Administrator Guide

Link Aggregation 1021
Routing is not supported across multiple MLAGs (i.e., in two-tier topology).
This is a fundamental limitation of MLAG, which is intended as a
replacement for other, less efficient layer-2 topologies. Should a multi-tier
layer-3 topology be desired, other well established and well understood
techniques, such as ECMP and redundant router pairs, will allow a layer-3
routed network to utilize bandwidth efficiently. Layer-3 routing is capable of
routing packets around failed links and failed routers.
Spanning tree (and LACP) PDUs are proxied from the secondary MLAG peer
to the MLAG primary switch. This implies that at least two spanning tree
roots will exist in the MLAG network: the root bridge for the MLAG member
ports/VLANs on the primary switch and the root bridge for the non-
redundant ports/VLANs that are not part of the MLAG.
The peer link requires a native VLAN to be configured. This is a limitation of
the peer-link keep alive protocol.
On primary switch failover, the secondary switch flushes the FDB MAC
addresses and uses the system virtual MAC address in spanning tree BPDUs
and in the LACP actor ID. This avoids rebuilding the link aggregation group
followed by spanning tree reconvergence.
MLAG-supported protocols are active only on the MLAG primary switch.
The protocols are proxied from the secondary peer switch to the primary
switch. The primary switch receives state information from the secondary
peer switch and programs the secondary peer hardware. It does not send
protocol state information to the secondary peer. This leads to a number of
seemingly inconsistent behaviors if these facts are ignored:
• MLAG port-channel state is maintained on the primary peer only. The
MLAG secondary peer has accurate state for the member links, but not for
an MLAG port-channel. The operator can shut down a MLAG port-
channel only from the primary MLAG peer.
• Shutting down a MLAG port-channel on the primary peer shuts down the
port-channel on both the primary and secondary MLAG peers.
• Shutting down a MLAG port-channel on the secondary MLAG peer has no
effect. The operator can shut down the individual links instead.
• The spanning tree status is only shown correctly on the primary MLAG
peer for the redundant links and associated VLANs. The spanning-tree
state on the secondary switch is accurate only for the non-redundant links
and associated VLANs.