HP Auto Port Aggregation Administrator's Guide HP-UX 11i v3 (766140-001, March 2014)

With proactive failover, the port that is the most efficient at carrying traffic is the active port.
Efficiency is determined by assigning a cost to each port in a failover group. This cost is divided
by the port's current link speed to yield a normalized port cost; link speed is the number of links
in a link aggregate multiplied by the speed of a member link, or in the case of a single link, only
the link speed. The lower the normalized port cost, the higher the link's efficiency. If two links have
the same normalized cost, the one with the higher priority is preferred.
For each failover group, if you assign a cost value to one link, you must assign a cost value to all
other links in the group. If you do not specify a cost value for any of the failover group's links, the
failover group uses the default failover behavior based on priority.
During certain LAN Monitor events (for example, link failure and link recovery), the normalized
port cost might change on the active or standby links. When these events occur, the normalized
port cost of the active link and the standby links are compared. If a standby link has a lower
normalized port cost than the active link, the standby link becomes the active link even if the current
active link is UP.
TCP Segmentation Offload
HP APA supports TCP Segmentation Offload (TSO), also known as Large Send, on link aggregates
and failover groups if all the Ethernet cards are capable of it. TSO is a mechanism by which the
host stack offloads certain portions of outbound TCP packet processing to the Network Interface
Card (NIC) thereby reducing host CPU utilization. This functionality can significantly reduce the
load on the server for certain applications which primarily transmit large amounts of data from the
system.
In link aggregates, TSO has the following behavior:
• If TSO is enabled on all of the physical ports in a link aggregate, TSO is enabled for the entire
link aggregate. If any of the ports within that link aggregate go DOWN or UP, the TSO status
of the link aggregate does not change. After the physical ports are added to the aggregate,
the TSO capability of the physical ports cannot be changed.
• If a port is removed from a link aggregate, the following occurs:
If TSO was supported on the link aggregate before removing the port, TSO remains
enabled on the link aggregate.
◦
◦ If TSO was disabled on the link aggregate before removing the port, TSO of the link
aggregate is based on remaining ports in the link aggregate. If all remaining ports support
TSO, TSO is enabled on the link aggregate; otherwise, TSO remains disabled.
• If a port is added to a link aggregate, the TSO settings are recalculated. If the added port
has TSO disabled, TSO is disabled on the link aggregate.
In failover groups, the TSO status depends on the TSO status of the current active port. When the
active port is changed, the TSO status of the failover group might change. For example, an active
port supports TSO and the standby port does not. Therefore, the failover group supports TSO. If
the active port goes down, the standby port becomes active and the failover group now no longer
supports TSO.
By default, TSO is disabled. To enable TSO on each specific interface, see Networking and
Communication section in the Ethernet Support Guide.
To verify if TSO is supported on an link aggregate or failover group, enter the following command:
# nwmgr -A vmtu -I instance -S apa
lan900 current values:
Virtual Maximum Transmission Unit = 0
In this example, a zero value indicates TSO is not enabled.
10 Introduction