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Cisco EtherChannel Technology

• The Cisco EtherChannel technology is Cisco’s method of aggregating or bundling parallel links to scale bandwidth.
  • Port Trunking or Link Aggregation is the standard term for this technology. It is not to be confused with Cisco’s use of the word “trunking” which refers to the network link used to carry multiple VLANs between switches.
• It is standards-based built upon the IEEE 802.3-compliant Ethernet mechanisms for full-duplex autonegotiation and autosensing.
• It allows you to bundle up to eight individual links to form one logical link:
  • 8 Fast Ethernet (full duplex) = 1600 Mbps
  • 8 Gigabit Ethernet (full duplex) = 16 Gbps
  • 8 10-Gigabit Ethernet (full duplex) = 160 Gbps
• Whenever parallel links between switches are present, the possibility of bridging loops increase. However, with EtherChannel, this problem is avoided by bundling these parallel links into a single logical link.
• Although an EtherChannel link is seen as a single logical link, the speed of the link is not always the sum of all the physical links’ speed.
  • For example, if we have 8 FastEthernet physcial links bundled together (1600 Mbps at full-duplex), the logical link doesn’t operate at that speed.
  • Although it is capable of carrying a total throughput of 1600 Mbps, traffic is not equally distributed to all the links.
  • Instead, traffic is distributed through each individual FastEthernet links within the EtherChannel. And each link operates at its inherent speed, that is 200 Mbps at full-duplex.
  • If one link within that bundle is used more than the other, the link will carry more load than the other links.
  • There is a way to balance the load more evenly among all other links (discussed at a later post).
• When bundling ports with EtherChannel:
  • All bundled ports must generally belong to the same VLAN.
  • If used as a trunk, bundled ports must be in trunking mode, have the same native VLAN, and pass the same set of VLANs.
  • Each port should have the same speed and duplex settings.
  • And each bundled port must be configured with identical spanning-tree settings.

EtherChannel Negotiation Protocols

I. Port Aggregation Protocol (PAgP)

• Cisco-proprietary.
• PAgP manages the automatic EtherChannel configuration and negotiation between switches.
  • Checks to make sure configuration is consistent on both ends of the link.
  • Manages link failures and addition of links.
  • Helps in network reliability by preventing STP loops or packet loss due to misconfiguration.
• EtherChannel formed only on ports that are configured for either identical static VLANs or trunking.
• If one of the bundled ports’ parameters are changed (ie VLAN, speed, duplex setting) PAgP also dynamically changes the parameters on the EtherChannel.

PAgP Modes

Options

The following options are available for the Auto and Desirable modes:

• Silent
  • Default option/keyword used for Auto or Desirable mode.
  • Allows ports to be added to an EtherChannel even if the other end of the link is silent and never transmits PAgP packets
  • Indicates that the switch does not expect PAgP frames from the partner device to prevent the switch from reporting the link to the STP as down.
  • This could be used to form an EtherChannel with a device such as a file server or network analyzer that does not participate in PAgP.
• Non-silent
  • Used with Auto or Desirable mode to indicate that the switch expects PAgP frames from the switch on the other end of the link.
  • This requires each port to receive PAgP packets before they can be added to the EtherChannel.
  • If a unidirectional link is detected (due to absence of PAgP packets received) the port status is reported to STP as down.

Mode Compatibilities

• On mode is compatible only with another port in the On mode.
• Auto mode is compatible only with Desirable mode.
• Desirable mode is compatible with Auto or Desirable.

II. Link Aggregation Control Protocol

• This is an industry standard based on IEEE 802.3ad (also known as IEEE 802.3 Clause 43, “Link Aggregation”)
• Just like PAgP, switches exchange LACP packets to learn parameters.
• On ports configured to use LACP, the protocol tries to configure the maximum number of compatibel ports in an EtherChannel, up to the maximum allowed by the hardware (eigth ports).
• There are different parameters that are used in LACP:
  • LACP System Priority
    • The lowest system priority is the one that makes decisions about what ports are actively participating in the EtherChannel at a given time.
    • The 2-byte system priority combined with the 6-byte switch MAC address form the system ID, which is used during negotiation with other switches.
    • This can be manually configured or automatically assigned by the swtich.
  • LACP Port Priority
    • LACP uses the port priority to decide which prots should be put in standy mode when there is a hardware limitation that prevents all compatible ports from aggregating.
      • For example, a set of 16 links defined for EtherChannel. Because only 8 are able to be bundled, the switch selects 8 ports with the lowest priorities and set as active. The other 8 goes on standby.
    • The 2-byte port priority + 2-byte port number makes up the port identifier.
    • The value can be determined by the switch or through manual configuration
  • LACP Administrative Key
    • Defines the ability of a port to aggregated with other ports, determined by:
      • Port’s physical characteristics such as: data rate, duplex settings, point-to-point or shared medium.
      • Manually configured restrictions.
    • LACP autmatically configures an administrative key value equal to teh channel group ID number on each port configured to use LACP.
    • It can also be manually configured.

LACP Modes

Resources:

1. Cisco EtherChannel Technology – Technology Whitepaper – Cisco Systems, Inc.
2. Understanding PAgP EtherChannel Configuration – Catalyst 6500 Configuration Guide 12.1E
3. Understanding LACP EtherChannel Configuration – Catalyst 6500 Configuration Guide 12.1E
4. IEEE 802.3 Standard - Click link –> Select User Type —> Accept/Begin Download. Go to “Clause 43, Link Aggregation”

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BCMSN course, consider purchasing Building Cisco Multilayer Switched Networks (BCMSN) (Authorized Self-Study Guide) (4th Edition)by Richard Froom, Balaji Sivasubramanian, and Erum Frahim and CCNP BCMSN Official Exam Certification Guide (4th Edition) by Dave Hucaby ; as well as following the links on the reference section of this entry.