Time to bust another myth! Supposedly PAgP runs in silent mode by default
in both desirable and auto mode. So what does silent mode do?

“Use the silent mode when the switch is connected to a device that is not PAgP-capable
and seldom, if ever, sends packets. An example of a silent partner is a file server
or a packet analyzer that is not generating traffic. In this case, running PAgP on a
physical port connected to a silent partner prevents that switch port from ever becoming
operational. However, the silent setting allows PAgP to operate, to attach the port to a
channel group, and to use the port for transmission.”

So now for the myth itself. The first quote is from David Hucabys CCNP SWITCH 642-813
Official Certification Guide.

“By default, PAgP operates in silent submode with the desirable and auto modes, and
allows ports to be added to an EtherChannel even if the other end of the link is silent
and never transmits PAgP packets. This might seem to go against the idea of PAgP,
in which two endpoints are supposed to negotiate a channel. After all, how can two
switches negotiate anything if no PAgP packets are received?”

And then from the Cisco configuration guide for 3560 12.12(58)SE.

“If your switch is connected to a partner that is PAgP-capable, you can configure the
switch port for nonsilent operation by using the non-silent keyword. If you do not specify
non-silent with the auto or desirable mode, silent mode is assumed.”

So even Cisco themselves claim that both modes operate in silent mode. Surely Cisco
can’t be wrong?! Doesn’t it seem strange to operate in silent mode by default? The
most common use must be to connect to other switches?

To test this we setup two switches with two trunks between themselves. One side is
set to auto and one side is set to desirable. Then debug pagp all is run to check
what mode they are running in.

Myth busted! Desirable runs in non silent mode but auto runs in silent mode.
So this myth exist book in official certification books and in Ciscos documents
which is bad. I’ll look into getting it update if I can. The point of forming
an etherchannel is to negotiate with the other side before forming it to make
sure that the links are not unidirectional and that they agree on all parameters.

Busting myths – PAgP desirable runs in silent mode by default
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7 thoughts on “Busting myths – PAgP desirable runs in silent mode by default

  • August 15, 2013 at 12:42 am
    Permalink

    Nice one. It always bugs me when books mention features and don’t explain their purpose. Like you said in your last sentence, the sub-modes are mainly for unidirectional link detection. Remembering stuff is a lot easier when you know how and why.

    Those were the best references I could find during my IE studies that clarify some things about non-silent vs silent mode.

    http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a00801b49a4.shtml#cg6

    https://supportforums.cisco.com/thread/155174

    Reply
    • August 15, 2013 at 6:52 am
      Permalink

      Thanks! Nice links Jochen. It bugs me that most books written on switching and spanning tree are mostly theoretical where the author did not do enough testing to accurately describe all features. Guess it’s mostly a problem at CCIE level though. Although switching hasn’t changed that much Cisco LAN switching shouldn’t still be the best book on switching considering its age.

      Reply
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  • September 29, 2014 at 11:47 am
    Permalink

    Nice one mate, caught me out too.

    Reply
  • Pingback: CCNP Notes – Link Aggregation/EtherChannel | Fredrik's Networking Blog

  • December 11, 2015 at 9:31 pm
    Permalink

    The silent mode PAgP is confusing me.
    If this is true:
    ““By default, PAgP operates in silent submode with the desirable and auto modes, and
    allows ports to be added to an EtherChannel even if the other end of the link is silent
    and never transmits PAgP packets. This might seem to go against the idea of PAgP,
    in which two endpoints are supposed to negotiate a channel. After all, how can two
    switches negotiate anything if no PAgP packets are received?”” Then an Auto-Auto should work as both do not send PAgP data and hence the silent function should still create it. It does not. So in the same book they say auto-auto does not work but they also say that auto is silent and if you have PAgP against a device not sending PAgP data you can form a connection.
    But thank you for proving that. I get the same result.

    Reply
  • January 4, 2017 at 10:09 pm
    Permalink

    Thanks for this one! I makes sense to add the Silent/Non-silent option for Auto mode, not on desirable.

    Reply

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