priority to multicast UDP packets on port 319 with the destination address
224.0.1.129 (IPv4) or FF02::181 (IPv6).
Spectralink IP-DECT Server 400/6500 and DECT Server 2500/8000 Synchronization and Deployment Guide
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