Port Trunking – (Link 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 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 ProCurve 5300xl Switch Series support 36 port trunks of up to 4 physical links each. There
are 2 ways to configure which ports on the switch participate in trunks: LACP (802.3ad) 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
ProCurve 5300xl Switch Series to work with trunks from a number of other vendors that do not
adhere to the LACP standard.
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
; each link is 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.
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.
To avoid this situation the ProCurve Switch 5300 Series uses the source and destination IP
addresses to determine which link a particular packet flow uses. This will provide a good overall
distribution of traffic across the different links in the trunk.
A Virtual LAN is a logical collection of ports or nodes that belong to a single broadcast/multicast
domain. VLANs were originally devised as a solution to limit the size of any one broadcast
domain to allow scaling of switched environments. With the advent of routing switch solutions,
however, use of VLANs in end user environments is now largely done for network policy or
For the ProCurve 5300xl Switch Series, VLANs are also used to provide entities to which to
attach the router functionality. All routing in the ProCurve 5300xl Switch Series is defined to be
ProCurve 5300xl Switch Series support 256 VLANs (8 default). VLAN membership can be
designated through either a particular port (untagged), or through an 802.1Q tag.
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