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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 27 Configuring EtherChannels
Unders tanding Ether Channels

PAgP Modes

Table 27-1 shows th e use r-configurab le Ethe rC hanne l m ode s f or t he channel-group interface
configurati on comm a nd: on, auto, and de si ra bl e. Switch interfaces exchange PAgP packets only with
partner i nterfaces configur ed in the auto or desirable modes; interfaces configured in the on m ode do
not exchange PAgP packets.
Both the auto and desirable modes allow interfaces to negotiate with partner interfaces to determine if
they can form an EtherChannel based on criteria such as interface speed and, for Layer 2 EtherChannels,
trunking stat e a nd VLAN numbe rs.
Inte rface s c a n form an E th er Cha nn el wh en th ey are in d ifferent PAgP m odes as long a s t he mod es ar e
compatible. For example:
•An interface in the desirable mode can form an EtherChannel with another interface that is in the
desirable or auto mode.
•An interface in the auto mode can form an EtherChannel with another interface in the desirable
mode.
An interface in the auto mode can not form an EtherC hannel with an other int erface tha t is also in the
auto mode bec ause neit her inter face starts PAgP negotiation .
An interface in the on mode that is added to a port channel is forced to have the same characteristics as
the already existing on mode interfaces in the channel.
Caution You should exercise care when setting the mode to on (manual configuration). All ports configured in
the on mode ar e bundled in the s a me gr o up an d are forc ed to h av e simi lar ch ar ac ter i stics. If th e gr o up is
misconfigured, packet loss or spanning-tree loops might occur.
If your switc h is conn ected to a partner that is PAgP-c ap able, you can con figure t he switch in ter f a ce f or
nonsilent op erat ion by using the non-silent keyword. I f you do n ot specif y non-silent with the auto or
desirable mode, silen t mo de is a ssu med.
The silent mode is used when the switch is connected to a device that is not PAgP-capable and seldom,
if ever, sends pa ckets. A n exam pl e of a sile nt par tn er i s a file se rver or a p ack et a nal yzer t hat is not
genera ting traffic. In this case , running PAgP on a phys ical port connect ed to a silent partner prevents
that switch port from ever becoming operational; however, the silent setting allows PAgP to operate, to
attach the interface to a channel group, and to use the interface for transmission.
Table 27-1 EtherChannel Mode s
Mode Description
auto Places an inter face into a pa ssi ve ne gotiating state, in which the interf ace responds to PAgP
packets it rece ives but does not start PAgP packet negotia tion. This setting mini mizes the
transmissi on of PAgP pa ckets.
desirable Places an interface into an active negotiating state, in which the interface starts
negotiations with other interfaces by sending PAgP packets.
on Forces the interface to channel without PAgP. With the on mode, a usable Eth erChanne l
exists only wh en an interfa ce group in the on mode is connected to another interface group
in the on mode .