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CCNA Routing and Switching Practice and Study Guide: Link Aggregation

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This chapter is from the book
To help you prepare for the ICND2 200-101 Certification Exam, this chapter explains the concepts behind and configuration strategies for link aggregation: creating one logical link using multiple physical links between two devices.

Link aggregation is the ability to create one logical link using multiple physical links between two devices. This allows load sharing among the physical links, rather than having a STP block one or more of the links.

Link Aggregation Concepts

One of the best ways to reduce the time it takes for STP convergence is to simply avoid STP. EtherChannel is a form of link aggregation used in switched networks.

EtherChannel Advantages

EtherChannel technology was originally developed by Cisco as a technique of grouping several Fast Ethernet or Gigabit Ethernet switch ports into one logical channel.

List at least three advantages to using EtherChannel:

EtherChannel Operation

You can configure EtherChannel as static or unconditional. However, there are also two protocols that can be used to configure the negotiation process: Port Aggregation Protocol (PAgP—Cisco proprietary) and Link Aggregation Control Protocol (LACP—IEEE 802.3ad).

These two protocols ensure that both sides of the link have compatible configurations—same speed, duplex setting, and VLAN information. The modes for each differ slightly.

For PAgP, briefly describe each of the following modes:

  • On:
  • Desirable:
  • Auto:

For LACP, briefly describe each of the following modes:

  • On:
  • Active:
  • Passive:

In Table 3-1, indicate the mode that is described.

Table 3-1 PAgP and LACP Modes

Mode

PAgP and/or LACP Mode Description

Initiates LACP negotiations with other interfaces.

Forces EtherChannel state without PAgP or LACP initiated negotiations.

Places an interface in a passive, responding state. Does not initiate PAgP negotiations.

Actively initiates PAgP negotiations with other interfaces.

Places an interface in a passive, responding state. Does not initiate LACP negotiations.

The mode that is configured on each side of the EtherChannel link determines whether EtherChannel will be operational.

In Table 3-2, two switches are using PAgP. Indicate with “yes” or “no” whether EtherChannel is established.

Table 3-2 EtherChannel Negotiation Using PAgP

Switch 1 Mode

Switch 2 Mode

EtherChannel Established?

Auto

Auto

Auto

Desirable

On

Desirable

On

Off

Desirable

Desirable

In Table 3-3, two switches are using LACP. Indicate with “yes” or “no” whether EtherChannel is established.

Table 3-3 EtherChannel Negotiation Using LACP

Switch 1 Mode

Switch 2 Mode

EtherChannel Established?

Passive

On

Passive

Active

On

On

Passive

Passive

On

Active

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