Information Technology Reference
In-Depth Information
channel was configured by using the PAgP on mode and one side of the connection has an error of some
kind or a reset, it could cause an errdisable state (shutdown) on the other side. With PAgP set in desirable
mode on each side. The channel will stabilize and renegotiate the EtherChannel connection.
Trunking and EtherChannel
EtherChannel is independent of trunking. You can turn trunking on, or you can leave trunking off. You
also can turn on trunking for all the ports before creating the channel, or you can turn it on after creating
the channel (as we will do here). As far as EtherChannel is concerned, it does not matter; trunking and
EtherChannel are completely separate features. What does matter is that all the ports involved are in the
same mode: Either they are all trunking before you configure the channel, or they are all not trunking
before you configure the channel. All the ports must be in the same trunking state before creating the
channel.
After a channel is formed, whatever is changed on one port is also changed for the other ports in the
channel. The modules used in this test bed can do ISL or 802.1q trunking. By default, the modules are
set to auto trunking and negotiate mode, which means that they will trunk if the other side asks them to
trunk, and they will negotiate whether to use the ISL or 802.1q method for trunking. If they are not asked
to trunk, they will work as normal nontrunking ports.
Switch-A (enable)
show trunk 2
Port Mode Encapsulation Status Native vlan
-------- ----------- ------------- ------------ -----------
2/1
auto
negotiate
not-trunking 1
2/2
auto
negotiate
not-trunking 1
2/3
auto
negotiate
not-trunking 1
2/4
auto
negotiate
not-trunking 1
There are a number of different ways to turn on trunking. For this example, we will set Switch A to
desirable. Switch A is already set to negotiate. The combination desirable/negotiate will cause Switch
A to ask Switch B to trunk and to negotiate the type of trunking to do (ISL or 802.1q). Because Switch
B defaults to autonegotiate, Switch B will respond to Switch A's request. The following results occur:
Switch-A (enable)
set trunk 2/1 desirable
Port(s) 2/1-4 trunk mode set to desirable.
Switch-A (enable)
1999 Dec 18 20:46:25 %DTP-5-TRUNKPORTON:Port 2/1 has become isl trunk
1999 Dec 18 20:46:25 %DTP-5-TRUNKPORTON:Port 2/2 has become isl trunk
1999 Dec 18 20:46:25 %PAGP-5-PORTFROMSTP:Port 2/1 left bridge port 2/1-4
1999 Dec 18 20:46:25 %PAGP-5-PORTFROMSTP:Port 2/2 left bridge port 2/1-4
1999 Dec 18 20:46:25 %DTP-5-TRUNKPORTON:Port 2/3 has become isl trunk
1999 Dec 18 20:46:26 %PAGP-5-PORTFROMSTP:Port 2/3 left bridge port 2/1-4
1999 Dec 18 20:46:26 %DTP-5-TRUNKPORTON:Port 2/4 has become isl trunk
1999 Dec 18 20:46:26 %PAGP-5-PORTFROMSTP:Port 2/4 left bridge port 2/1-4
1999 Dec 18 20:46:28 %PAGP-5-PORTTOSTP:Port 2/1 joined bridge port 2/1-4
1999 Dec 18 20:46:29 %PAGP-5-PORTTOSTP:Port 2/2 joined bridge port 2/1-4
1999 Dec 18 20:46:29 %PAGP-5-PORTTOSTP:Port 2/3 joined bridge port 2/1-4
1999 Dec 18 20:46:29 %PAGP-5-PORTTOSTP:Port 2/4 joined bridge port 2/1-4
Switch-A (enable)
show trunk 2
Port Mode Encapsulation Status Native vlan
-------- ----------- ------------- ------------ -----------
2/1
desirable
n-isl
trunking
1
2/2
desirable
n-isl
trunking
1
2/3
desirable
n-isl
trunking
1
2/4
desirable
n-isl
trunking
1
