CCNA – EtherChannel

Notes:

The Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP) facilitate the automatic creation of EtherChannels by exchanging packets between Ethernet interfaces. The Port Aggregation Protocol (PAgP) is a Cisco-proprietary solution, and the Link Aggregation Control Protocol (LACP) is standards based.

LACP modes:

+ on: the link aggregation is forced to be formed without any LACP negotiation. A port-channel is formed only if the peer port is also in “on” mode.
+ off: disable LACP and prevent ports to form a port-channel
+ passive: the switch does not initiate the channel, but does understand incoming LACP packets
+ active: send LACP packets and willing to form a port-channel

The table below lists if an EtherChannel will be formed or not for LACP:

LACP	Active	Passive
Active	Yes	Yes
Passive	Yes	No

PAgP modes:

+ on: The link aggregation is forced to be formed without any PAgP negotiation. A port-channel is formed only if the peer port is also in “on” mode.
+ off: disable PAgP and prevent ports to form a port-channel
+ desirable: send PAgP packets and willing to form a port-channel
+ auto: does not start PAgP packet negotiation but responds to PAgP packets it receives

The table below lists if an EtherChannel will be formed or not for PAgP:

PAgP	Desirable	Auto
Desirable	Yes	Yes
Auto	Yes	No

An EtherChannel in Cisco can be defined as a Layer 2 EtherChannel or a Layer 3 EtherChannel.
+ For Layer 2 EtherChannel, physical ports are placed into an EtherChannel group. A logical port-channel interface will be created automatically. An example of configuring Layer 2 EtherChannel can be found in Question 1 in this article.

+ For Layer 3 EtherChannel, a Layer 3 Switch Virtual Interface (SVI) is created and then the physical ports are bound into this Layer 3 SVI.

For more information about EtherChannel, please read our EtherChannel tutorial.

Question 1

Explanation

From the configuration of SW1, we see it is using Port Aggregation Protocol (PAgP) with “auto” mode so the other end (SW2) must also runs PAgP with “desirable” mode to actively sending request to form an Etherchannel.

Question 2

Explanation

All interfaces in an EtherChannel must be configured identically to form an EtherChannel. Specific settings that must be identical include:
+ Speed settings
+ Duplex settings
+ STP settings
+ VLAN membership (for access ports)
+ Native VLAN (for trunk ports)
+ Allowed VLANs (for trunk ports)
+ Trunking Encapsulation (ISL or 802.1Q, for trunk ports)

In the output of the “show interface fa0/1” commands we see the speed of interface Fa0/1 of SW1 is “100Mb/s” while that of SW2 is “10Mb/s” so the speed is mismatched here -> an Etherchannel will not be formed.

Question 3

Explanation

When an EtherChannel is created, a logical interface will be created on the switches or routers representing for that EtherChannel. You can configure this logical interface in the way you want. For example, assign access/trunk mode on switches or assign IP address for the logical interface on routers… An example of a Layer 3 Etherchannel port is shown below:

interface PortChannel12 description Link to R2 ip address 10.2.4.13 255.255.255.252

Question 4

Explanation

All interfaces in an EtherChannel must be configured identically to form an EtherChannel. Specific settings that must be identical include:
+ Speed settings
+ Duplex settings
+ STP settings
+ VLAN membership (for access ports)
+ Native VLAN (for trunk ports)
+ Allowed VLANs (for trunk ports)
+ Trunking Encapsulation (ISL or 802.1Q, for trunk ports)

-> DTP negotiation settings can be different on ports within an EtherChannel.

Question 5

Explanation

From the output we see these lines:

+ Port-channel = Po2 -> must use the command “channel-group 2 …”
+ Mode = Passive -> must set the mode to passive.
+ Protocol = LACP -> In fact, from the “passive” mode we have already learned it is running LACP.

Therefore the correct command should be “channel-group 2 mode passive”.