VLAN Trunks
At the admission layer, end user accessories affix to about-face ports that accommodate simple connectivity
to a distinct VLAN each. The absorbed accessories are blind of any VLAN anatomy and simply
attach to what appears to be a accustomed concrete arrangement segment. Remember, sending
information from an admission articulation on one VLAN to addition VLAN is not accessible afterwards the
intervention of an added device—either a Band 3 router or an alien Band 2 bridge.
VLAN Trunks 107
NOTE Note that a about-face anchorage can abutment added than one IP subnet for the accessories absorbed to it. For
example, accede a aggregate Ethernet hub that is affiliated to a distinct Ethernet about-face port. One
user accessory on the hub may be configured for 192.168.1.1 255.255.255.0, while addition is
assigned 192.168.17.1 255.255.255.0. Although these subnets are altered communicating on
one about-face port, they cannot be advised abstracted VLANs. The about-face anchorage supports one
VLAN, but assorted subnets can abide on that distinct VLAN.
A block link, however, can carriage added than one VLAN through a distinct about-face port. Trunk
links are best benign back switches are affiliated to added switches or switches are
connected to routers.
A block articulation is not assigned to a specific VLAN. Instead, one, many, or all alive VLANs can
be transported amid switches application a distinct concrete block link. Connecting two switches
with abstracted concrete links for anniversary VLAN is possible. Amount 4-3 shows how two switches
might be affiliated in this fashion.
As VLANs are added to a network, the cardinal of links can bound grow. A added able use
of concrete interfaces and cabling involves the use of trunking. The adapted bisected of the amount shows
how one block articulation can alter abounding alone VLAN links. A block articulation can be associated
with a built-in VLAN, which is acclimated if the block articulation fails for some reason.
Cisco supports trunking on both Fast Ethernet and Gigabit Ethernet about-face links, as able-bodied as
aggregated Fast and Gigabit EtherChannel links. To analyze amid cartage acceptance to
different VLANs on a block link, the about-face charge accept a adjustment of anecdotic anniversary anatomy with
the adapted VLAN. Several identification methods are accessible and are discussed in the
next section.
108 Affiliate 4: VLANs and Trunking
Figure 4-3 Casual VLAN Cartage Application Distinct Links Versus Block Links
VLAN Anatomy Identification
Because a block articulation can be acclimated to carriage abounding VLANs, a about-face charge analyze frames with
their VLANs as they are beatific and accustomed over a block link. Anatomy identification, or tagging,
assigns a altered user-defined ID to anniversary anatomy transported on a block link. This ID can be
thought of as the VLAN cardinal or VLAN “color,” as if anniversary VLAN was fatigued on a network
diagram in a altered color.
VLAN anatomy identification was developed for switched networks. As anniversary anatomy is transmitted
over a block link, a altered identifier is placed in the anatomy header. As anniversary about-face forth the way
receives these frames, the identifier is advised to actuate to which VLAN the frames
belong.
VLAN 1
VLAN 2
3
1
2
VLANs
VLAN 3
VLAN 1
VLAN 2
VLAN 3
VLAN 1
VLAN 2
VLAN 3
VLAN 1
VLAN 2
VLAN 3
VLANs 1,2,3
Trunk link
VLAN Trunks 109
If frames charge be transported out addition block link, the VLAN identifier is retained in
the anatomy header. Otherwise if frames are destined out an admission link, the about-face removes
the VLAN identifier afore transmitting the frames to the end station. Therefore, all traces of
VLAN affiliation are hidden from the end station.
VLAN identification can be performed application several methods. Anniversary uses a altered frame
identifier mechanism, and some are ill-fitted for specific arrangement media. These methods are
described in the sections that follow.
Inter-Switch Articulation Protocol
The Inter-Switch Articulation (ISL) agreement is a Cisco proprietary adjustment for attention the source
VLAN identification of frames casual over a block link. ISL performs anatomy identification in
Layer 2 by encapsulating anniversary anatomy amid a attack and trailer. Any Cisco about-face or router
device configured for ISL can action and accept the ISL VLAN information. ISL is
primarily acclimated for Ethernet media, although Cisco has included accoutrement to backpack Token Ring,
FDDI, and ATM frames over Ethernet ISL. (A frame-type acreage in the ISL attack indicates the
source anatomy type.)
When a anatomy is destined out a block articulation to addition about-face or router, ISL adds a 26-byte header
and a 4-byte bivouac to the frame. The antecedent VLAN is articular with a 10-bit VLAN ID in the
header. The bivouac contains a circadian back-up analysis (CRC) to assure the abstracts candor of the
new encapsulated frame. Amount 4-4 shows how Ethernet frames are encapsulated and
forwarded out a block link. Because tagging advice is added at the alpha and end of
each frame, ISL is sometimes referred to as bifold tagging.
Figure 4-4 ISL Anatomy Identification
If a anatomy is destined for an admission link, the ISL encapsulation (both attack and trailer) is
removed afore transmission. This abatement preserves ISL advice alone for block links and
devices that can accept the protocol.
IEEE 802.1Q Protocol
The IEEE 802.1Q agreement can additionally be acclimated to bottle VLAN associations over block links.
However, this anatomy identification adjustment is standardized, acceptance VLAN trunks to abide and
operate amid accessories from assorted vendors.
Access articulation Block link
Frame
(variable length)
ISL header
(26 bytes)
CRC
(4 bytes)
110 Affiliate 4: VLANs and Trunking
In particular, the IEEE 802.1Q accepted defines an architectonics for VLAN use, services
provided with VLANs, and protocols and algorithms acclimated to accommodate VLAN services. Further
information about the 802.1Q accepted can be begin at grouper.ieee.org/groups/802/1/pages/
802.1Q.html
Like Cisco ISL, IEEE 802.1Q can be acclimated for VLAN identification with Ethernet trunks.
Instead of encapsulating anniversary anatomy with a VLAN ID attack and trailer, 802.1Q embeds its
tagging advice aural the Band 2 frame. This adjustment is referred to as single-tagging or
internal tagging. 802.1Q additionally introduces the abstraction of a built-in VLAN on a trunk. Frames
belonging to this VLAN are not encapsulated with tagging information. In the accident that an end
station is affiliated to an 802.1Q block link, the end base will be able to accept and
understand alone the built-in VLAN frames.
In an Ethernet frame, 802.1Q adds a four-byte tag aloof afterwards the antecedent abode field, as shown
in Amount 4-5.
Figure 4-5 IEEE 802.1Q Anatomy Tagging Standard
The aboriginal two bytes are acclimated as a Tag Agreement Identifier (TPID). The aboriginal two bytes additionally always
have a amount of 0x8100 to announce an 802.1Q tag. The actual two bytes are acclimated as a Tag
Control Advice (TCI) field. The TCI advice contains a 3-bit antecedence field, which is
used to apparatus chic of account functions in the accompanying 802.1Q/802.1p prioritization
standard. One bit of the TCI is a Approved Architecture Indicator (CFI), abatement whether the MAC
addresses are in approved format. The aftermost 12 $.25 are acclimated as a VLAN Identifier (VID) to
indicate the antecedent VLAN for the frame. The VID can accept ethics from 0 to 4095, but VLAN
0, 1, and 4095 are reserved.
Access articulation Block link
Frame
(variable length)
Dest Addr
Src Addr
Type/Length
Data burden FCS
802.1Q Tag
(+4 bytes)
VLAN Block Configuration 111
NOTE Note that both ISL and 802.1Q tagging methods accept one implication: they add to the breadth of
an Ethernet frame. ISL adds a absolute of 30 bytes to anniversary frame, while 802.1Q adds 4 bytes.
Because Ethernet frames cannot beat 1518 bytes, the added VLAN tagging information
can account the anatomy to be too large. Frames that almost beat the MTU admeasurement are alleged baby
giant frames. Switches will usually address these frames as Ethernet errors or billowing frames.
LAN Emulation (LANE)
Trunking VLANs amid switches over an Asynchronous Transfer Approach (ATM) articulation is
possible. Here, VLANs are transported application the IEEE LAN Emulation (LANE) standard.
LANE is discussed in greater detail in Affiliate 6, “Trunking with ATM LANE.”
IEEE 802.10
Cisco offers a proprietary adjustment for alteration VLAN advice central the accepted IEEE
802.10 FDDI frame. The VLAN advice is agitated in the Security Affiliation Identifier
(SAID) acreage of the 802.10 frame.
Dynamic Trunking Protocol
Trunk links on Catalyst switches can be manually configured for either ISL or 802.1Q mode.
However, Cisco has implemented a proprietary point-to-point agreement alleged Dynamic
Trunking Agreement (DTP) that will accommodate a accepted trunking approach amid two switches.
DTP is accessible in Catalyst administrator agent software Release 4.2 and later. DTP negotiation
should be disabled if a about-face has a block articulation affiliated to a router because the router cannot
participate in the DTP agreement protocol.
NOTE A block articulation can be adjourned amid two switches alone if both switches accord to the same
VLAN Trunking Agreement (VTP) administration domain. VTP is discussed in the “VTP
Configuration” area of this chapter. If the two switches are in altered VTP domains and
trunking is adapted amid them, the block links charge be set to on or nonegotiate mode. This
setting will force the block to be established. These options are explained in the abutting section.