DELL N3000 User Manual

Configuring Link Aggregation

such as ECMP and redundant router pairs, will allow a L3 routed network to 

utilize bandwidth efficiently. L3 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. Spanning tree on the secondary switch 

shows as the root bridge because, in a properly configured network, it will 

have no ports attached. In an improperly configured network, one in which 

orphan ports and orphan port VLANs are configured on one or both MLAG 

switches, three root bridges may appear in the network: the root bridge for the 

MLAG member ports/VLANs on the primary switch, the port-less root bridge 

on the secondary switch, and the root bridge for the orphan ports/VLANs that 

are not part of the MLAG. Careful planning may be required when 

configuring the network to ensure that orphan ports are isolated on the 

MLAG peers, and segregating the MLAG VLANs from the non-MLAG 

VLANs to not cross the peer link can reduce the number of root bridges in the 

network to two.
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 its own MAC address in spanning tree BPDUs and in the 

LACP actor ID. This will cause LACP to rebuild 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.