Cisco VTP: VLAN Trunking Protocol 5:21 AM

VLAN Trunking Protocol (VTP) is a Cisco Layer 2 messaging protocol that manages the addition, deletion, and renaming of VLANs on a network-wide basis. Virtual Local Area Network (VLAN) Trunk Protocol (VTP) reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst Family products.

VTP ensures that all switches in the VTP domain are aware of all VLANs. There are occasions, however, when VTP can create unnecessary traffic. All unknown unicasts and broadcasts in a VLAN are flooded over the entire VLAN. All switches in the network receive all broadcasts, even in situations where few users are connected in that VLAN. VTP pruning is a feature used to eliminate (or prune) this unnecessary traffic.

By default, all Cisco Catalyst switches are configured to be VTP servers. This is

suitable for small-scale networks where the size of the VLAN information is small and easily stored in all switches (in NVRAM). In a large network, a judgment call must be made at some point when the NVRAM storage needed is wasted, because it is duplicated on every switch. At this point, the network administrator should choose a few well-equipped switches and keep them as VTP servers. Everything else participating in VTP can be turned into a client. The number of VTP servers should be chosen so as to provide the degree of redundancy desired in the network.

There are three version of VTP so far. VTP Version 2 (V2) is not much different than VTP Version 1 (V1). The major difference is that VTP V2 introduces the support for Token Ring VLANs. If you are using Token Ring VLANs, you need to enable VTP V2. Otherwise, there is no reason to use VTP V2. VTP version 3 differs from earlier VTP versions in that it does not directly handle VLANs. VTP version 3 is a protocol that is only responsible for distributing a list of opaque databases over an administrative domain. When enabled, VTP version 3 provides the following enhancements to previous VTP versions:

• Support for extended VLANs.
• Support for the creation and advertising of private VLANs.
• Improved server authentication.
• Protection from the "wrong" database accidentally being inserted into a VTP domain.
• Interaction with VTP version 1 and VTP version 2.
• Provides the ability to be configured on a per-port basis.
• Provides the ability to propagate the VLAN database andother databases.

Protocol Structure - VTP: VLAN Trunking Protocol

The format of the VTP header can vary depending on the type of VTP message. However, they all contain the following fields in the header:

• VTP protocol version: 1 or 2 or 3
• VTP message types:
• Summary advertisements
• Subset advertisement
• Advertisement requests
• VTP join messages
• Management domain length
• Management domain name

Summary Advertisements

When the switch receives a summary advertisement packet, it compares the VTP domain name to its own VTP domain name. If the name is different, the switch simply ignores the packet. If the name is the same, the switch then compares the configuration revision to its own revision. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent.

• Followers indicate that this packet is followed by a Subset Advertisement packet.
• The updater identity is the IP address of the switch that is the last to have incremented the configuration revision.
• Update timestamps are the date and time of the last increment of the configuration revision.
• Message Digest 5 (MD5) carries the VTP password if it is configured and used to authenticate the validation of a VTP update.

Subset Advertisements

When you add, delete, or change a VLAN in a switch, the server switch where the changes were made increments the configuration revision and issues a summary advertisement, followed by one or several subset advertisements. A subset advertisement contains a list of VLAN information. If there are several VLANS, more than one subset advertisement may be required in order to advertise them all.

The following formatted example shows that each VLAN information field contains information for a different VLAN (ordered with lowered-valued ISL VLAN IDs occurring first):

Most of the fields in this packet are easy to understand. Below are two clarifications:

• Code- The format for this is 0x02 for subset advertisement.
• Sequence number- This is the sequence of the packet in the stream of packets following a summary advertisement. The sequence starts with 1.

Advertisement Requests

A switch needs a VTP advertisement request in the following situations:

• The switch has been reset.
• The VTP domain name has been changed.
• The switch has received a VTP summary advertisement with a higher configuration revision than its own.

Upon receipt of an advertisement request, a VTP device sends a summary advertisement, followed by one or more subset advertisements. Below is an example.

• Code- The format for this is 0x03 for an advertisement request
• Start Value - This is used in cases where there are several subset advertisements. If the first (N) subset advertisement has been received and the subsequent one (N+1) has not, the Catalyst only requests advertisements from the (N+1)th one.

Related Protocols

IEEE 802.1Q

Sponsor Source

VLAN Trunking Protocol (VTP) is a Cisco protocol.

Reference

http://www.cisco.com : Understanding and Configuring VLAN Trunking Protocol

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CCNA Exam Troubleshooting 12:14 PM

Refer to the exhibit. The network administrator is in a campus building distant from Building B. WANRouter is hosting a newly installed WAN link on interface S0/0. The new link is not functioning and the administrator needs to determine if the correct cable has been attached to the S0/0 interface. How can the administrator accurately verify the correct cable type on S0/0 in the most efficient manner?

A. Telnet to WANRouter and execute the command show interfaces S0/0

B. Telnet to WANRouter and execute the command show processes S0/0

C. Telnet to WANRouter and execute the command show running-configuration

D. Telnet to WANRouter and execute the command show controller S0/0

E. Physically examine the cable between WANRouter S0/0 and the DCE.

F. Establish a console session on WANRouter and execute the command show interfaces S0/0

 Answer: D

Explanation:

The show controller command displays the information about the physical interface itself and the type of serial cable plugged into a serial port. In this case, it should be a DTE cable that plugs into a type of data service unit (DSU).

For your understanding, below is the output of this command:

From the output, we notice that serial 0/0 has a DTE cable and would get its clocking from the DSU.

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Two routers named Atlanta and Brevard are connected by their serial interfaces as shown in the exhibit, but there is no data connectivity between them. The Atlanta router is known to have a correct configuration.

Given the partial configurations shown in the exhibit, what is the problem on the Brevard router that is causing the lack of connectivity?

A. A loopback is not set

B. The IP address is incorrect.

C. The subnet mask is incorrect.

D. The serial line encapsulations are incompatible.

E. The maximum transmission unit (MTU) size is too large.

F. The bandwidth setting is incompatible with the connected interface.

 Answer: B

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Refer to the exhibit. The two exhibited devices are the only Cisco devices on the network. The serial network between the two devices has a mask of 255.255.255.252. Given the output that is shown, what three statements are true of these devices? (Choose three)

A. The Manchester serial address is 10.1.1.1.

B. The Manchester serial address is 10.1.1.2.

C. The London router is a Cisco 2610.

D. The Manchester router is a Cisco 2610.

E. The CDP information was received on port Serial0/0 of the Manchester router.

F. The CDP information was sent by port Serial0/0 of the London router.

 Answer: A C E

Explanation:

From the output, we learn that the IP address of the neighbor router is 10.1.1.2 and the question stated that the subnet mask of the network between two router is 255.255.255.252. Therefore there are only 2 available hosts in this network (2² - 2 = 2). So we can deduce the ip address (of the serial interface) of Manchester router is 10.1.1.1 -> A is correct

The flatform of the neighbor router is cisco 2610, as shown in the output -> C is correct

Maybe the most difficult choice of this question is the answer E or F. Please notice that "Interface" refers to the local port on the local router, in this case it is the port of Manchester router, and "Port ID (outgoing port)" refers to the port on the neighbor router -> E is correct.

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A network administrator has configured two switches, named London and Madrid, to use VTP. However, the switches are not sharing VTP messages. Given the command output shown in the graphic, why are these switches not sharing VTP messages?

A. The VTP version is not correctly configured.

B. The VTP operating mode is not correctly configured.

C. The VTP domain name is not correctly configured.

D. VTP pruning mode is disabled.

E. VTP V2 mode is disabled.

F. VTP traps generation is disabled.

 Answer: C

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The network shown in the diagram is experiencing connectivity problems. Which of the following will correct the problems? (Choose two.)

A. Configure the gateway on Host A as 10.1.1.1.

B. Configure the gateway on Host B as 10.1.2.254.

C. Configure the IP address of Host A as 10.1.2.2.

D. Configure the IP address of Host B as 10.1.2.2.

E. Configure the masks on both hosts to be 255.255.255.224.

F. Configure the masks on both hosts to be 255.255.255.240.

Answer: B D

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Refer to the exhibit:

The network administrator has created a new VLAN on Switch1 and added host C and host D. The administrator has properly configured switch interfaces FastEthernet0/13 through FastEthernet0/24 to be members of the new VLAN. However, after the network administrator completed the configuration, host A could communicate with host B, but host A could not communicate with host C or host D. Which commands are required to resolve this problem?

A. Router(config)# interface fastethernet 0/1.3

Router(config-if)# encapsulation dot1q 3

Router(config-if)# ip address 192.168.3.1 255.255.255.0

B. Router(config)# router rip

Router(config-router)# network 192.168.1.0

Router(config-router)# network 192.168.2.0

Router(config-router)# network 192.168.3.0

C. Switch1# vlan database

Switch1(vlan)# vtp v2-mode

Switch1(vlan)# vtp domain cisco

Switch1(vlan)# vtp server

D. Switch1(config)# interface fastethernet 0/1

Switch1(config-if)# switchport mode trunk

Switch1(config-if)# switchport trunk encapsulation isl

 Answers: A

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Refer to the exhibit. Hosts on the same VLAN can communicate with each other but are unable to communicate with hosts on different VLANs. What is needed to allow communication between VLANs?

A. a switch with a trunk link that is configured between the switches

B. a router with an IP address on the physical interface that is connected to the switch

C. a switch with an access link that is configured between the switches

D. a router with subinterfaces configured on the physical interface that is connected to the switch

 Answer: D

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The show interfaces serial 0/0 command resulted in the output shown in the graphic. What are possible causes for this interface status? (Choose three)

A. The interface is shut down.

B. No keepalive messages are received.

C. The clockrate is not set.

D. No loopback address is set.

E. No cable is attached to the interface.

F. There is a mismatch in the encapsulation type.

 Answer: B C F

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While troubleshooting a connectivity issue from a PC you obtain the following information:Local PC IP address: 10.0.0.35/24Default Gateway: 10.0.0.1

Remote Sever: 10.5.75.250/24

You then conduct the following tests from the local PC:

Ping 127.0.0.1 - Successful

Ping 10.0.0.35 - Successful

Ping 10.0.0.1 - Unsuccessful

Ping 10.5.75.250 - Unsuccessful

What is the underlying cause of this problem?

A. A remote physical layer problem exists.

B. The host NIC is not functioning.

C. TCP/IP has not been correctly installed on the host.

D. A local physical layer problem exists.

 Answer: D

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