Operating instructions

118 Cisco Systems Intelligent Gigabit Ethernet Switch Module
Use of the terms trunk and aggregation
Some industry terms and acronyms have proven to be a source of confusion. One such term
is the word
trunk or trunking. This term has been used interchangeably to describe several
technologies, most commonly the act of bundling links together to increase performance and
reliability, and the act of carrying multiple VLANs on a single connection. Another such term of
confusion is the term
aggregation or link aggregation.
For all discussions in this chapter we follow IEEE definitions for these terms:
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Trunk or trunking: The act of carrying multiple VLANs on a single connection. The
connection might be a single link or a group of links aggregated to form a Link Aggregation
Group. The IEEE specification for VLAN trunking is 802.1Q.
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Aggregation or link aggregation: The act of bundling multiple physical links into one
logical link for the purposes of increasing throughput or offering increased reliability or
both. Link aggregation is often referred to as EtherChannel in the Cisco world. The IEEE
specification for link aggregation is 802.3ad (now part of 802.3-2002).
Use of the term native VLAN
The term native VLAN is used throughout this chapter to describe a single, designated,
untagged VLAN in an 802.1Q trunk. The 802.1Q specification does not define this term, but
the concept of untagged VLANs on a trunk is defined within the specification. Cisco has
adopted this term to describe a VLAN that provides, among other things, backward
compatibility with devices that might not understand 802.1Q tagging, such that at least some
communications can take place across this link.
In many Cisco networks, this native VLAN has often defaulted to VLAN 1 for 802.1Q trunk
connections. If using a native VLAN, it is important that both sides of a trunk link agree to use
the same native VLAN. Note that the Cisco Systems IGESM defaults all of its blade server
and external connections to native VLAN 2, while most other Cisco switches default the native
VLAN to VLAN 1. To prevent native VLAN mismatch messages, this must be taken into
account.
Link aggregation (EtherChannel) support
Although the Cisco Systems IGESM supports Port Aggregation Protocol (PAgP), Link
Aggregation Control Protocol (LACP) is the preferred method for performing link aggregation
between the Cisco Systems IGESM and Cisco devices, as this is the IEEE standard. If the
external Cisco switch being used does not support LACP (for 6500, LACP support began in
IOS Version 12.1(13)E), you have the choice of upgrading the switch to newer code that
supports LACP or using PAgP or static aggregation. (Use of PAgP and static aggregation are
not covered in this chapter, but both are supported with the IGESM).
It is always advisable to check the release notes of the revision of Cisco IOS being used to
ensure that the feature set you require is present.
The examples in this paper use a single module in each 6500 to connect each module over
an LACP channel. This is only for convenience in this document; in reality, we highly
recommend that you split this aggregation over multiple 6500 modules to increase high
availability if a single module should fail.
Important: Although you can attach the Cisco Systems IGESMs to external switches at
100 Mb speeds, in production environments, we strongly recommend that you use
1000BaseT connections (available on all Cisco platforms suitable for data center
environments) to ensure the best possible throughput.