Information Technology Reference
In-Depth Information
(Xmit-Unicast) shows how Switch A distributed the traffic across the channel to Switch B.
We also see here that a small amount of switch-generated multicast traffic (Dynamic ISL,
CDP) goes out all four ports. The broadcast packets are ARP queries (for the default
gateway, which doesn't exist in our lab here). If we had workstations sending packets
through the switch to a destination on the other side of the channel, we would expect to
see traffic going over each of the four links in the channel. You can monitor the packet
distribution in your own network using the show mac command.
Switch-A (enable)
clear counters
This command will reset all MAC and port counters reported in CLI and SNMP.
Do you want to continue (y/n) [n]? y
MAC and Port counters cleared.
Switch-A (enable)
show mac
Port Rcv-Unicast Rcv-Multicast Rcv-Broadcast
-------- -------------------- -------------------- --------------------
2/1
9
320
183
2/2
0
51
0
2/3
0
47
0
2/4
0
47
0
(...)
Port Xmit-Unicast Xmit-Multicast Xmit-Broadcast
-------- -------------------- -------------------- --------------------
2/1
8
47
184
2/2
0
47
0
2/3
0
47
0
2/4
0
47
0
(...)
Port Rcv-Octet Xmit-Octet
-------- -------------------- --------------------
2/1
35176
17443
2/2
5304
4851
2/3
5048
4851
2/4
5048
4851
(...)
Last-Time-Cleared
--------------------------
Wed Dec 15 1999, 01:05:33
Using PAgP to Automatically Configure EtherChannel (Preferred Method)
The Port Aggregation Protocol (PAgP) facilitates the automatic creation of EtherChannel links by
exchanging packets between channel-capable ports. The protocol learns the capabilities of port groups
dynamically and informs the neighboring ports.
When PAgP identifies correctly paired channel-capable links, it groups the ports into a channel. The
channel is then added to the spanning tree as a single bridge port. A given outbound broadcast or
multicast packet is transmitted out one port in the channel only, not out every port in the channel. In
addition, outbound broadcast and multicast packets transmitted on one port in a channel are blocked
from returning on any other port of the channel.
Four user-configurable channel modes exist: on, off, auto, and desirable. PAgP packets are exchanged
only between ports in auto and desirable modes. Ports configured in on or off modes do not exchange
PAgP packets. The recommended settings for switches that you want to form and EtherChannel is to
have both switches set to desirable mode. This gives the most robust behavior in case one side encounters
error situations or must be reset. The default mode of the channel is auto.
