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VLAN Synchronization

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VLAN Synchronization

Last time we used a simple two-story topology to demonstrate how to configure a trunk link to carry multiple traffic. Now we will talk about VLAN synchronization

Imagine that you are working in a skyscraper and need to adjust the configuration of the VLAN as required within a limited time. After observing, you find the building is very majestic, which has 108 floors above the ground, and 7 floors underground, while each floor has 100 access switches, which plus the convergence layer and core layer of equipment, at least more than 10,000 devices in total to configure  …………

But, calm down, we can still make it, because there is a technology that allows switches to automatically learn VLAN configuration information!

Vlan Trunk Protocol (VTP) is cisco’s proprietary VLAN synchronization technology. Let’s do the experiment while understanding the basic concepts and applications of VTP.

Test topology::

As shown in the figure, the links between all switches are trunk links. Note that VTP technology must be used to ensure normal operation of the links between all switches. We will complete the configuration on Switch1 and verify on the other switches.

First to see Show VTP status. By default, Cisco switches enable VTP in Server mode. VTP has four modes: server, client, transparent, and off.

In the default server mode, the switch supports adding, modifying, and deleting vlans. But how do we synchronize vlans to other switches?

The default VTP domain name is empty. In the empty domain name state, you can learn the domain name of other switches. As you can see, when we configure the VTP domain name on Switch1, Switch2 (and other switches) VTP can automatically learn the domain name and  the VLAN on Switch1.

So what are the basic requirements to synchronize a VLAN?

1. The VTP domains of all switches must be consistent

2. Links between switches must be trunk

3. The VTP mode of the switch must be Server or Client

4. If VTP authentication is configured, the authentication passwords must be the same

5. The low configuration revision number learns the high configuration revision number

Client mode: When the Client mode is set to Client, the switch can not support adding, modifying, and deleting vlans. This mode is suitable for access switches 3, 4, 5, and 7 in the figure. This mode enables to access switches to learn from vlans of other switches and prevents misoperations led by accessing personnel from affecting the global network.

Note: Learning a VLAN is relevant only to configuration revision, not to whether the mode is Server or client. Let’s verify that

First, we shutdown the trunk link in sw3 to observe the phenomenon. Then we add the VLAN and delete it. At this time, we can see that the configuration revision number increase to 10, while the configuration revision number of other switches is 8.Now, we change the VTP mode into client mode, and do no shutdown on the trunk link to observe.

Second, if the basic conditions are met but the VLAN is not synchronized, you can check the MD5 check value of VTP status. The MD5 check value is obtained based on the VLAN information on the local switch and the configuration revision number. The reason that the MD5 values are different is that the synchronization may be caused by the same configuration version. The solution is to add and delete vlans on switches with correct VLAN information and increase the size of the configuration revision number to achieve information synchronization.

Transparent: When we don’t want a device in the network to participate in the VTP synchronization, you can set it to transparent mode, which will put the configuration revision number to 0, can and can add, modify or delete vlan, but not synchronize other switches vlan information, also will not synchronize their own vlan information to other switches. However, if a switch in transparent mode receives a VTP packet, it still sends it to other switches. That is so-called transparent.

VTP also has an off mode. If the OFF mode is enabled, the VTP function of the switch is disabled and VTP packets can be isolated.

VTP pruning is enabled by running the VTP pruning command. The purpose of VTP pruning is to reduce unnecessary broadcast flooding. After VTP pruning is enabled, if no access interface associated with a VLAN is detected behind a trunk link, related traffic flooding is automatically blocked to save hardware resources.

VTP version1 and version2 only support standard VLANS, while version3 adds support for extended vlans.

Does Huawei have corresponding VLAN synchronization technology?

Huawei currently has two technologies for VLAN synchronization. One is the GVRP Universal VLAN Registration protocol, which belongs to the GARP protocol set. You only need to run the “GVRP” command in system mode and interface to enable the GVRP protocol. However, the registered VLAN information can ensure that the trunk in the network can pass traffic normally. Huawei later launched the VCMP protocol, which has many similarities with Cisco’S VTP. The biggest difference between VCMP and GVRP is that VCMP registers static VLANS, while GVRP registers dynamic vlans. VCMP requires a later version of the system to support, and BOTH VCMP and GVRP are Huawei proprietary protocols. Note that both VTP GVRP and VCMP require that the cascading line between switches be a trunk link.

VLAN Synchronization

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