Thursday, September 29, 2016

CCNAv3 Chapter 3 Notes - Link Aggregation

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This post contains notes for
CCNA3 Chapter 3 Notes
Image result for link aggregation

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Link Aggregation
  • What is Link Aggregation?
    • The ability to create one logical link using multiple physical links between two devices.
  • What does Link Aggregation do?
    • Link aggregation allows us to have load sharing amongst the physical links, rather than having STP block one or more of the links.

  • What is EtherChannel?
    • A form of link aggregation used in switched networks.

    • Can be manually configured or negotiated by two protocols:  
      • Port Aggregation Protocol (PAgP)
      • Link Aggregation Control Protocol (LACP) (an IEEE 802.3ad standard)

  • What is the original purpose of EtherChannel?
    • The original purpose of EtherChannel is to increase speed capability on aggregated links between switches.

  • Advantages of EtherChannel:
    • Advantage #1: Many configuration tasks can be done on the EtherChannel interface in place of individual ports, ensuring consistency throughout the links.

    • Advantage #2: EtherChannel relies on existing switch ports, meaning there is no need to upgrade the link to a faster (and more expensive) connection for more bandwidth.

    • Advantage #3: Load balancing takes place between links that are apart of the same EtherChannel.
      • The methods used to implement Load Balancing are...
        • Source MAC to Destination,
        • MAC Load Balancing, or
        • Source IP to Destination IP

    • Advantage #4: EtherChannel creates aggregation that is seen as one logical link.
      • Where there is only one EtherChannel link, all physical links in the EtherChannel are active because STP sees only one (logical) link (whereas others are blocked).

    • Advantage #5: EtherChannel provides redundancy because the overall link is seen as one logical connection.

    • Advantage #6: The loss of one physical link within the channel does not create a change in the topology

  • Implementation and Restrictions of EtherChannel:
    • EtherChannel can be implemented by grouping multiple physical ports into one or more logical EtherChannel links.

    • Restriction #1: Interface types cannot be mixed; for example, Fast Ethernet and Gigabit Ethernet cannot be mixed within a single EtherChannel.

    • Restriction #2: The individual EtherChannel group member port configuration must be consistent on both devices.

  • Port Channel
    • What is “Port Channel”?
      • Term used to describe the resulting virtual interface when EtherChannel is configured.

Port Aggregation Protocol
EtherChannels can be formed through negotiation using one of two protocols:
  • PAgP
    • Cisco-proprietary protocol.
    • Aids in the automatic creation of EtherChannel links.
    • Groups links into EtherChannel by sending packets.
    • Manages the EtherChannel when enabled.
    • Every 30 seconds, PAgP packets are sent out.
      • PAgP checks for configuration consistency and manages links.
      • Ensures when an EtherChannel is created, all ports have the same configuration.

Forces the interface to channel without PAgP. Does not exchange PAgP packets.
Actively asking if the other side can or will participate. Negotiates with PAgP packets.
Interface responds to the PAgP packets, but doesn't initiate the PAgP negotiation.

Channel member without negotiation (no protocol).
Actively asking if the other side can or will participate.
Passively waiting on the other side to initiate negotiation. Does not send LACP packets, but will respond to them (in hopes for initiation).

Channel Establishment

Channel Establishment

  • What is LACP?
    • LACP is a protocol similar to PAgP.
    • LACP allows several physical ports to be bundled to form a single logical channel.
    • LACP allows a switch to negotiate an automatic bundle by sending LACP packets to the peer.

PAgP and LACP Negotiation Term Description
Places interface in a passive, responding state. Does NOT initiate PAgP negotiations.
Initiates LCP negotiations with other interfaces.
Actively initiates PAgP negotiations with other interfaces.
Forces EtherChannel state without PAgP or LACP initiated negotiations.
Places an interface in a passive, responding state. Does not initiate LACP negotiations.

Configuration Guidelines
The following guidelines and restrictions exist when configuring EtherChannel:

  • EtherChannel Support
    • All interfaces must support EtherChannel and must not require that they must be 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.

  • VLAN Match
    • All interfaces in the EtherChannel bundle must be assigned to the same VLAN, or be configured as a trunk.

  • Range of VLAN
    • An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking 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.

Configuring EtherChannel with LACP
  • Step 1: Specify the interfaces that comprise the EtherChannel group using the interface range interface global configuration mode command.
    • Before continuing, It’s suggested to shut down the ports you’re configuring prior to doing so to prevent errors.

  • Step 2: Create the port channel interface with the channel-group identifier mode active command in interface range configuration mode.
    • The identifier specifies a channel group number. The mode active keywords identify this as an LACP EtherChannel configuration.

  • Table & Diagram as Example to Step 1 & Step 2:
    • Diagram of two switches, with VLANs (as trunks, of course):

    • The commands used to configure the diagram above (up-page).
S1(config)# interface range FA0/1-2
S1(config-if-range)# channel-group 1 mode active
S1(config-if-range)# interface port-channel 1
S1(config-if)# switchport mode trunk
S1(config-if)# switchport trunk allowed vlan 1,2,20

Troubleshooting EtherChannel
  • All interfaces within an EtherChannel must have the same configuration of…
    • Speed and duplex mode,
    • Native and allowed VLANs on trunks, and
    • Access VLAN on access ports.

  • Causes & How To Avoid:
    • Assign all ports in the EtherChannel to the same VLAN, or configure them as trunks.
    • Ports with different native VLANs cannot form an EtherChannel.
    • PAgP and LACP negotiation options must be compatible on both ends.

  • Verifying Examples:
    • When configuring an EtherChannel from trunk ports, verify that the trunking mode is the same on all the trunks.
    • An EtherChannel supports the same allowed range of VLANs on all the ports.

  • Penalties of Bad Troubleshootings
    • Inconsistent trunk modes on EtherChannel ports can cause EtherChannel not to function and ports to be shutdown (“error disable state”).
    • If the allowed range of VLANs is not the same, the ports do not form an EtherChannel even when PAgP is ‘automatic’ or ‘desirable’ mode.

  • Knowing the Differences
    • PAgP and LACP are used for link aggregation (EtherChannel).
    • DTP is used for automating the creation of trunk links.
    • It is very easy to confuse DTP with PAgP and LACP:
      • When an EtherChannel trunk is configured, typically EtherChannel (PAgP or LACP) is configured first and then DTP.
  • Reminders
    • EtherChannel and spanning tree must interoperate. For this reason, the order in which EtherChannel-related commands are entered is important,


  • EtherChannel aggregates multiple switched links together to load balance over redundant paths between two devices.
    • All ports in one EtherChannel must have the same speed, duplex setting, and VLAN information on all interfaces on the devices at both ends.
    • Settings configured in the port channel interface configuration mode will also be applied to the individual interfaces in that EtherChannel.
    • Settings configured on individual interfaces will not be applied to the EtherChannel or to the other interfaces in the EtherChannel.

  • PAgP is a Cisco-proprietary protocol that aids in the automatic creation of EtherChannel links.
    • PAgP modes are on,
      • PAgP desirable, and
      • PAgP auto.

  • LACP is part of an IEEE specification that also allows multiple physical ports to be bundled into one logical channel.
    • The LACP modes are on, LACP active and LACP passive.
    • PAgP and LACP do not interoperate.
    • The on mode is repeated in both PAgP and LACP because it creates an EtherChannel unconditionally, without the use of PAgP or LACP.
    • The default for EtherChannel is that no mode is configured.


  1. can I get the ccna final pdf, thanks

    1. Nadia,
      Which final were you looking for?

      CCNA1's can be found in the CCNA1 tab here:

      CCNA2's final exam questions, as well as the practice, can be found here:

      CCNA3's and CCNA4's, of course, is in progress, so Google might be your best bet otherwise.

      Hope this helps.
      ~ Seeseenayy


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