CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Both protocols utilize a 12-bit VLAN ID field, and hence support the same number of VLANs.
ISL Header
(26 bytes)
ISL Header
Encapsulated
Ethernet Frame
CRC
(4 bytes)
ISL Trailer
Figure 7-2: Trunking Protocol Encapsulation Formats
- Below shows the trunking actions for different types of switch port modes:
Switch Port Mode Trunking Action
access Never try to trunk.
trunk Permanent trunking mode – always becomes a trunk link even if the
interface at the other end is not configured as a trunk port.
dynamic desirable Trunks to trunk, dynamic desirable, and dynamic auto interfaces.
dynamic auto Trunks to trunk and dynamic desirable interfaces.
Note: A trunk link will never be established if the interfaces at both ends of a trunk link were
configured as the dynamic auto mode!
- The switchport nonegotiate interface subcommand prevents an interface from generating
Dynamic Trunking Protocol (DTP) frames, which are used for trunking negotiation.
- Per-VLAN Spanning Tree Plus (PVST+) allows each VLAN to have its own instance of
spanning tree. Figure 7-3 shows 6 interfaces on 3 switches with 2 VLANs. STP parameters in
each VLAN are configured to block different interfaces in different spanning trees for VLAN
2 and VLAN 3 on SW3. SW3 would use the link to SW1 for traffic in VLAN 3 and link to SW2
for traffic in VLAN 2. STP will converge to find a new path for a VLAN if one of the links fails.
Figure 7-3: Per-VLAN Spanning Tree Plus (PVST+)
- 802.1Q defines a VLAN on a trunk as the native VLAN, where frames associated with the
native VLAN are not tagged. A switch recognizes a native VLAN frame when a frame
without 802.1Q tag is received from an 802.1Q trunk. ISL does not have this concept –
every frame will be encapsulated and has an ISL header.
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Dest Src Data FCS
Original
Frame
Dest Src Tag Data FCS
802.1Q Tag
SW1 SW2
Forwarding – VLAN 3 Forwarding – VLAN 2
Blocking – VLAN 2
SW3
Blocking – VLAN 3
Tagged
Frame
VLAN 3
VLAN 2
Root Bridge
Root Bridge
(Odd VLANs) (Even VLANs)
(eg: 1. 3. 5) (eg: 2, 4, 6)
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com