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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter6 Starting Interface Configuration (ASA 5510 and Higher)
Information About Starting ASA 5510 and Higher Interface Configuration
•On—The EtherChannel is always on, and LACP is not used. An “on” EtherChannel can only
establish a connection with another “on” EtherChannel.
LACP coordinates the automatic addition and deletion of links to the EtherChannel without user
intervention. It also handles misconfigurations and checks that both ends of member interfaces are
connected to the correct channel group. “On” mode cannot use standby interfaces in the channel group
when an interface goes down, and the connectivity and configurations are not checked.
Load Balancing
The ASA distributes packets to the interfaces in the EtherChannel by hashing the source and destination
IP address of the packet (this criteria is configurable; see the “Customizing the EtherChannel” section
on page 6-29). The hash result is a 3-bit value (0 to 7).
The eight hash result values are distributed in a round robin fashion between the channel group
interfaces, starting with the interface with the lowest ID (slot/port). For example, all packets with a hash
result of 0 go to GigabitEthernet 0/0, packets with a hash result of 1 go to GigabitEthernet 0/1, packets
with a hash result of 2 go to GigabitEthernet 0/2, and so on.
Because there are eight hash result values regardless of how many active interfaces are in the
EtherChannel, packets might not be distributed evenly depending on the number of active interfaces.
Table 6 -2 shows the load balancing amounts per interface for each number of active interfaces. The
active interfaces in bold have even distribution.
If an active interface goes down and is not replaced by a standby interface, then traffic is rebalanced
between the remaining links. The failure is masked from both Spanning Tree at Layer 2 and the routing
table at Layer 3, so the switchover is transparent to other network devices.
EtherChannel MAC Address
All interfaces that are part of the channel group share the same MAC address. This feature makes the
EtherChannel transparent to network applications and users, because they only see the one logical
connection; they have no knowledge of the individual links.
The port-channel interface uses the lowest numbered channel group interface MAC address as the
port-channel MAC address. Alternatively you can manually configure a MAC address for the
port-channel interface. In multiple context mode, you can automatically assign unique MAC addresses
Table6-2 Load Distribution per Interface
# of Active
Interfaces
% Distribution Per Interface
12345678
1 100% ———————
2 50% 50% — — — — — —
3 37.5% 37.5% 25% — — — — —
4 25% 25% 25% 25% — — — —
5 25% 25% 25% 12.5% 12.5% — — —
6 25% 25% 12.5% 12.5% 12.5% 12.5% — —
7 25% 12.5% 12.5% 12.5% 12.5% 12.5% 12.5% —
8 12.5% 12.5% 12.5% 12.5% 12.5% 12.5% 12.5% 12.5%