What
is Telnet?
Telnet is a terminal emulation program for TCP/IP networks such as the Internet.
The Telnet program runs on your computer and connects your PC to a server on
the network. You can then enter commands through the Telnet program and they
will be executed as if you were entering them directly on the server console.
This enables you to control the server and communicate with other servers on
the network. To start a Telnet session, you must log in to a server by entering
a valid username and password. Telnet is a common way to remotely control Web
servers. Telnet use port 23.
The
Telnet Protocol
The Telnet protocol is designed to
provide a bi-directional, eight-bit byte oriented communications facility to
allow for a standard method of interfacing terminal devices and processes. Additional
information on the Telnet protocol specification can be found RFC854.
Recall that a trunk’s frame
tagging protocol can be auto negotiated, through the use of the Dynamic
Trunking Protocol (DTP). DTP can also negotiate whether a port becomes a trunk
at all.
Examples demonstrated how to
manually configure a port to trunk:
Switch(config)# interface gi2/24
Switch(config-if)# switchport
mode trunk
DTP has two modes to dynamically decide
whether a port becomes a trunk:
Desirable
– the
port will actively attempt to form a trunk with the remote switch. This is the
default setting.
Auto
–
the port will passively wait for the remote switch to initiate the trunk.
What is VTP (VLAN Trunking Protocol)?
VTP (VLAN Trunking Protocol) is a Cisco proprietary
protocol used by Cisco switches to exchange VLAN information. With VTP, you can
synchronize VLAN information (like VLAN ID or VLAN name) with switches inside
the same VTP domain.
VTP Versions
There are three versions of VTP. VTP version 1 supports
the standard 1 – 1005 VLAN range. VTP version 1 is also default on Catalyst
switches.
VTP version 2 introduces
some additional features:
Token Ring support
VLAN
consistency checks
Domain-independent
transparent pass through VTPv1 and v2 are not compatible. The VTP version is
dictated by the VTP server, discussed in detail shortly. If the VTP server is
configured for VTPv2, all other switches in the VTP domain will change to v2 as
well. Until recently, VTP Version 3 was supported on only limited Cisco switch platforms.
VTPv3 was built to be flexible, and can forward both VLAN and other database
information, such as Multiple Spanning Tree (MST) protocol.
Other enhancements
provided by VTPv3 include:
Support for the extended 1006-4094 VLAN range.
Support for private VLANs.
Improved VTP authentication.
Protection from accidental database overwrites, by
using VTP primary and secondary servers.
Ability to enable VTP on a per-port basis.
Each switch can use one of
three different VTP modes:
1. VTP client mode –
a switch using this mode can’t change its VLAN configuration. That means that a
VTP client switch can’t create or delete VLANs. Received VTP updates are
processed and forwarded.
2. VTP server mode –
a switch using this mode can create and delete VLANs. A VTP server switch will
propagate VLAN changes. This is the default mode for Cisco switches.
3. VTP transparent mode –
a switch using this mode doesn’t share its VLAN database, but it forwards
received VTP advertisements. You can create and delete VLANs on a VTP
transparent switch, but the changes are not sent to other switches.
Local Area Network
(LAN) Switches support different Switching Methods. Important Switching Methods
are store and forward, cut-through and fragment-free. Switching Methods
determine how a switch receives, processes, and forwards a Layer 2
Ethernet frame.
Store and Forward Switching
In Store and forward switching, Switch copies
each complete Ethernet frame into the
switch memory and computes a Cyclic Redundancy Check (CRC) for errors. If a
Cyclic Redundancy Check (CRC) error is found, the Ethernet frame is dropped and if there is no Cyclic Redundancy
Check (CRC) error, the switch forwards the Ethernet frame to the destination device. Store and forward
switching can cause delay in switching since Cyclic Redundancy Check (CRC) is
calculated for each Ethernet frame.
Cut-through Switching
In cut-through switching, the switch copies into
its memory only the destination MAC address
(first 6 bytes of the frame) of the frame
before making a switching decision. A switch operating in cut-through switching
mode reduces delay because the switch starts to forward the Ethernet
frame as soon as it reads the destination MAC
address and determines the outgoing switch
port. Problem related with cut-through switching is that the switch may forward
bad frames.
Fragment-Free Switching
Fragment-free (runt less switching) switching is
an advanced form of cut-through switching. The switches operating in
cut-through switching read only up to the destination MAC address field in the Ethernet frame before making a switching decision. The switches
operating in fragment-free switching read at least 64 bytes of the Ethernet
frame before switching it to avoid forwarding Ethernet
runt frames (Ethernet frames smaller than 64
bytes).
A collision domain is simply defined as any physical segment where a collision
can occur. Hubs can only operate at half-duplex, and thus all ports on a hub
belong to the same collision domain. Layer-2 switches can operate at full
duplex. Each individual port on a switch belongs to its own collision domain.
Thus, Layer-2 switches create more collision domains, which results in fewer
collisions. Like hubs though, Layer-2 switches belong to only one broadcast
domain. A Layer-2 switch will forward both broadcasts and multi-casts out every
port but the originating port. Only Layer-3 devices separate broadcast domains.
VLAN (Virtual Local Area Network)
A virtual LAN (VLAN) abstracts the idea of the LAN. A VLAN might comprise a
subset of the ports on a single switch or subsets of ports on multiple
switches. By default, systems on one VLAN don't see the traffic associated with
systems on other VLANs on the same network.
VLANs allow network administrators to partition their networks to match the
functional and security requirements of their systems without having to run new
cables or make major changes in their current network infrastructure. IEEE
802.1Q is the standard defining VLANs; the VLAN identifier or tag consists of
12 bits in the Ethernet frame, creating an inherent limit of 4,096 VLANs on a
LAN.
Advantages of VLANs
BroadcastControl– eliminates unnecessary broadcast traffic, improving
network performance and scale-ability. Security– logically separates users and departments,
allowing administrators to implement access-lists to control traffic between VLANs. Flexibility – removes the physical
boundaries of a network, allowing a user or device to exist anywhere.
Access & trunk ports
Each port on a switch can be
configured as either an access or a trunk port. An access port is a port that
can be assigned to a single VLAN. This type of interface is configured on
switch ports that are connected to devices with a normal network card, for
example a host on a network. A trunk interface is an interface that is
connected to another switch. This type of interface can carry traffic of multiple VLANs.
Range of VLANs Standard range –VLANs number is 1 – 1005 Extended range –VLANs number is 1006 – 4094
Standard VLAN Vs Extended VLAN VLANs numbered from 1 to 1005 are considered as Standard VLANs and the VLANs
range from 1006 to 4094 are considered as Extended VLANs. Extended VLANs are
not stored in the vlan.dat file they are stored in running config.
Secondly, if you create extended VLANs in Version 1 & 2, then your switch
must be in transparent mode as these VLANs cannot be sent in VTP updates.
VLAN 0 is reserved and not available for use
The VLAN 0 is used when a device needs to send priority tagged frames but
does not know in which particular VLAN it resides.
VLAN 1 is reserved and not available for use
VLAN 2 to 1001 Normal range VLANs (Standard VLANs)
VLAN 1002 to 1005 are used for FDDI and TR (Token Ring) translational
bridging and shouldn’t be used for anything other than these purposes, they are
not advertised by VTP
VLAN 1006 to 4094 are extended VLANs which can’t be advertised by VTP and
the switch must be configured in VTP transparent mode, epically if you using
VTP version 1 & 2.
It’s also important to know that when a switch starts up it checks the VTP
mode and domain name from the startup and vlan.dat file; if they are different
it ignores the startup config and uses the vlan.dat file.
