Dynamic Trunk Protocol (DTP)- Overview

Here is a quick overview/cheat-sheet of Dynamic Trunk Protocol (DTP).  The table below shows which combination of port modes will create a successful trunk via DTP.  (and before anyone says anything, I do realize that the modes with no-negotiate do not actually use DTP)

	Access + No-negotiate	Access	Dynamic Auto	Dynamic Desirable	Trunk	Trunk + No-negotiate
Access + No-negotiate	No	No	No	No	No	No
Access	No	No	No	No	No	No
Dynamic Auto	No	No	No	Yes	Yes	No
Dynamic Desirable	No	No	Yes	Yes	Yes	No
Trunk	No	No	Yes	Yes	Yes	Yes
Trunk + No-negotiate	No	No	No	No	Yes	Yes

Here are a few important points regarding DTP.

• The default port mode on 3550 switches is Dynamic Desirable.
• The default port mode on 3560 switches is Dynamic Auto.
• If both switches support ISL and Dot1q trunks, and neither is specified, ISL will be preferred.
• The no-negotiate option is used when you do not want to send out DTP frames.
• The no-negotiate option is only usable with the Access and Trunk modes.

I have seen Cisco documentation that Dynamic Auto is the default switchport mode.  The Cisco Press Exam Certification Guide says that Dynamic Desirable is the default mode.  It seems to be depndant on the switch model.  New models appear to use Dynamic Auto.  So I guess I would stick with that as the default mode of a switchport if you were asked on a test (unless a specific model is mentioned).

VLANs and VLAN Trunking- Notes

Here are my notes on the VLANs and VLAN Trunking chapter of the Cisco Press Exam Certification Guide book.  This is foundational knowledge for switching that needs to be known backwards and forwards.

Best practices dictate a 1-to-1 relationship between VLANs and IP subnets

For a VLAN to be operational, the following must be true

• VLAN has been created
• Switchports must be assigned to VLAN

VLANs can be created in VLAN Database mode (depreciated), global config mode, and interface config mode

Private VLANs

• Requires a Primary VLAN and one or more Secondary VLANs
• Ports in a primary VLAN are promiscuous and can talk with any secondary VLAN ports
• Secondary Community VLAN ports can talk to other community ports in the same secondary VLAN and ports in the primary VLAN. Need a community secondary VLAN for each community group.
• Secondary Isolated VLAN ports can only talk to ports in the primary VLAN. Only need one isolated secondary VLAN.

VTP Modes

• Server
  • Can create/edit VLANs (1-1005)
  • Can act upon and originate VLAN updates
  • Default VTP mode
  • Stores info in vlan.dat file on flash

• Client
  • Can learn VLANs (1-1005)
  • Can act upon and originate VLAN updates
  • Stores info in vlan.dat file on flash

• Transparent
  • Can create/edit VLANs (1-4094)
  • Can not originate or act upon VLAN updates
  • Will forward on received updates from other switches
  • Stores info in vlan.dat file on flash  and also in config

Switches are in server mode by default.

They do not send out advertisements until they have a domain configured.

They will learn a domain from the first advertisement that they receive.

For switches to be able to update each other, VTP domain and password must match.

VTP updates only sent over trunks

VTP version 2 really only needed for Token Ring support

VLANs 1, 1002-1005 are special and cannot be deleted

ISL trunk adds a 26 byte header and a 4 byte trailer.

ISL header destination address is 0100.0C00.0000 or 0300.0C00.0000

ISL is the preferred trunking encapsulation if both ISL and 802.1Q are supported

802.1Q adds 4 byte tag after source address field. First 2 bytes is an ethernet type value of 0×8100.

802.1Q supports native VLAN concept (does not insert a tag).

Dynamic Trunk Protocol (DTP)

• Trunk- always sets up a trunk
• Desirable- will set up a trunk with Trunk, Desirable, and Auto
• Auto- will setup a trunk with Trunk and Desirable
• Access- will not setup a trunk
• No-negotiate- used with Access and Trunk. Does not negotiate using DTP. If used with Trunk, the other side must also be Trunk in order for a trunk to be fully operational

Trunking on routers

• Routers do not support DTP. So trunk must be statically configured on both ends.
• Typically uses subinterfaces for each VLAN on the router
• Uses the encapsulation command under each subinterface
• Native VLANs configured on subinterfaces require the native keyword in the encapsulation command
• If native VLAN is not configured on a subinterface, the router assumes it is on the physical interface. In this case, the physical interface does not need the encapsulation command

802.1Q-in-Q trunking

• Allows a service provider to preserve 802.1Q VLAN tags across a WAN service
• CDP and VTP traffic passes transparently over the Q-in-Q service

Ethernet Basics- Notes

Here are my notes on the Ethernet Basics chapter of the Cisco Press Exam Certification Guide book.  This is pretty much one of those chapters that is all theory and not very helpful in relation to the lab.  But it’s stuff that everyone should know.

Also, an FYI on how I’ll categorize my future posts.  “Notes” posts will be my own study notes cut and pasted into the post.  So they won’t be real descriptive or polished.  “Overview” posts will be focused on a certain topic and will be written a more reader-friendly manner.

10/100 Ethernet uses pins 1-3 and 6 for communications.

Pins 1-2 are transmit and pins 3,6 are receive for a PC/Router (switches are opposite)

Auto-MDIX on switches can compensate for using wrong cable types (straight-through or cross-over)

Speed/duplex auto-negotiation

• Cisco switches sense speed using the Fast Link Pulses (FLP) of the auto-negotiation process
• If one side has auto-negotiation turned off, the other switch can still detect speed based on incoming electrical signal
• Duplex auto-negotiation requires both sides to participate
• If one side does not participate, 10/100 links default to half-duplex and Gig links default to full
• To turn off auto-negotiation on a switch port, statically set a speed/duplex setting in the interface config mode
• Cannot set duplex on an interface until speed has been set
• CDP can detect duplex mismatches and send notification, but cannot remedy

CSMA/CD process

• Device with a frame to send listens to the wire and waits for a free line (no carrier signal sensed)
• When the line is free, the device starts sending the frame
• Sender listens to make sure no collision occurred
• If there was a collision, all devices that sent a frame send a jamming signal
• After the jamming signal is complete, each device that sent a frame that collided set a random back-off timer
• Once the timer expires, the devices can begin sending again

Ethernet hubs

• Operate at layer 1
• Repeat (regenerate) signals to improve cabling distances
• Forward data received on one port out all others
• Creates a single collision domain

When switches receive multiple frames of different switch ports, they stores frames in memory to prevent collisions

NICs operating in half-duplex mode use loopback circuitry when transmitting a frame. This loops the transmitted frame back to the receive side of the NIC, so that when the NIC receives a frame over the cable, the combined loopback signal and received signal allows the NIC to notice that a collision occurred

In switch interface stats, collisions are detected in first 64 bytes. Late collisions are detected after 64 bytes have been sent. Deferred packets mean that the half-duplex switch port waited until it was not receiving data to send it.

Ethernet type fields

• Ethernet DIX
  • 2 byte Type field

• 802.3
  • 1 byte DSAP

• 802.3 with SNAP header
  • 2 byte type field in SNAP header, DSAP of 0xAA

Multicast packets identified by I/G bit being a 1 (8th bit of the address)

U/L bit (7th bit) identifies if address is vendor assigned or locally assigned

Switch MAC address (CAM table) aging timeout is 300 seconds by default

802.3u defines 100MB Ethernet over fiber and copper

802.3z (optical) and 802.3ab (copper) define Gig Ethernet

Switch internal processing

• Store and forward- switch fully receives frame before forwarding
• Cut-through- switch performs table lookup once destination address field is received. Does not allow switch to drop frames that fail FCS check
• Fragment free- like cut-through, but waits to receive first 64 bytes before forwarding. Should avoid forwarding frames damaged by collisions