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Virtual LANs 175
Step 6
Switch 2 considers port E11 to be in VLAN1, so it performs table
lookup for 0200.1111.0002 in that address table.
Or . . .
Step 7
Switch 2 does not consider port E11 to be in any particular VLAN,
so it does table lookup in all tables and forwards out all ports
matched.
Or . . .
Step 8
Before Switch 1 forwards the frame in Step 4, it adds a header that
identifies the VLAN. Then, Switch 2 can look at the frame header
to identify the VLAN number and can do table lookup just in that
VLAN's address table.
The third option for Step 6 is the one that actually was implemented. The first option would
work fine for one VLAN and is used when connecting multiple switches without using VLANs.
However, the logic in this first option fails when devices in VLAN2 send frames because their
addresses would never be found in VLAN1's address table. The second option would work well
for unicasts, particularly because a unicast address should be found in only a single address
table. However, broadcasts would be sent on all interfaces, regardless of VLAN, which would
cause horrendous side effects for OSI Layer 3 processes. So, the third option, called VLAN
tagging
, is used.
ISL is one of the tagging options used in switches; Figure 4-24 shows ISL framing details.
Understanding all the values in the ISL header fields is not vital. However, there are two very
important features. First, the ISL header encapsulates the LAN frame, which lengthens the
frame. 802.1Q, the IEEE-defined Ethernet VLAN protocol, actually modifies the existing
header to accomplish the same tagging goal. The second important feature is the VLAN ID
field, which identifies the VLAN to which the encapsulated frame belongs. The source address
field in the ISL header is the address of the sending switch, and the destination address is a
special multicast address, whose first 5 bytes are 0100.0C00.00 and whose last byte is actually
comprised of the values shown in the type and user fields of Figure 4-24. The two ISL features
most important for CCNAs, however, are that ISL encapsulates the orignal frame and that there
is a VLAN-ID field in the ISL header.
Figure 4-24
ISL Framing
ISL Header
26 bytes
CRC
4 bytes
Encapsulated Ethernet Frame
VLAN
DA Type User SA LEN AAAA03 HSA VLAN BPDU INDEX RES
BPDU
ch04.fm Page 175 Monday, March 20, 2000 5:02 PM