VLAN Trunk Protocols (VTP) One Picture Summary

What is VTP? Subnet adv:

How VLAN database is stored:
VTP uses lay 2 multicast frames on Native VLAN (VLAN 1) to IOS switch in Server & Client mode: VLAN databases into Response to request from client, includes detailed info for
synchronize VLAN information among a group of switches in vlan.dat file in Flash, to clear all configurations, you need specific vlan.
the same VTP domain. to delete the vlan.dat file in flash after command “erase
VTP versions: startup-config”.
 VTP version 1&2: only support VLAN ID range 1-1005, in IOS switch in Transparent mode: VLAN database is stored
which VLAN 1, 1002-1005 are set aside for dedicated in configuration file in NVRAM, thus “erase
use. In order to support extended-range VLANs (1-4094), startup-config” will clear all VLANs.
the only way for VTP version 1&2 node is to enable VTP CatOS switch: VLAN database stored in memory. VLAN
Transparent mode. database disappears after reloading and configuration
 VTP version 3: natively supports extended-range VLANs. version becomes 0.
However, it can be enabled only on CatOS based VTP Pruning:
switches.  It makes more efficient use of Trunk link BW.
 VTP version 1 is by default enabled, and switching the  Broadcast and unknown unicast frames on a VLAN are
whole VTP domain to VTP Ver 2 only requires you to forwarded over a trunk link only if the receiving switch
enable VTP ver 2 on VTP Server mode switch. currently has active port on that VLAN.
 Note that SVI of a VLAN will also be shutdown Adv Request from client:
How does VTP works? automatically if there is no Active port on that VLAN. Request updates when higher config version number in
summary adv is heard.
VTP mechanism relies on 3 VTP Roles (Server / Client /
Transparent; and 3 VTP Advertisement (Summary / Request / Basic Operational Parameters
Response). Layer-2 multicast address: 01-00-0C-CC-CC-CC
VTP Operation VLAN range: Vlan 1 to 1005
VTP Server: VTP message Interval: 5 mins
 Originate VTP Summary adv every 300 secs, but never
forward VTP Summary adv.
 VTP version change on VTP server will automatically VTP Advertisement Types
propagate throughout the whole domain.
VTP Client: Summary adv:
Every 300 secs, and change triggered. Configuration Commands
 Originate VTP Summary adv every 300 secs, and also
forward VTP Summary adv originated from other VTP Switch(config)# vtp version {1 | 2}
Server Nodes. Switch(config)# vtp mode server
VTP Transparent: Switch(config)# vtp domain Enterprise
 Configuration version value always equals 0. Switch(config)# vtp password cisco
 Only forwards VTP Advertisements but never originates, Switch(config)# vtp pruning
and its local VLAN database is never overwritten Switch(config-if)# switchport trunk pruning vlan {none |
according to VTP Advertisements. {add | except | remove (vlan-list)} }
 Normally used to clear all local VLAN Information.
Troubleshooting
Switch# show vtp status
Switch# show vtp counters
Switch# show vlan brief
Dynamic Trunking Protocols (DTP) One Picture Summary
Basic Concepts
Default Behaviors
What is DTP?
 Cisco proprietary point-to-point protocol that negotiates Default Configuration on Trunk port:
whether a link should be a trunk link, and whether ISL or Switch(config)# interface fa0/1
802.1Q should be used for encapsulation Switch(config-if)# switchport mode dynamic desirable
How does it work? Switch(config-if)# switchport trunk encapsulation
 DTP frames are sent out per 30 secs to keep informing negotiate
neighbor of its link mode, regardless of which trunk  ISL is preferred over 802.1Q.
mode (trunk | dynamic desirable | dynamic auto) the  In case of 802.1Q, Native VLAn is 1 by default.
switchport is on.  By default, all VLANs (1-4094) are allowed on trunk.
 The only way to disable these DTP frames exchange is
“switch(config-if)# switchport nonegotiate”.
 A trunk negotiation only happens between two switches DTP Configuration
who belong to the same VTP domain or if one or both Switch(config)# interface fa0/1
switches have not defined their VTP domain (that is, the Switch(config-if)# switchport
NULL domain). Switch(config-if)# switchport mode {trunk | dynamic
(desirable | auto)}
Types of switchport mode Switch(config-if)# switchport trunk encapsulation {isl |
dot1q | negotiate}
Switchport mode trunk: Switch(config-if)# switchport trunk native vlan (vlan-id)
 Place port in permanent trunking mode unconditionally, Switch(config-if)# switchport trunk allowed vlan {vlan-list |
regardless all other configs (VTP domain, etc). However, all | (add | except | remove) vlan-list}
DTP frames is still sent out.
 In other words, setting up the trunk forcely, and DTP Disable unnecessary DTP frames: You should disable DTP
frames can be manually disabled in this mode. negotiation if a switch has a trunk link connected to a
Switchport mode dynamic desirable (default): non-trunking router or firewall interface because those devices
 Proactively ask the far-end to setup the trunk link, and cannot participate in DTP negotiation.
will succeed when dynamic desirable or dynamic auto is
configured on the far-end. Troubleshooting
Switchport mode dynamic auto:
Switch# show interface fa0/0 trunk
 Passively wait far-end to proactively attempt to setup
the trunk link when dynamic desirable is configured on
the far-end.