Split Trunking Support with HP Auto Port Aggregation

APA Split Trunking Support
The example in this paper was run on an HP-UX 11i v3 system, using the J4240AA Auto-Port
Aggregation product from the December 2007 AR release (version B.11.31.10). This version of APA
will also provide support for split trunks on the following versions of HP-UX:
• 11i v3: version B.11.31.02 or greater
• 11i v2: version B.11.23.25 (PHNE_36403) or greater, Sept. 2007 Application Release or later
• 11i v1: version B.11.11.25 (PHNE_36483) or greater, Sept. 2007 Application Release or later
Split Trunk Support on Switches
As described in the introduction of this document, HP APA has been verified to work with split
trunking on Cisco and Nortel switches. You can configure Cisco switches as a stack of switches
(StackWise switch stacking) that then operate like a single switch. With Cisco StackWise switch
stacking, you can use both manual mode and LACP mode link aggregation on stacked switches.
PAgP is not supported by Cisco on stacked switches.
On Nortel switches, split trunking is supported using the SMLT (Split Multi-Link Trunking) features.
Only LACP mode is supported.
HP APA supports Nortel SMLT and the Cisco StackWise cross-stack EtherChannels. The configuration
examples are performed on the Nortel 6000 Passport switches and the Cisco 3750 switches.
When to Use Split Trunking
If you want protection against switch failures when using link aggregates, you must set up a failover
group consisting of two or more link aggregates, with ports within each link aggregate connected to
a single switch. The switches are then connected to each other (see Figure 2). As described earlier,
the disadvantage with this configuration is that the link aggregation serving as the standby port is not
carrying traffic during normal conditions, reducing the maximum available bandwidth capability.
Split trunking support offers several advantages over the Figure 2 configuration (failover-group of link-
aggregates). First, only the link aggregate needs to be created; the configuration task is simpler.
Second, the split trunking solution provides a highly available and active-active failover solution. You
can fully employ the bandwidth provided by the ports used in the link aggregation for normal traffic
all the time. Bandwidth capacity is reduced only in the case of switch failure. With priority or cost
based failover groups, less than the full bandwidth capacity can be utilized at any one time. The
remainder is idling, held in reserve while waiting for the primary link-aggregate to fail.
For split trunks, Link Aggregation Control Protocol (LACP) link aggregates are preferred over the
Manual mode link aggregates. Manual mode link aggregates might not remove ports connected to
failed switches. As a result, traffic might still go to these ports, causing unexplained traffic losses. On
the other hand, with a LACP link aggregate, APA removes ports connected to the failed switches from
the link aggregate, which eliminates the possibility of traffic losses.
6