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Chapter 5
Using Spanning Tree with VLANs
form the connections into a channel. In this case, ports 0/26 and 0/27 are
connected to the same device ID (hostname). By using the command show
spantree 1
, you can see that ports 0/26 and 0/27 are now one port:
1900A#show spantree 1
[output cut]
Port PortChannel of VLAN1 is Forwarding
Port path cost 10, Port priority 128
Designated root has priority 32768, address
0030.80CC.7B40
Designated bridge has priority 32768, address
0030.80CC.7B40
Designated port is 26, path cost 0
Timers: message age 20, forward delay 15, hold 1
Ports 0/26 and 0/27 are now just listed as Port PortChannel. To verify the
EtherChannel on the 5000 series switch, use the show port channel
command:
Todd5000> (enable) sh port channel
Port Status Channel Channel Neighbor Neighbor
mode status device port
----- ---------- --------- ----------- ------------------ ----------
1/1 connected on channel cisco 1900 1900A A
1/2 connected on channel cisco 1900 1900A B
----- ---------- --------- ----------- ------------------ ----------
Todd5000> (enable)
The preceding switch output shows the port numbers, status, mode, channel
status, neighbor device, and neighbor port ID. Our EtherChannel is working!
Port Aggregation Protocol (PAgP)
The Port Aggregation Protocol (PAgP) is used to add more features to the
EtherChannel technology. This protocol is used to learn the capabilities of
the neighbors' EtherChannel ports. By doing this, it allows the switches to
connect via Fast EtherChannel automatically.
The PAgP protocol groups the ports that have the same neighbor device
ID and neighbor group capability into a channel. This channel is then added
to the Spanning Tree Protocol as a single bridge port.
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