The PAgP (and LACP) are protocols used to negotiate the creation of an EtherChannel. Their purpose is to find out if the opposite device is also willing to bundle the ports into a single channel, if all ports are truly connected to the same opposite device, whether they belong into the same group... these kinds of verifications that are necessary before bundling the ports together. After the EtherChannel is established, PAgP/LACP make infrequent checks whether the opposite device still bundles the ports together (by listening to PAgP/LACP PDUs). Once again, the PAgP/LACP are signalling protocols intended to negotiate bundling of parallel links into an EtherChannel.
However, neither PAgP nor LACP change the frame format (i.e. the encapsulation) of data carried over an EtherChannel bundle, and PAgP/LACP are themselves encapsulated into ordinary Ethernet frames. Hence, using a double WiFi interconnection between two switches and bundling it into an EtherChannel is indeed possible.
Care must be taken to configure the WiFi in a way that does not allow any other parties to join either of your WiFi links. In other words, both WiFi connections must behave in a point-to-point fashion and should never leak one into another, nor should they ever allow an external third party WiFi client to associate.
As with all wireless technologies, this setup is inherently prone to packet losses and variable delay. Interconnecting two switches together with a parallel WiFi interconnect should, in my personal opinion, be used only as a temporary measure. Your mileage may vary - but I would be very thankful if you shared your experience with running this setup!
I am perhaps answering for David himself, but a couple of observations:
This doesn't make sense. Wireless as a backup for what? What is your primary link?
David obviously has these two switches connected via two WiFi links, and wants to use one of them as a backup for the other. He indicates that in his original post: "... if it is good idea to use PAGP as backup for wireless link with another wireless link."
What speed do you see yourself using this so-called "backup wireless link"?
What is the distance between both Switch 1 wireless and Switch 2 wireless?
You are probably referring to the fact that wireless links are notorious for being slow, lossy, quite unpredictable in their performance, even two parallel wireless links can differ quite significantly. That is all true. However, thanks to the nature of load-balancing algorithms in Cisco's EtherChannel implementation, a single flow always traverses one particular link only, so even though the performance may strongly vary for different flows, at least no frame reordering should happen. We are otherwise accustomed to the performance of wireless links so that should not be anything new.
What flavour of "wireless" are you talking about? 802.11 a/b/g/n? BT? WiMax? Satellite?
Well, in my understanding, the WiFi specifically refers to 802.11 family of wireless technologies.
I am not saying that running PAgP/LACP over a bunch of wireless links is a particularly good idea, but then again, I do not see any reason why not try it
Cisco now has an IOS that can support wireless bridge using 802.11n protocol. Alot faster but you need to ensure you have a pair of directional antenna if Switch 1 and 2 are located at a fair distance. One must also ensure that there is line-of-sight. Another thing alot of people fail to consider: trees GROW.
Agree - WiFi is WiFi, but a cable is a cable That is also why I indicated in my very first response that such a setup should be considered only a temporary measure until a copper/fiber interconnection can be made. Then again, to me, it is a nice experiment and I would like to see its results.
One must also ensure that there is line-of-sight.
Sure, and all the Fresnel zones... There are lots of things to consider in wireless technologies, aren't they?
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