Link Aggregation

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Part 1: Introduction

Link Aggregation: the ability to create one logical link using multiple physical links between two devices.

EhterChannel is a form of link aggregation used in switched netwroks.

• Port Aggregation Protocol(PAgP, Cisco Propretary)
• Link Aggregation Control Protocol(LACP, IEEE802.3ad-defined)

Note: it is also possible to configure a static or unconditional EtherChannel without PagP or LACP.

What does EtherChannel look like?

EtherChannel Advanages:

1. Most configuration tasks can be done on the EtherChannel interface instead of on each individual port, ensuring configuration consistency throughout the links.
2. EtherChannel relies on existing switch ports. There is no need to upgrade the link to a faster and more expensive connection to have more bandwidth.
3. Load balancing takes place between links that are part of the same EtherChannel. Depending on the hardware platform, one or more load-balancing methods can be implemented. These methods include source MAC to destination MAC load balancing, or source IP to destination IP load balancing, across the physical links.
5. EtherChannel creates an aggregation that is seen as one logical link. When several EtherChannel bundles exist between two switches, STP may block one of the bundles to prevent switching loops. When STP blocks one of the redundant links, it blocks the entire EtherChannel. This blocks all the ports belonging to that EtherChannel link. Where there is only one EtherChannel link, all physical links in the EtherChannel are active because STP sees only one (logical) link.
6. EtherChannel provides redundancy because the overall link is seen as one logical connection. Additionally, the loss of one physical link within the channel does not create a change in the topology; therefore a spanning tree recalculation is not required. Assuming at least one physical link is present; the EtherChannel remains functional, even if its overall throughput decreases because of a lost link within the EtherChannel.

EtherChannel can be implemented by grouping multiple physical ports into one or more logical EtherChannel links.

Note: Interface types cannot be mixed;

Currently each EhterChannel can consist of up to 8 compatibly-configured Ethernet Ports.(FastEhterChannel:800Mb/s, GigabitEtherChannel:8Gb/s)

The individual EtherChannel group member port configuration must be consistent on both devices. If the physical ports of one side are configured as trunks, the physical ports of the other side must also be configured as trunks within the same native VLAN. Additionally, all ports in each EtherChannel link must be configured as Layer 2 ports.

Part 2: PAgP

PAgP is a cisco-proprietary protocol that aids in the automatic creation of EhterChannel links.

When an EtherChannel link is configured using PAgP, PAgP packets(Sent every 30s) are sent between EtherChannel-capable ports to negotiate the forming of a channel.

When PAgP identifies matched Ethernet links, it groups the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single port.

Note: In EtherChannel, it is mandatory that all ports have the same speed, duplex setting, and VLAN information. Any port modification after the creation of the channel also changes all other channel ports.

PAgP Modes:

• on - Channel member without negotiation(no protocol)
• desirable - Actively asking if the other side can or will participate
• auto - Passively wating for the other side.

Link Aggregation

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原文地址：http://www.cnblogs.com/elewei/p/4721396.html