Administrator Guide

Audio Video Bridging 1607
the same as the delay from responder to the requestor. The peer delay
mechanism also requires that there are no transparent devices (bridges) that
can add extra delay between the peers.
As part of the PDELAY exchange, the requestor computes the ratio of the
frequency of the responder’s local clock at the other end of the link and the
frequency of the requestor’s local clock. To account for the frequency offset
between the clocks at each end, the peer delay is adjusted based on the
computed ratio.
Caveats and Limitations
IEEE 802.1AS PTP requires the presence of a grand master clock. Dell
Networking N-Series AVB switches are not capable of acting as grand master
clocks.
IEEE 802.1AS PTP does not operate across a stack. Both the master and all
slave ports must be located on the same stack unit.
CoS queues 5–7 should not be used for SRP traffic, as they are also used for
control plane traffic. Use CoS queues 2–4 instead.
Dell Networking N-Series MSRP supports MMRP/MSRP only for
identification of multicast sources. GMRP and IGMP are not supported.
The MRP protocols are not tied to the spanning-tree protocols. A spanning-
tree must exist, but MRP operates independently of the spanning-tree
protocols and does not interact with them.
The MRP “New” message can be used to discard learned information without
using the service registration (pruning) state machines of MVRP.
The IEEE 1588 Precision Time Protocol supports the logical grouping of PTP
clocks into multiple sub-domains, with each sub-domain having a best master
clock. PTP clocks are synchronized to the grandmaster clock in that sub-
domain. There is no such grouping of clocks in an 802.1AS domain.
The time synchronization mechanism is applicable only after a grandmaster is
elected and port states are set to Master/Slave. In other words, SYNC and
FOLLOWUP messages are not sent when the Best Master Clock Algorithm
selection is in progress.