EtherChannel has specific requirements and additional
platform (hardware) related requirements. A partial list is:
·
EtherChannel
can have a maximum of eight physical interfaces per port-channel
·
All
interfaces within the port-channel must have the same speed and duplex settings
·
An
EtherChannel does not form if one interface is a Switch Port Analyzer (SPAN)
port
·
An
interface within a port-channel cannot be configured as a secure port (do not
activate port security)
·
All
interfaces within a port-channel must be assigned to the same access VLAN or if
they are Dot1q trunks they must be assigned the same native VLAN
·
If
interfaces are trunking then they must all have the same allowed range of VLANs
Catalyst 2960 switches can leverage a
protocol to dynamically establish and maintain the EtherChannel bundle. The channel-group
mode command allows you to decide if the EtherChannel group uses Port
aggregation Protocol (PAgP), Link Aggregation Protocol (LACP), or
to simply force the interface to channel without PAgP or LACP. Forcing interfaces to channel may create
problems if any interfaces have dissimilar configurations. PAgP allows the switches to learn the
capabilities of each interface assigned to an EtherChannel bundle and reliably
activates interfaces of similar configuration to form a port-channel. PAgP
transmits and receives messages on all interfaces in the EtherChannel bundle
and restricts the PAgP traffic to the native VLAN if the ports are in trunking
mode. LACP is similar in operation to PAgP and standards based while PAgP is
Cisco proprietary.
Other local link protocols such as DTP,
VTP, CDP, and STP still transmit and receive frames over a port-channel. STP
only sends frames out the first interface in the port-channel and views the
port-channel as a single physical port. Cisco uses Per-VLAN Spanning-Tree
(PVST) and if the port-channel is a Dot1q trunk then multiple STP messages might
require transmission (with appropriate Dot1q tags).
Spanning-tree reflects the increased
bandwidth provided by EtherChannel. The default cost for a 100 Mbps link is 19
and if a port-channel is created that has only two 100 Mbps links the
spanning-tree cost will be 9. If more physical ports are assigned to the port-channel
the cost decreases. Note: the spanning-tree cost does not dynamically change
when links go down or when a link is brought back up within the port-channel;
the cost is calculated based on the number of ports assigned to the
port-channel.