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Chapter 11

Troubleshooting Switched Ethernet

802.1q Trunking

Although the IEEE 802.1q standard is similar to the Cisco proprietary ISL
protocol in terms of function, as a standard it may be used to connect non-
Cisco trunks to Cisco equipment. Note that the 802.1q encapsulation is
accessed with the command encapsulation dot1Q, which is available in
IOS versions 12.0.1(t) and higher on routers and IOS 4.1 on the Catalyst
5000 switches.

ISL provides additional functions, when compared with 802.1q. For

example, spanning trees are handled somewhat better in ISL. However,
802.1q should be recommended in any network that does not adhere to a
strict Cisco-only policy, given the proprietary concerns.

From a troubleshooting perspective, 802.1q requires the same under-

standing of the VLAN's relationships to the subnets that are beneficial in all

Reserved

The reserved bits are set to zero for
Ethernet frames. However, when
ISL encapsulates Token Ring, the
access control (AC) and frame infor-
mation (FC) octets are duplicated
here. When encapsulating FDDI, the
frame control octet is prefixed with
0x00 and copied in this field.

Original frame

The original frame field may be
24,575 octets long and includes
Ethernet, Token Ring, or FDDI
frames--along with the original
CRC value for the encapsulated
frame.

ISL CRC

The ISL CRC field is a new 32-bit
CRC that is calculated for the entire
ISL frame. It is calculated from the
entire ISL frame, including the orig-
inal frame.

T A B L E 1 1 . 4

Key for Figure 11.2 (continued)

Figure Symbol

Definition

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