VLAN (Virtual LANs) are sometimes difficult to troubleshoot and you face a lot of confusion in isolation a problem especially in an environment where there are alot of VLANs. And this is common in today’s networks, there are hundreds of VLANs on switches and you are required to grasp a knowledge of troubleshooting VLANs in an affective way i-e isolation of problem in minimum ammount of time available. There are some CISCO power point slides which help in gaining and sharpening this knowledge. You can find the slides as follows:

http://rapidshare.com/files/226416774/Troubleshooting_VLANS.ppt

The designated cost is the cost of the directly connected router to the root bridge. Example is explained as follows:

For example when we change the cost of F1/1 of SWC to 5 for VLAN 2 and cost of F1/2 of SWB for VLAN 2 to 11, SWD shows 16 (11+5) as a designated cost for VLAN 2 as shown below:

See some explanation below:

>> If you see the value of Root Bridge and Bridge ID to be the same for some switch, that will indicate that the current switch is the root bridge. Its shown below:

The value 32768 is the default value . The root path cost is the cost of the path to the root bride. Since this is the root bridge itself, so you see the Root Path Cost value of Zero.

>> There is one more thing that STP operates on Layer 2 (Data Link Layer). It doesn’t contain the information for Layer 3 (Network Layer). See the snapshot below:

It can be seen clearly that there is a MAC Address information displayed for STP packets instead of IP Address information which is displayed for other layer 3 packets.

>> The Root port is the one which is connected to the Root Bridge. As shown in the output of Switch 2 below, the Role of Ethernet 0/1 is “Root” which proves that this port connects to Switch1 (through Cross-over cable).

The designated port is the one which has the lower cost. The designated port is regarded as the forwarding port as compared to Non-Designated port which can be alternative to the designated port.

>> We can set a protection for each port of the switch such as we can set a specific mac-address which can access the port and if a device with some other MAC Address tries to connect to that port, the port can be turned-off or it can restrict the user.

>> The Switch with lowest priority is selected as a Root Bridge. If all the Switches have the same priority, then MAC Address serves as a tie for Root Bridge Election process. For small networks, the default settings work fine but if the network is larger, you can set the priority of the bridge yourself and set the priority of the Switch which occurs to be in the centre of the network lower. In this way, this switch will automatically become the root bridge and we will get an advantage that the STP will converge faster. The default time for the convergence of STP is 50 seconds. The priority can from 0 to 61440 where zero means the switch will always be the root bridge and 61440 means it can never be the root bridge. This value can be set in the increments of 4096.

>> The forward delay is the time it takes the switch port for the transition from Listening to Learning state. It can be seen in the Ethereal packets. The default value used in Switches is 15 Seconds as shown below:

To be continue …

>> The Root ID is the combination of bridge priority and base MAC Address as shown below:

>> There are 3 modes of STP

(a) MST (Multiple Spanning Tree)

(b) PVST (Per-VLAN Spanning Tree Mode)

(c) Rapid PVST

Rapid PVST uses IEEE 802.1w which gives a good package to all the issues which are addressed by using uplink fast, port fast and backbone fast commands. Others use IEEE 802.1d.

>> BPDU is Bridge Protocol Data Unit. These are the packets transferred between the switches with STP being enabled. These are used for the operation of STP.

>> If there is a topology change, the switch ports come in the state of listening and learning. In normal operation, the switch ports are in forwarding states. The topology change is determined by the BPDU Flag option as shown below:

>> The Root Bridge cannot have any blocked port. After convergence, all its ports come in the Forwarding state as shown below:

Root Bridge Output (Switch 1):

Non-Root Bridge Output (Switch 2):

To be continue …

The acronym VLAN expands to Virtual Local Area Network. A VLAN is a logical local area network (or LAN) that extends beyond a single traditional LAN to a group of LAN segments, given specific configurations. Because a VLAN is a logical entity, its creation and configuration is done completely in software.

More info on VLANs from the following links:

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CBT Nuggets is the best video training available about Cisco and other technology certifications. IP Subnetting is an important part of CCNA 640-802 or ICND2 640-816 and ICND1/CCENT 640-822 exams. Actually it is the basis of all the future Cisco certifications and rather a base of your networking skills. Here are the complete online video tutorial regarding IP Subnetting. It is divided into three parts. All three parts are given below:

A very good video explanation of IPv6 protocol. Todd Lammle explains how IPv6 works.

IPv6 stands for Internet Protocol version 6. It is the second version of the Internet Protocol to be used generally across thevirtual world. The first version was IPv4. IPv5 was a protocol of a different sort, intended to support video and audio rather than all-purpose addressing. IPv6 is also known as IPng, which stands for IP Next Generation.

If you want to practice your CCNA skills, you should use some kind of exam simulation in order to see that how competent you are and in which topics you still need to work. Here is a link where you can find different kinds of free exams related to CCNA. For example, if you are going for an upgrade exam, you should click on the CCNA upgrade exam and if you want to go for the new 640-802 exam, you should click that. Access the practice exams from here:

http://www.howtonetwork.net/public/department33.cfm

The video below shows you how to access the free exams.