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EtherChannel Questions

November 22nd, 2019 Go to comments

Quick overview of EtherChannel:

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

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

LACP Active Passive
Active Yes Yes
Passive Yes No

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

PAgP Desirable Auto
Desirable Yes Yes
Auto Yes No

To form an Etherchannel both sides must use the same Etherchannel protocol (LACP or PAgP). According the two tables above we can see only “desirable” and “auto” (of PAgP) can form an Etherchannel bundle.

Note: If we want to use “on” mode, both ends must be configured in this “on” mode to create an Etherchannel bundle.

Question 2

Explanation

To form an Etherchannel both sides must use the same Etherchannel protocol (LACP or PAgP).

Question 3

Explanation

In this case the EtherChannel bundle was configured to load-balance based on the destination IP address but there is only one web server (means one destination IP address). Therefore only one of the EtherChannel links is being utilized to reach the web server. To solve this problem we should configure load-balancing based on source IP address so that traffic to the web server would be shared among the links in the EtherChannel bundle with different hosts.

Question 4

Question 5

Explanation

If one end is passive and another end is active then the EtherChannel will be formed regardless the two interfaces in the same switch use different modes and different load-balancing method. Switch 1 will load-balance based on destination IP while Switch2 will load-balance based on source MAC address.

Question 6

Explanation

When storm control is configured on an EtherChannel, the storm control settings propagate to the EtherChannel physical interfaces. In the “show etherchannel” command output, The storm control settings appear on the EtherChannel but not on the physical port of the channel.

Note: You cannot configure storm control on the individual ports of that EtherChannel.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_22ea/SCG/scg/swtrafc.html

Question 7

Explanation

Issue the port-channel load-balance {src-mac | dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port | dst-port | src-dst-port | mpls} global configuration command in order to configure the load balancing.

Question 8

Explanation

A LACP port priority is configured on each port using LACP. The port priority can be configured automatically or through the CLI. LACP uses the port priority with the port number to form the port identifier. The port priority determines which ports should be put in standby mode when there is a hardware limitation that prevents all compatible ports from aggregating.

The syntax of LACP port priority is (configured under interface mode):

lacp port-priority priority-value

The lower the range, the more likely that the interface will be used for LACP transmission.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2sb/feature/guide/gigeth.html

Question 9

Explanation

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

PAgP Desirable Auto
Desirable Yes Yes
Auto Yes No

For “on” mode, 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.

Question 10

Explanation

Interfaces Fa0/13 to Fa0/15 are bundled into Port-channel 12 and it is running with “desirable” mode -> it is using PAgP.

Comments
  1. Peach
    January 29th, 2020

    Q4:
    “After configuring SPAN to monitor this port” – says that the etherchannel is the source and not the destination port. So I think C is not correct, because it is not the question.

    So I think A is the correct answer, because the traffic of the whole etherchannel is too much for a single destination port. This cause some packets are dropped by the Destination.

  2. null
    February 8th, 2020

    Boilami is correct on Q10.

    when you configure mode on you are not using a control protocol, this mean that you are NOT using PAGP or LACP. the switch simply put the port and bundle them together. NO alternatives are correct, i bet that this question is different in the exam.

    for example:

    Switch(config-if)#do sho run int gig 0/0
    Building configuration…

    Current configuration : 96 bytes
    !
    interface GigabitEthernet0/0
    media-type rj45
    negotiation auto
    channel-group 1 mode on
    end

    Switch(config-if)#channel-protocol lacp
    Command rejected ( Gi0/0 ): is already part of a channel with a different type of protocol enabled

    Switch(config-if)#channel-protocol pagp
    Command rejected ( Gi0/0 ): is already part of a channel with a different type of protocol enabled

    Switch(config-if)#

  3. suntzu
    February 15th, 2020

    I think Q10 might be a question about is it suspended or down or something like that. IT doesn’t make sense to ask about mode.

    The int trunk command doesn’t add anything to the question except to confuse the test taker

  4. suntzu
    February 22nd, 2020

    @Peach
    January 29th, 2020
    Q4:
    “After configuring SPAN to monitor this port” – says that the etherchannel is the source and not the destination port. So I think C is not correct, because it is not the question.

    So I think A is the correct answer, because the traffic of the whole etherchannel is too much for a single destination port. This cause some packets are dropped by the Destination.

    ^^The problem is that A is about VLANs and this is an etherchannel question. Cisco is tricky and you have to know what TYPE of question they are asking about. There is no way that they would have the words ETHERCHANNEL in a question and the right answer has to deal with a VLAN.

    Maybe on the CCIE test but not the CCNP level.

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