Alteon Application Switch Operating System Application Guide
Port Trunking
90
Document
ID:
RDWR-ALOS-V2900_AG1302
Statistical Load Distribution
Network traffic is statistically load balanced between the ports in a trunk group. Alteon uses both the
Layer 2 MAC address and Layer 3 IP address information present in each transmitted frame for
determining load distribution.
The addition of Layer 3 IP address examination is an important advance for traffic distribution in
trunk groups. In some port trunking systems, only Layer 2 MAC addresses are considered in the
distribution algorithm. Each packet's particular combination of source and destination MAC
addresses results in selecting one line in the trunk group for data transmission. If there are enough
Layer 2 devices feeding the trunk lines, then traffic distribution becomes relatively even. In some
topologies, however, only a limited number of Layer 2 devices (such as a handful of routers and
servers) feed the trunk lines. When this occurs, the limited number of MAC address combinations
encountered results in lopsided traffic distribution, which can reduce the effective combined
bandwidth of the trunked ports.
By adding Layer 3 IP address information to the distribution algorithm, a far wider variety of address
combinations are seen. Even with just a few routers feeding the trunk, the normal source and
destination IP address combinations (even within a single LAN) can be widely varied. This results in
a wider statistical load distribution and maximizes the use of the combined bandwidth available to
trunked ports.
The Trunk Hash Algorithm
In order to distribute the load across all active ports in a trunk group, the following algorithm is used
to determine which port within the trunk group to use for frame forwarding, where x is the number
of active ports within the trunk group:
hash_idx = (A xor B)
port = (lower 6 bits of hash_idx) mod x
The values of parameters A and B are defined below for the different types of forwarding and
frames. These two parameters are XORed together to give the hash index. The modulus (mod) x of
the lower 6 bits of the hash index is then taken to give the port of the trunk group.
Note:
The same algorithm is used across all Alteons.
•
For Layer 2 forwarding of non-IP frames:
A = lower 16 bits of destination MAC address
B = lower 32 bits of source MAC address
•
For Layer 2 forwarding of IP frames:
A = lower 16 bits of source IP address
B = lower 32 bits of source MAC address
•
For Layer 3 forwarding (enabled in WSM platform and Cheetah 20.1):
A = lower 32 bits of destination IP
B = lower 16 bits of source MAC
•
For Layer 4 trunking (traffic towards the real servers in SLB and WCR):
A = lower 32 bits of source IP
B = lower 16 bits of destination MAC
Note:
Layer 4 trunk hashing is currently supported only in Alteon 21.0 and higher.
