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.
SW1(config)#int range f0/13 - 14 SW1(config-if-range)#sh SW1(config-if-range)#channel-group 1 mode des
SW2(config)#int range f0/13 - 14 SW2(config-if-range)#sh SW2(config-if-range)#channel-group 1 mode auto
PAgP: Fa0/13 enabling PAgP with mode desirable-nonsl PAgP: set hello interval from 0 to 1000 for port Fa0/13 PAgP: Fa0/14 enabling PAgP with mode desirable-nonsl PAgP: set hello interval from 0 to 1000 for port Fa0/14
PAgP: Fa0/13 enabling PAgP with mode auto-sl PAgP: set hello interval from 0 to 1000 for port Fa0/13 PAgP: Fa0/14 enabling PAgP with mode auto-sl PAgP: set hello interval from 0 to 1000 for port Fa0/14
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.