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2.6 Bandwidth Management
2.6.1 Port Trunking – (Port Aggregation)
Link Aggregation is the industry term for the ability to combine multiple coterminous links (links that
begin at the same point and end at the same point) as one logical link.
Link aggregation allows two HP ProCurve switches to be interconnected by 2-4 of the same type of
links, with all links acting as one higher-speed link. Since the number of links in a trunk is configurable,
the bandwidth is scaleable to the needs of a particular network. For example, 4 links at 100Mbps can
be trunked to provide the equivalent of a 400 Mbps (800 Mbps full-duplex) link between two switches
or 4 individual Gigabit links can be trunked for the equivalent of a 4 Gigabit (8 Gigabit full-duplex) link.
Fiber-optic links can be trunked to interconnect switches across large campuses. Port trunking also
provides redundancy on links between the two switches or switch and server. If one of the links fails,
the traffic is moved to another link in the trunk in under one second.
The HP ProCurve Switch 5300xl Series support 36 port trunks of up to 4 physical links each. There are
3 ways to configure which ports on the switch participate in trunks: LACP (802.3ad), Cisco Fast
®
EtherChannel
, and manually. Although an easy process, manually configured trunks do require the
user to configure them directly into the switch. Any changes in the links used will require a manual
change in this configuration. The advantage of manual configuration is that it allows the HP ProCurve
Switch 5300xl Series to work with trunks from a number of other vendors that do not adhere to the
LACP standard or support Fast EtherChannel
2.6.1.1 802.3ad – LACP
Automatic configuration of port trunks happens when using LACP, Link Aggregation Control Protocol.
LACP, in the active form, operates by sending out packets looking for LACP running on the other end
of each connection. The switch, however, has a default configuration of passive LACP
listening for an active LACP connection on the other end. For LACP to dynamically configure the ports
in a trunk, one or both ends of the trunks need to have LACP configured in its active mode. Once the
user configures active LACP, links can be moved to different ports, or new links can be added, with
LACP detecting this and reconfiguring to reestablish the LACP trunk.
LACP, like the other forms of trunking, supports 4 links per trunk. LACP does allow the configuration
of standby links. Standby links carry no data unless one of the active links in the trunk fails. Standby
links are used in situations where the loss of even one of the active links would cause an unacceptable
traffic load on the remaining active links.
2.6.1.2 Cisco Fast EtherChannel
Another form of port trunk configuration protocol is Cisco's proprietary Fast EtherChannel
®
Gigabit EtherChannel
Protocol. It essentially performs the same function as LACP but is not standards based. Since many
Cisco products do not currently support LACP, PAgP can be used to automatically configure the trunk
links between the HP ProCurve Switch 5300xl Series and Cisco switches.
2.6.1.3 Trunking in a Layer 3 Environment
Traditional trunking uses MAC (Layer 2) addresses to determine which link in the trunk a particular
traffic flow travels over to avoid the problem of out-of-sequence packets. In a Layer 3 environment
between two routing switches this would cause all packets to flow over only one link because the
source and destination MAC addresses for all packets would be the same – the MAC address of the two
connected routing switches.
2
The passive form of LACP is the default because active LACP sends out packets periodically on each port looking for
LACP on the other end. While this traffic level is very low, most users don't want any traffic that doesn't directly relate to
their environment.
© Hewlett-Packard Co. 2002, 2003
HP ProCurve Switch 5300xl Series Reviewer's Guide
®
. Under EtherChannel
Rev 1.1 – 2/11/2003
http://www.hp.com/go/hpprocurve
®
.
®,
the control protocol is called PAgP, Port Aggregation
2
; each link is
®
and
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