EtherChannel Requirements CCNA

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.