Introduction to Ether-Channel : PAGP and LACP Protocols

Today I am going to talk about the Ether-channel standards named as PAgP and LACP protocols. As I earlier wrote about Ether-channel and the purpose of the ether-channel but we can cover the same again in this article.

What is Ether-channel ? 
Ether-channel port channel is the way to connect two different switches with two or more links and make them bundled in such a way that it looks like to be a single link.

Below is the picture showing the Ether-channel between the switches as 

Fig 1.1- Network Scenario

So in the above mentioned diagram showing the two links bundled between one device to another device and logically it is a single link between the devices, this is called as Ether-channel.

Now some of you thinking, in the above scenario there are two links between NDNA router and NDNA multi-layer switch, if one of the link down then what happens ?

So if you look on the topology, it is showing the aggregate bandwidth of 400 Mbps which means two links of 200 Mbps each, if one of the link down, the connectivity between them will not be effected but you will get the bandwidth of 200 Mbps. Hope it clears the concept of Ether-channel now.

LACP Protocol 
Lets talk about LACP protocol first, so LACP is a Standards-based (IEEE 802.3ad) negotiation protocol. Link Aggregation Control Protocol, is used to dynamically build an Ether-channel. There are different modes of LACP.

  • Active: The active end of the group sends out a LACP frame and initiates the negotiation to form the Ether-channel. Both ends could be active and the result would be the same.
  • Passive: Passive Mode does not initiate the negotiation. It just responds to LACP packets initiated by other end. So if both ends were passive, the Ether-channel would not be formed. 
PAgP Protocol
PAgP is a Cisco proprietary negotiation protocol. Port Aggregation Protocol is used to achieve the same goal as LACP. PAgP has 2 modes:

  • Desirable: In this mode, the negotiation will be initiated by sending the special frames to form Ether-channel with the other end.
  • Auto: In this mode, negotiation is not initiated. It only responds to the special frames received for negotiation form the other end. So, if both ends are in Auto mode, Ether-channel would not be formed