Users Guide

Link Aggregation 1079
DCPDP and Peer Link Failures
DCPDP is intended to provide a secondary layer of protection against peer
link failures. If the peer-link goes down while the DCPDP protocol is enabled
and remains up, the MLAG links on the MLAG secondary peer are disabled.
The primary switch continues to forward traffic and, if LACP is enabled, send
LACPDUs using the system MAC of the MLAG. Spanning tree
reconvergence on the partner devices is avoided.
In the case where there are no keep-alive messages detected from the peer and
DCPDP is disabled, but both peer units remain up, two primary switches
result, each with the MLAG system MAC address, and each operating over its
part of the former MLAG. In this situation, the selection of dynamic or static
LAGs determines the MLAG behavior.
On a peer-link failure with DCPDP disabled and the MLAG configured with
dynamic LAGs to the partners, traffic forwarding continues through the
primary MLAG switch. The secondary switch brings down the MLAG
interfaces and brings them up with a new (different from the primary) MLAG
system ID. LACP running on the partner device detects that the links in the
port-channel connected to the secondary MLAG switch are sending
LACPDUs with a different system ID and does not bring up the links
connected to secondary MLAG peer. This behavior reduces or eliminates
spanning tree reconvergence due to the MLAG switches sending BPDUs with
different bridge IDs to the partner switch.
On a peer-link failure with DCPDP disabled and the MLAG configured with
static LAGs to the partners, traffic forwarding continues through both the
primary and secondary MLAG switches. Spanning tree sends BPDUs with
different bridge IDs to the partner switch, resulting in serial spanning tree
reconvergence. For this reason, dynamic MLAGs are strongly recommended.
MLAG and Redundant Peer Links
A redundant peer link is a link other than the peer link between the MLAG
peer switches. Typically, these links cause a loop in the network and may cause
the peer link to become blocked by spanning-tree. Dell EMC Networking can
support traffic flow over redundant links with some additional configuration.
Multiple spanning tree must be configured, the redundant link must be
assigned a VLAN that is NOT in the MLAG domain (but can be configured
on the peer link), and the VLAN assigned to the redundant link must be