摘自:
https://en.wikipedia.org/wiki/Link_aggregation
http://blog.xuite.net/zerofirst/blog/239023010-LACP+(Link+Aggregation+Control+Protocol)+
All sorts of things about LACP and LAGs
EtherChannel(PAgP、LACP)基本配置
在一般伺服器或者交換機的網路在大量傳輸過程會遭遇到瓶頸，通常要解決這種問題直觀方式就是儘可能的加大每條channel的頻寬囉～ 但每個Port的上線頻寬都是固定的阿！所以就有人提出所謂的「Trunking」的概念。
Trunking意即為將幾條port channel「邏輯的」想像成合併成一條大的channel。好比從甲地開車到乙地原本是只有一條單行道可以通行（一個port），Trunk就是將這條馬路拓寬為三線道四線道，白話點的解釋可以這樣想像。
而當大家遇到這些問題的時候各家廠商就會去各自提出解決的方式。在過去，INTEL叫LINK
AGGREGATION，CISCO叫FAST
ETHERCHANNEL，3COM叫PORT
TRUNKING。百家爭鳴的結果所以衍生一個國際標準：IEEE802.3ad
LACP
但Trunking為static的，沒有設定就不會有這功能。LACP則差別是有Learning的概念，透過一段時間後可以找出最適合現在網路狀況的LACP Group，動態的改善網路頻寬的使用
LACP(Link Aggregation Control Protocol)為IEEE 標準規格 802.3ad. 可以將上許多不同的實體連接埠(port)邏輯的共同結合在一起, 視為一條線路[1]。LACP有幾個限制條件：
1)full-duplex links
2)相同的media type, speed, duplex mode, flow
control settings
Link Aggregation ID
Link Aggregation ID：system
priority + MAC + KEY + port priority + port Number
同一個LACP的成員埠可以均衡負載，相互備援。LACP通常用於需要提高主幹網路頻寬的應用，這是一個提高網路傳輸資料量的最經濟的方式。
EtherChannel(乙太通道)是由Cisco研發的，應用於交換機之間的多鏈路捆綁技術。
它的基本原理是：
將兩個設備間多條相同特性的快速乙太或十億位元乙太物理鏈路捆綁在一起組成一條邏輯鏈路，從而達到帶寬倍增的目的。除了增加帶寬外，EtherChannel還可以在多條鏈路上均衡分配流量，起到負載分擔的作用；當一條或多條鏈路故障時，只要還有鏈路正常，流量將轉移到其他的鏈路上，整個過程在幾毫秒內完成，從而起到冗餘的作用，增強了網路的穩定性和安全性。
在EtherChannel中，負載在各個鏈路上的分佈可以根據源IP位址、目的IP位址、源MAC位址、目的MAC位址、源IP位址和目的IP位址組合，以及源MAC位址和目的MAC位址組合等來進行分佈。兩台交換機之間是否形成EtherChannel也可以用協議自動協商。目前有兩個協商協議：PAgP和LACP，PAgP(埠彙聚協議 Port
Aggregation Protocol)是Cisco私有的協議，而LACP(鏈路彙聚控制協議 Link
Aggregation Control Protocol)是基於IEEE 802.3ad的國際標準。
標準化:
Initial release 802.3ad in 2000
As of 2000, most gigabit channel-bonding schemes use the IEEE standard of Link Aggregation which was formerly clause 43 of the IEEE 802.3 standard added in March 2000 by the IEEE 802.3ad task force. Nearly every network equipment manufacturer quickly adopted this joint standard over their proprietary standards.
Move to 802.1 layer in 2008
David Law noted in 2006 that certain 802.1 layers (such as 802.1X security) were positioned in the protocol stack below Link Aggregation which was defined as an 802.3 sublayer. This discrepancy was resolved with formal transfer of the protocol to the 802.1 group with the publication of IEEE 802.1AX-2008 on 3 November 2008.
~
LACP
Advantages over static configuration
- Failover occurs automatically: When a link fails and there is (for example) a media converter between the devices, a peer system will not perceive any connectivity problems. With static link aggregation the peer would continue sending traffic down the link causing the connection to fail.
- Dynamic configuration: The device can confirm that the configuration at the other end can handle link aggregation. With Static link aggregation a cabling or configuration mistake could go undetected and cause undesirable network behavior.
Practical notes
LACP works by sending frames (LACPDUs) down all links that have the protocol enabled. If it finds a device on the other end of the link that also has LACP enabled, it will also independently send frames along the same links enabling the two units to detect multiple links between themselves and then combine them into a single logical link. LACP can be configured in one of two modes: active or passive. In active mode it will always send frames along the configured links. In passive mode however, it acts as "speak when spoken to", and therefore can be used as a way of controlling accidental loops (as long as the other device is in active mode)
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