You are designing your network to be able to use trunks. As part of this process you are
comparing the ISL and 802.1 Q encapsulation options. All of these statements about the two
encapsulation options are correct except which one?
A.
Both support normal and extended VLAN ranges.
B.
ISL is a Cisco proprietary encapsulation method and 802.1 Q is an IEEE standard.
C.
ISL encapsulates the original frame
D.
Both support native VLANs.
E.
802.1 Q does not encapsulate the original frame.
Explanation:
ISL is a Cisco proprietary protocol for the interconnection of multiple switches and maintenance of
VLAN information as traffic goes between switches. ISL provides VLAN trunking capabilities while
it maintains full wire-speed performance on Ethernet links in full-duplex or half-duplex mode. ISL
operates in a point- to-point environment and can support up to 1000 VLANs. In ISL, the original
frame is encapsulated and an additional header is added before the frame is carried over a trunk
link. At the receiving end, the header is removed and the frame is forwarded to the assigned
VLAN. ISL uses Per VLAN Spanning Tree (PVST), which runs one
instance of Spanning Tree Protocol (STP) per VLAN. PVST allows the optimization of root switch
placement for each VLAN and supports the load balancing of VLANs over multiple trunk links.
802.1Q is the IEEE standard for tagging frames on a trunk and supports up to 4096 VLANs. In
802.1Q, the trunking device inserts a 4-byte tag into the original frame and recomputes the frame
check sequence (FCS) before the device sends the frame over the trunk link. At the receiving end,
the tag is removed and the frame is forwarded to the assigned VLAN. 802.1Q does not tag frames
on the native VLAN. It tags all other frames that are transmitted and received on the trunk. When
you configure an 802.1Q trunk, you must make sure that you
configure the same native VLAN on both sides of the trunk. IEEE 802.1Q defines a single instance
of spanning tree that runs on the native VLAN for all the VLANs in the network. This is called
Mono Spanning Tree (MST). This lacks the flexibility and load balancing capability of PVST that is
available with ISL. However, PVST+ offers the capability to retain multiple spanning treetopologies with 802.1Q trunking.