Hi Sulaiman,
1) what is the default running protocol on switches? DTP or VTP ?
Both.
2) what these both do ?
DTP tries to make sure that on a link that interconnects two switches, both ports operate in the same mode - either access or trunk. In other words, DTP allows two interconnected switches to negotiate the operating mode of a link - either an access link or a trunk.
VTP is responsible for synchronizing the VLAN database across multiple switches. When you create, rename, delete a VLAN or change its state (active or suspended) on a single switch, VTP will propagate this change across all switches in a VTP domain. This way, the management of VLANs on a network with multiple switches is greatly simplified - you make a change to the VLAN database on a single switch, and all switches will learn about the change. So in other words, VTP tries to make sure that all switches in a VTP domain have an identical VLAN database and propagates any changes to it.
3) When wee say DTP has some modes(on, off, desireable, auto and negitiate) and when we say individual switchport has some modes(trunk, access, dynamic desireable, dynamic auto, nonegotiate). what's that mean?
Let's start with the switchport modes. A switchport can either belong to a single VLAN only, or it can be a link that carries data in many VLANs at the same time. If the port belongs to a single VLAN, we call it an access port. If the port belongs to multiple VLANs simultaneously, we call it a trunk. So, switchport modes are either access or trunk - there are no others. Access ports are usually used to connect individual end devices to the network, such as PCs, network printers, webcams, etc. Trunk ports are used to interconnect switches because then the VLANs on both these switches can be made contiguous and access ports in the same VLAN across multiple switches can communicate together as if they were on a single switch. Trunk ports must use frame tagging to distinguish the VLAN frames belong into: when a frame is sent out a trunk port, it receives a tag that says which VLAN does this frame belong into. The receiving trunk port will analyze this tag, remove it, and forward the frame in the appropriate VLAN. Access ports do not use tags because they do not need them - the configuration of a trunk port says which single VLAN it belongs into.
The switchport mode, either access or trunk, is also called the operating mode of a port.
DTP modes tell the port whether it is capable of negotiating the operating mode with the neighboring port via DTP, and if so, what is the preferred operating mode of that port. DTP modes are translated into operating modes of ports.
- DTP mode on is a synonym for saying that the port is statically set to trunk and the DTP has not been deactivated. It is confusingly named, as the DTP is also "on" (i.e. in operation) on dynamic ports. This mode does not allow the port to negotiate its own operating mode (as it is already a static trunk) but the neighboring device can adapt itself if it is in dynamic mode.
- DTP mode off means that the DTP has been deactivated. As a result, the port is not able to tell the neighboring device about its operating mode, and is not able to adapt to the neighboring device's mode. The DTP mode is set to off automatically on static access ports, and can be manually deactivated on static trunk ports using switchport nonegotiate command. Hence, sometimes a static trunk with the switchport nonegotiate command is said to be in DTP nonegotiate mode but DTP itself does not recognize that mode - it is a misnomer.
- DTP mode dynamic means that the switchport is not statically configured for its operating mode and has to dynamically negotiate it with the connected devices. There are two submodes to this mode:
- dynamic desirable means that the port can dynamically adapt to the operating mode of its neighbor, but it prefers to be a trunk
- dynamic auto means that the port can dynamically adapt to the operating mode of its neighbor, but it prefers to be an access port
The resulting operating mode of a link between two ports as negotiated by DTP (if possible) would be:
- dynamic desirable + dynamic desirable = trunk
- dynamic desirable + dynamic auto = trunk
- dynamic auto + dynamic auto = access
- trunk (on) + dynamic (either desirable or auto) = trunk
- access (off) + dynamic (either desirable or auto) = access
- trunk with nonegotiate (off) + dynamic (either desirable or auto) = access on the dynamic side - MODE MISMATCH
Of course, DTP is not capable of correcting mode mismatches if it does not run or if the ports are statically set to different operating modes.
Best regards,
Peter
