Cisco CCNA ICND2 (Interconnecting Cisco Networking Devices Part 2)Exam Cram

3. WAN Technologies

3.4 EtherChannel Configuration

Two protocols, namely PAgP or LACP may used for negotiating EtherChannel and Link Aggregation. We can configure Etherchannel in three ways in Cisco Switches.

1. Port Aggregation Protocol (PAgP) - Cisco Proprietary protocol

2. IEEE Link Aggregation Protocol (LACP) - Industry Standard

3. Manual Etherchannel Configuration - Without using any negotiation protocol listed above

PAgP : PAgP stands for Port Aggregation Protocol. PAgP helps in the automatic creation of Fast EtherChannel links. PagP is a Cisco proprietary link aggregation protocol used in Catalyst switches.

The following are features of Fast EtherChannel that is running PAgP (Port Aggregation Protocol):

1. PAgP helps in the automatic creation of Fast EtherChannel.

2. PAgP does not group ports configured for dynamic VLANs. PAgP requires that all ports in a channel must belong to the same VLAN or should be configured as trunk ports.

3. PAgP does not group ports that work at different speeds or port duplexes.

The following are available PAgP modes and the corresponding action:

1. ON mode does not send or receive PAgP packets. Therefore, both ends should be set to ON mode to form an EtherChannel.

2. Desirable mode tries to ask the other end in order to bring up the EtherChannel.

3. Auto mode participates in the EtherChannel only if the far end asks for participation. Two switches in auto mode will not form an EtherChannel.

If you are using Port Aggregation Protocol (PAgP) for EtherChannel negotiation, EtherChannel will be formed only if two ends are configured under following modes.

  Desirable Auto
Desirable Yes Yes
Auto Yes No

EtherChannel can support from 2 to 8 links to be bundled into one logical link. Therefore, if Gigabit Ethernet links are bundled, 8 links represent 8 Gbps of one-way bandwidth, and 16 Gbps for full-duplex operation.

The load distribution algorithm in EtherChannel can use source IP, destination IP, a combination of source and destination IPs, Source MAC, destination MAC, or TCP.UDP port numbers for decision process. If there are only two links in the EtherChannel, only 1 bit in the IP are required. If there are 4 links in the EtherChannel, 2 bits are required. Similarly, for an 8 link EtherChannel, 3 bits are required.

LACP: Link Aggregation Control Protocol (LACP) is a standards based protocol and conforms to IEEE standard 802.3ad. Note that PAgP is Cisco proprietary protocol. If your network consists of both Cisco and non-Cisco devices, LACP is the desired option for configuring EtherChannel. If your network consists of all Cisco switches then PAgP would be recommended.

1. Active: The active end of the group sends out a LACP frame and initiates the negotiation to form the EtherChannel. Both ends could be active and the result would be the same.

2. Passive: Passive Mode does not initiate the negotiation. It just responds to LACP packets initiated by other end. So if both ends were passive, the EtherChannel would not be formed.

Passive Mode in Link Aggregation Control Protocol (LACP) does not start Link Aggregation Control Protocol (LACP) packet negotiation.

  Desirable Auto
Desirable Yes Yes
Auto Yes No

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