VLAN Trunking: ISL and 802.1Q
VLAN trunking allows switches, routers, and alike PCs with the adapted NICs to accelerate traffic
for assorted VLANs beyond a distinct link. In adjustment to apperceive to which VLAN a anatomy belongs, the
sending switch, router, or PC adds a attack to the aboriginal Ethernet frame, with that attack having
a acreage in which to abode the VLAN ID of the associated VLAN. This area describes the protocol
details for the two trunking protocols, followed by the capacity of how to configure trunking.
ISL and 802.1Q Concepts
If two accessories are to accomplish trunking, they charge accede to use either ISL or 802.1Q, because there
are several differences amid the two, as abbreviated in Table 2-7.
ISL and 802.1Q alter in how they add a attack to the Ethernet anatomy afore sending it over
a trunk. ISL adds a new 26-byte header, additional a new bivouac (to acquiesce for the new FCS value),
encapsulating the aboriginal frame. This encapsulating attack uses the antecedent abode (listed as SA
in Figure 2-3) of the accessory accomplishing the trunking, instead of the antecedent MAC of the aboriginal frame.
ISL uses a multicast destination abode (listed as DA in Figure 2-3) of either 0100.0C00.0000 or
0300.0C00.0000.
802.1Q inserts a 4-byte header, alleged a tag, into the aboriginal anatomy (right afterwards the Antecedent Address
field). The aboriginal frame’s addresses are larboard intact. Normally, an Ethernet ambassador would expect
to acquisition either an Ethernet Blazon acreage or 802.3 Length acreage appropriate afterwards the Antecedent Abode field. With
an 802.1Q tag, the aboriginal 2 bytes afterwards the Abode fields holds a registered Ethernet blazon amount of
0x8100, which implies that the anatomy includes an 802.1Q header. Because 802.1Q does not
actually abbreviate the aboriginal frame, it is generally alleged anatomy tagging. Figure 2-3 shows the
contents of the headers acclimated by both ISL and 802.1Q.
Table 2-7 Comparing ISL and 802.1Q
Feature ISL 802.1Q
VLANs accurate Normal and continued range1 Normal and continued range
Protocol authentic by Cisco IEEE
Encapsulates aboriginal anatomy or inserts tag Encapsulates Inserts tag
Supports built-in VLAN No Yes
1ISL originally accurate alone normal-range VLANs, but was after bigger to abutment extended-range VLANs as well.
ISL and 802.1Q Frame Marking Methods
Finally, the last row from Table 2-7 refers to the native VLAN. 802.1Q does not tag frames sent
inside the native VLAN. The native VLAN feature allows a switch to attempt to use 802.1Q
trunking on an interface, but if the other device does not support trunking, the traffic for that one
native VLAN can still be sent over the link. By default, the native VLAN is VLAN 1.
ISL Header
26 bytes
CRC
4 bytes
Encapsulated Ethernet Frame
VLAN
DA Type User SA LEN AAAA03 HSA VLAN BPDU INDEX RES
BPDU
Dest Src Len/Etype Data FCS
Dest Src Etype Tag Len/Etype Data FCS
Priority VLAN-ID
Original
Frame
Tagged
Frame