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EtherChannel Questions 2

November 22nd, 2018 in SWITCH 300-115 Go to comments

Question 1


From the output we see currently the Server_Switch is load balancing via source MAC address. By changing load-balance to another method the problem can be solved. In this case C is the best choice because other answers are surely incorrect.

Question 2


Configuration changes applied to the port-channel interface apply to all the physical ports assigned to the port-channel interface. Configuration changes applied to the physical port affect only the port where you apply the configuration. To change the parameters of all ports in an EtherChannel, apply configuration commands to the port-channel interface, for example, spanning-tree commands or commands to configure a Layer 2 EtherChannel as a trunk.

Note: If we only change the parameters on a physical port of the port-channel, the port-channel may go down because of parameter mismatch. For example, if you only configure “switchport trunk allowed vlan …” on a physical port, the port-channel will go down.

Question 3


The EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host.

Each EtherChannel can consist of up to eight compatibly configured Ethernet interfaces. All interfaces in each EtherChannel must be the same speed, and all must be configured as either Layer 2 or Layer 3 interfaces.

Note: 800 Mbps full-duplex means data can be transmitted at 800 Mbps and received at 800 Mbps (1600 Mbps in total).

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-1_13_ea1/configuration/guide/3550scg/swethchl.html

Question 4


From the last line of the output, we learn physical ports Fa0/13, Fa0/14, and Fa0/15 are bundled into Port-channel 1 and use LACP which is an open standard protocol.

Question 5


The EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host. Therefore if we have 10 Gigabit Ethernet connections, only 8 links will be used.

Question 6


Multichassis LACP (mLACP) is also supported on 7600 and ASR9000 series -> A is not correct.

mLACP supports both FastEthernet & GigabitEthernet -> B is not correct.

VSS mode does not support the mLACP for server access feature only. But mLACP is available in Virtual Switching Systems (VSS). An example of combination of VSS and mLACP is shown below:


In the topology above, the mLACP is a port channel that spans the two chassis of a VSS. Notice that the two chassis of this VSS is connected via a Virtual Switch Link (VSL). VSL is a special link that carries control and data traffic between the two chassis of a VSS. In this case the VSL is implemented as an EtherChannel with two links.

Some of the restrictions for mLACP are mentioned at http://www.cisco.com/c/en/us/td/docs/routers/asr920/configuration/guide/lanswitch/lanswitch-ethernet-channel-xe-3s-asr920-book/lsw_mlacp.html

+ mLACP does not support Fast Ethernet.
+ mLACP does not support half-duplex links.
+ mLACP does not support multiple neighbors.
+ Converting a port channel to mLACP can cause a service disruption (in a short time) -> D is not correct.

Question 7


When enabled, LACP tries to configure the maximum number of LACP-compatible ports in a channel, up to a maximum of 16 ports. Only eight LACP links can be active at one time. The software places any additional links in a hot-standby mode. If one of the active links becomes inactive, a link that is in the hot-standby mode becomes active in its place.
If you configure more than eight links for an EtherChannel group, the software automatically decides which of the hot-standby ports to make active based on the LACP priority. The software assigns to every link between systems that operate LACP a unique priority made up of these elements (in priority order):
+ LACP system priority
+ System ID (a combination of the LACP system priority and the switch MAC address)
+ LACP port priority
+ Port number
In priority comparisons, numerically lower values have higher priority. The priority decides which ports should be put in standby mode when there is a hardware limitation that prevents all compatible ports from aggregating.
Ports are considered for active use in aggregation in link-priority order starting with the port attached to the highest priority link. Each port is selected for active use if the preceding higher priority selections can also be maintained. Otherwise, the port is selected for standby mode.

(Reference: http://www.cisco.com/c/en/us/td/docs/switches/metro/me3600x_3800x/software/release/12-2_52_ey/configuration/guide/3800x3600xscg/swethchl.html#wp1144010)

Question 8


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

LACP Active Passive
Active Yes Yes
Passive Yes No

Therefore if switch 1 is configured LACP in active mode, the other end must be configured as Active or Passive mode.

Note: If the other end is configure with “On” mode, the EtherChannel will not be formed because in “On” mode, no negotiation is sent so the neighbor cannot receive any EtherChannel information.

Question 9


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 the way you want. For example, assign access/trunk mode on switches or assign IP address for the logical interface on routers/Layer 3 switches… An example of a Layer 3 Etherchannel port is shown below:

interface PortChannel12
description Link to R2
ip address

Question 10


To configure EtherChannel load balancing, “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”. Therefore only the “source MAC address and destination MAC address” answer is correct.

Reference this link: http://www.cisco.com/c/en/us/support/docs/lan-switching/etherchannel/12023-4.html

Question 11


You can use EtherChannel guard to detect an EtherChannel misconfiguration between the switch and a connected device. A misconfiguration can occur if the switch interfaces are configured in an EtherChannel, but the interfaces on the other device are not. A misconfiguration can also occur if the channel parameters are not the same at both ends of the EtherChannel.

If the switch detects a misconfiguration on the other device, EtherChannel guard places the switch interfaces in the error-disabled state, and displays an error message.

You can enable this feature by using the “spanning-tree etherchannel guard misconfig” global configuration command.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swstpopt.html

Question 12


You can enable EtherChannel guard to detect an EtherChannel misconfiguration if your switch is running PVST+, rapid PVST+, or MSTP.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swstpopt.html

Question 13


Follow these guidelines and restrictions when configuring EtherChannel interfaces:
+ EtherChannel support: All Ethernet interfaces on all modules support EtherChannel, with no requirement that interfaces be physically contiguous or on the same module.
+ Speed and duplex: Configure all interfaces in an EtherChannel to operate at the same speed and in the same duplex mode. Also, if one interface in the bundle is shut down, it is treated as a link failure, and traffic will traverse other links in the bundle.
+ VLAN match: All interfaces in the EtherChannel bundle must be assigned to the same VLAN or be configured as a trunk.
+ Range of VLANs: An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking Layer 2 EtherChannel.

If the allowed range of VLANs is not the same, the interfaces do not form an EtherChannel, even when set to auto or desirable mode.

Reference: http://www.ciscopress.com/articles/article.asp?p=2348266&seqNum=3

Question 14


If all devices sends traffic to only one destination MAC address then we should load-balance with source MAC

if the traffic on a channel is going only to a single MAC address, using the destination MAC address always chooses the same link in the channel; using source addresses or IP addresses might result in better load balancing.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/54sg/configuration/guide/config/channel.html

Note: The answer “dest-source-MAC” is acceptabe but as we know there is only one destination MAC address so this answer is the same as “destination-MAC”. Therefore the answer “source-IP” is better.

Comment pages
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    August 21st, 2017

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  2. Hank
    September 19th, 2017

    Yes, question #3 answer is wrong. The link provided is no longer available… In the CCNP Switch book in chp 10 it explicitly states 16Gb/s.

    Answer is C.

    From the CCNP Book:
    Cisco offers another method of scaling link bandwidth by aggregating, or bundling,
    parallel links, termed the EtherChannel technology. Two to eight links of either Fast
    Ethernet (FE), Gigabit Ethernet (GE), or 10-Gigabit Ethernet (10GE) can be bundled as
    one logical link of Fast EtherChannel (FEC), Gigabit EtherChannel (GEC), or 10-Gigabit
    Etherchannel (10GEC), respectively. This bundle provides a full-duplex bandwidth of up
    to 1600 Mbps (eight links of Fast Ethernet), 16 Gbps (eight links of GE), or 160 Gbps
    (eight links of 10GE).

    This also provides an easy means to “grow,” or expand, a link’s capacity between two
    switches, without having to continually purchase hardware for the next magnitude of
    throughput. For example, a single Fast Ethernet link (200 Mbps throughput) can be
    incrementally expanded up to eight Fast Ethernet links (1600 Mbps) as a single Fast
    EtherChannel. If the traffic load grows beyond that, the growth process can begin again
    with a single GE link (2 Gbps throughput), which can be expanded up to eight GE links
    as a Gigabit EtherChannel (16 Gbps). The process repeats again by moving to a single
    10GE link, and so on.

  3. Mark
    October 7th, 2017

    The C answer say: up to 800 Mbps ONLY for Fast EtherChannel or 16 Gbps ONLY for Gigabit EtherChannel.
    ONLY is the problem, because the cisco ccnp book say: Two to eight links of either Fast
    Ethernet (FE), Gigabit Ethernet (GE)

    so, for me A is correct

  4. Mark
    October 7th, 2017


    What is the maximum number of 10 Gigabit Ethernet connections that can be utilized in an EtherChannel for the virtual switch link?
    C. 8

    Same as 3

  5. Hank
    October 13th, 2017

    @Mark: The ONLY statement is the same in answer A or C.
    Q5 is answer C for 8.
    BUT…the links can be 2Gbps each, which equals 16Gbs throughput.
    The 8Gbps is an old standard and the link provided no longer exists because of the new 16Gbps standard. The key is the statement about full-duplex. That means 8Gbps x 2.

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    November 6th, 2017

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  7. Marcus
    December 21st, 2017

    A small clarification about Q14:

    The answer “dest-source-MAC” is NOT acceptabe, because no such method. We can use src-dst-mac, but NOT dst-src-mac.


  8. Anonymous
    February 13th, 2018

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  9. Anonymous
    February 13th, 2018

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  11. 1WAY
    February 14th, 2018

    @Anonymous. Look at 9. Under the ‘Switch FAQ & Tips’ URL link for questions.

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    May 30th, 2018

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  13. patzen
    July 12th, 2018

    Q14 @Marcus, well spotted

  14. patzen
    July 12th, 2018

    Q3 I don’t understand what is the fuss about here, answer A clearly states fast ethernet = 100Mbs and gigabit ethernet=1000Mbs, the question doesn’t even mention 2Gbs interfaces

    100Mbs x 8 = 800Mbs
    1Gbs x 8 = 8Gbs

    some nexus ( they don’t run IOS) models can have 16 active interfaces in the bundle then yes you would have 16Gbs

    on switches not supporting 16 active interfaces in ehterchannel you can have 8 active and 8 standby, maybe that’s how cisco is trying to confuse us here

    in regards @ Hank September 19th, 2017

    maybe by “up to 16Gbs” they mean other models that are capable of running 16 interfaces concurrently

    also neither question nor the part of the book you cited mentions throughput

  15. harry
    July 28th, 2018

    Do you have any link for ether channel tutorial. I don’t understand which load balancing method to use. thank you in advance

  16. Dany1
    August 10th, 2018

    patzen is not 16 . LACP is only protocol and have 8 active ports and 8 hot standby. The issue is embedded in CIsco hardware. Can be use maximum 3 bits for load-balance . 2^3=8 active ports So, not depends on equipment type. Which is really depend is Number of Etherchannel (16, 64, 128 … )

  17. NetShit
    July 14th, 2019

    In the explanation of question 6, it says that both fast eth and gig eth are supported. However, according to Cisco’s mLACP restrictions (see link below), this is not true. Fast ethernet is not supported in mLACP. Only gig eth is.


  18. NetShed
    July 14th, 2019

    Q9: Layer 2 Etherchannel with an IP address??? Wouldn’t that be a LAYER 3 etherchannel???? This is driving me crazy, layer 2 interfaces, logical or physical, are not supposed to have ip addresses.
    Please can somebody explain???

  19. Cisco…lol
    September 15th, 2019

    RE: Q9 – The question has a typo. It is a layer 3 port channel, hence the IP address and lack of “switchport” config.

    RE: Q3 – 8 active interfaces = 8 Gbps if they are gig, theoretical max. UNIDIRECTIONALLY!!!

    Cisco is lame. They license their ISR routers with “aggregate” throughput, so they consider a 1 Gbps interface as passing 2 Gbps of traffic (1 in, 1 out).

    SO, based off that logic, and since they state “full-duplex bandwidth,” one can surmise they are trying to be clever (annoying) and an 8 link bundle running gig interfaces would have 16 Gbps theoretical max throughput BIDIRECTIONALLY.

    Q3 therefore is C, in agreeance with Hank and his snippet.

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