Port aggregation protocol

A port aggregation protocol is a Cisco-proprietary network protocol that facilitates the automatic combination of EtherChannels by exchanging PAgP packets between Ethernet ports. PAgP packets are sent between Fast EtherChannel-capable ports in order to form a channel.

When any of the active ports fail, a standby port becomes active. By using PAgP, the controller learns the identity of partners that support PAgP and the capabilities of each port.

PAgP then dynamically groups similarly configured ports (on a single device in a stack) into a single logical link (channel or aggregate port). Similarly configured ports are grouped based on hardware, administrative, and port parameter constraints.

A port channel interface number is a configuration identifier that uniquely distinguishes a port channel logical interface. It defines allowable ranges and limits depending on the controller model.

From Cisco IOS XE Bengaluru 17.5.1 onwards, port channel interface numbers from one to 64 are supported, with model-dependent limits on the following Cisco Catalyst 9800 Series Wireless Controllers:

• Cisco Catalyst 9800-CL Wireless Controller for Cloud: The available range on the CLI is one to 64. The maximum supported port channel interfaces are 64.

• Cisco Catalyst 9800-L Wireless Controller: The available range on the CLI is one to 64. The maximum supported port channel interfaces are 14.

• Cisco Catalyst 9800-40 Wireless Controller: The available range on the CLI is one to 64. The maximum supported port channel interfaces are 16.

• Cisco Catalyst 9800-80 Wireless Controller: The available range on the CLI is one to 64. The maximum supported port channel interfaces are 64.

For example on the Cisco Catalyst 9800-L Wireless Controller, port-channel interface numbers can be anywhere between one and 64, provided that no more than 14 port-channel interfaces are configured.

Device(config)#
                Dec 15 08:58:22.209 CST: %ETC-5-CANNOT_ALLOCATE_AGGREGATOR: Aggregator limit reached, cannot allocate aggregator for group 17
            

interface Port-channel29
                ^% Invalid input detected at '^' marker.
                interface Port-channel35
                ^% Invalid input detected at '^' marker.

Note that the HA pairing remains intact after downgrade.

A flow control mechanism is a network protocol feature that:

• Enables connected Ethernet ports to control traffic rates during congestion by allowing congested nodes to pause link operation at the other end.

• Uses pause frames for signaling congestion and allows congested nodes to pause traffic from other nodes to prevent packet loss.

Flow control is enabled by default for both bay0 and bay1 ports as a first-day behavior. If you see overruns on a Cisco Catalyst 9800-L Wireless Controller due to burst traffic, enable flow control on the uplink switch. This change helps the uplink switch honor the flow control settings enabled on the controller.

Although the show command might indicate that flow control is off or unsupported in the controller’s CLI, the actual default behavior is that flow control is enabled. For more information, check https://bst.cloudapps.cisco.com/bugsearch/bug/CSCwk52366.