Snabay Networking

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INTER-SWITCH LINK VLAN TAGGING

The article written here describes the inter-switch link (ISL) VLAN (Virtual Local Area Network) tagging and its uses in detail. For the functioning of the frame tagging in VLAN (Virtual Local Area Network) we use a proprietary protocol. One such protocol is named Inter-Switch Link (ISL). Inter-Switch Link protocol can only be used with Cisco switches because they are a proprietary protocol. The Inter-Switch Link (ISL) has ISL (Inter-Switch Link) header and ISL (Inter-Switch Link) trailer. The ISL (Inter-Switch Link) works by combining or wrapping the Ethernet frame in the ISL (Inter-Switch Link) header and the ISL (Inter-Switch Link) trailer. The ISL (Inter-Switch Link) header contains the VLAN (Virtual Local Area Network) id. The wrapped Ethernet frames will not change, they remain constant.

The frame before wrapping contains destination MAC, source MAC, length/type, data; FCS after wrapping with the ISL header the frame contains the ISL header, destination MAC, source MAC, length/type, data, FCS and ISL FCS. About 1000 VLANS are supported by this ISL. This ISL (Inter-Switch Link) can only be used in the type like Fast Ethernet type and the Gigabyte Ethernet links. In the ISL header it contains the header part is 24 byte and the Frame Check Sequence (FCS) is 4 byte, the overall byte is 30 byte contains in the ISL header section. Mainly ISL (Inter-Switch Link) are function occurs in the OSI (Open Systems Interconnection) model. In the OSI Open Systems Interconnection) model data link layer is the layer used for ISL (Inter-Switch Link) functioning. ISL manages the VLAN (Virtual Local Area Network) information’s in the frames used.

Inter-Switch Link Protocol (ISL) fields are described briefly in the below part, they are:

Destination Address (DA) - multicast MAC addresses are used in the Destination address. 40 bits of fields are available in the destination address field. The packets send the signal in the ISL format.

Type- the type of field describes which type of frame is used to the combination. Examples provided are Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).

User- four-bit codes are used in the user field. This field is used to expand the type of field available. 0000 is the default user value.

Source Address (SA) - this field contains the address of the source and is used in ISL (Inter-Switch Link).

Len- packet length is described in this field.

HSA- High bits of Source Address is a 24-bit field which represents the upper 3 layers.

VLAN (Destination VLAN id)-VLAN (Virtual Local Area Network) contains the VLAN id of the packet. It contains a 15-bit field and this field is used to identify different VLANs (Virtual Local Area Network) used.

BPDU- which frame is used is identified.

Index- it contains the source address index number.

Res- For the additional information, to store it the field used is the reserved field.

ISL CRC- to check that whether the information’s in the packets are corrected or any error occurs, for knowing it such fields are used. It is a 4-byte field.

Read More: https://snabaynetworking.com/

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VLAN Trunking Protocol (VTP) Advertisement Messages

VLAN Trunking Protocol (VTP) Advertisement Messages

This article is going to discuss about VLAN Trunking Protocol (VTP) Advertisement Messages. In the below part, a brief introduction of all these topics discussed before sections are written. And the main portions are discussed and explained in detail.

VLAN is the abbreviated form of Virtual Local Area Network. It will wrap together with the devices in the local area and starts the communication. Mainly these are used in large companies to control the flow of data means to control the traffic. Three different types of Virtual Local Area Network (VLAN) are used based on their functions. The use of VLAN (Virtual Local Area Network) will reduce the work of network administrators also.

 VTP is the expanded form of the VLAN Trunking Protocol. VTP carries VLAN information from the switches. It is one of the Cisco Protocol. In most of the Cisco products, this protocol is used. Different versions of VTP are introduced. They are version 1, version 2, and version 3. To communicate VLAN information with VTP some of the requirements are required. VTP versions need to be the same as the user. The VTP domain name must be the same as the switches used. Authentication needs to be matched. One of the switch used must be a server.

 When the VTP (VLAN Trunking Protocol) gets configured with the switches the switch will start to exchange the information about the VTP (VLAN Trunking Protocol) to different trunk ports. The VTP (VLAN Trunking Protocol) domains manage the different information. Ethernet Frames are used to send the VTP messages. The different devices located near will receive the Ethernet Frames.

There were three different types of VTP (VLAN Trunking Protocol) advertisement messages, they are as follows:

Client     advertisement Request

Summary     advertisement

Subset     advertisement

 Client advertisement Request: - From the name itself it defines that the message that is generated from the client-side for VLAN (Virtual Local Area Network) information to a server. With both the summary and subset advertisement the server responds.

 Summary advertisement: - The message defines here is the summarized message of VLAN (Virtual Local Area Network) information by default it will provide summarized messages each 5 min, or if any changes occur in the configuration.

 Subset advertisement: - it provides specific information about the VLAN (Virtual Local Area Network). About each VLAN (Virtual Local Area Network) the details were provided. If any changes happen in the server switch the subset advertisement used.

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Spanning Tree Path Cost Value and How is Spanning Tree Cost Value Calculated

As we learn in early topics, a value called Path Coast Value is used for the purpose of identifying the Root Port. This identification of the Root Port is for transmitting the data. For this, we have to identify the root port in every switch other than the root switch. If there is more than one ports are present in the switch, it is necessary to find the best port from the all available ports. This is done by calculating the value called Path Coast Value. The best path from the port to the destination device is selected on the basis of the value. The path which has the minimum cost will be selected and known as the Root Port.

The bandwidth and the Spanning Tree Path Cost Value are inversely proportional. That is, if a path has having high bandwidth will have the minimum coast value and it is more preferable than any other. Like, the path having the lowest bandwidth will have a large cost value and therefore, it is less preferable.

The following are the most value for the path with their corresponding bandwidth.

Link Speed: 10 Gbps  - Cost Value: 2

Link Speed: 1 Gbps  - Cost Value: 4

Link Speed: 100 Mbps  - Cost Value: 19

Link Speed: 10 Mbps  - Cost Value: 100

We can explain the concept of cost value by the following  diagram.

Consider that switch 4 wants to transmit the packet to the Root Switch, switch 1, then there two paths occur. Either the packet can be transmitted through the switch 2 or through the switch 3. If we opted for the first path, the Path Cost Value of the Spanning Tree will be 19 + 4 = 23. If we choose the second path for transmission, then the Path Cost Value of the Spanning Tree will be 4 + 4 = 8. Therefore, we choose the second path. Because it has the lowest cost value.

Then the packet from switch 4 to switch 1 will be transmitted through the switch 3. In this example, the root port in the switch 4 will be the port which is connected to the switch 3.

Finally, we can define the Spanning three path cost value as it is the sum of ports cost value from the switch which transmits the data to the Root Switch. When a BPDU reaches a switch, its path cost value will be added to the cost of the incoming port. Read More: https://snabaynetworking.com/

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Introduction to link-state routing protocol

The complete routing path of a network in a router that runs the protocol called link-state routing protocol can be maintained by the link-state routing protocol. The information about the router is created by each of the router that runs this protocol. This originated message is then sent to all of the routers in that particular network. This is done as multicast messages. The protocol, Link-state routing protocol will always keep the complete network topology. This is done by updating the network topology by itself always when there occur any changes.    

           The routers that are present in a network will always keep a copy of the topology. And it should be kept without any modification to it. All the router will calculate the best path to the destination from the router by itself. This calculation is processed only after it gets the complete topology of the network.

           To find the best path from the source to destination, the link-state protocols are work based on the algorithm named SPF algorithm(Shortest Path First). This SPF algorithm is also named as Dijkstra Algorithm. The reason behind this name is that the Dijkstra's are used for conceptualization.

           If the state of any link changes, the routing update will call in the case of an SPF(Shortest Path First) algorithm. And then the LSAs are exchanged between the routers, Where, LSA Stands for Link-State Advertisement. The all the routers that receive this LSA routing update will again calculate the smallest path to the destination from there. The Link-Sate algorithm is used for this recalculation purpose. And Each of these networks will again make the new complete network structure.

eg: OSPF

Some of the important terms that we can see in this topic are explained below:

SPF Algorithm: As already explained above its full name is the Shortest Path First Algorithm, another name is Dijkstra's algorithm. This algorithm performs operations over the database and this calculation will create a tree called SPF tree.

Routing tables: The routing table is a table that contains all the interfaces and the paths for transmission that are known.

Topological Database: Like all other databases it also keeps the collection of information where these data are collected from the Link-State Advertisements(LSAs).

Link-Sate Advertisement(LSA):  This is a packet of information that is small and is transferred between the routers.

Characteristics:

Some of the major characteristics of Link State Routing Protocol are below:

•         This protocol converges more rapidly than others.

•         There is only less chance to occur routing loops as compared to the Distance vector routing protocol.

•         Uses More Memory and CPU power.

•         This protocol uses a hierarchical structure. The advantage of using this structure is that it controls the distance traveled by the LSA.

•         To share routing information Link State Routing Protocol uses the feature called, multicast.

•         The updates can only be processed by the router which runs the Link State Routing Protocol.

•         Only if there any changes occur in the state of the network, it will send the update. And these updates are known as incremental updates.

•         This Protocol is highly expensive to support than others.

Link state algorithms are also difficult to implement. 

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Root Guard, BPDU Guard, and BPDU Filter

Root Guard

Root Guard is a feature that is used during network anomalies for avoiding Layer 2 loops. For preventing the switches at the surrounding of the port from becoming root switches, the Root guard feature will compel the interface to change to a designated port. In a network, this feature will provide a way to enforce the root bridge placement. This feature will not allow the Designation Port to become a Root Port. If the Root Guard features receive a superior BPDU on port, it changes the port into root-inconsistent state (which is a listening state), a trough that the Root Bridge status is maintained.

Other than STP enhancement (enabled on a global basis), Root Guard is enabled on every port (that don’t want Root Bridge) manually. Due to this, it is important to make sure to have a deterministic topology when there is a plan to design and implement STP in the LAN. The switch won’t allow the port to become an STP port after the Root Guard is enabled on that port. If a port enabled with Root Guard receives a better BPDU, other than processing the BPDU the Root Guard disables the port. Is an unauthorized user starts to send BPDU which has a good ID, the STP process will select a new switch to become a root switch, and when the port is disabled the network topology will get protected.    

BPDU Guard

BPDU Guard is also a feature that is used on an access port that is configured with Port Fast. When a port that is enabled with BPDU Guard and received a BPDU from the device that is connected to it, then BPDU will disable the port and also the state of the port will be changed to Err disable state.

This feature can also be globally enabled at Global configuration mode or by interface at Interface configuration mode.

BPUD Filter

BPUD filter is also a feature that is implemented on an access port that is configured to Port Fast. BPUD filter feature not allow you to generate BPUDs on the access port which is configured with the Port Fast.

BPUD filter also can be globally enabled at Global configuration mode or by interface at Interface configuration mode.

When it is configured as a Global level or Interface configuration mode, this feature will act in two different forms. When the BPDU Filter feature is enabled with the Global level, then all ports will be applied with BPDU Filter that is enabled by Spanning Tree Protocol (STP) Port Fast. The Port Fast feature will be disabled when a BPDU is received on that port and that port will become as like an STP port.

BPDU Filter will not send BPDUs when the BPDU filter is enabled on the interface level and it will avoid the processing of the received BPDU. Spanning Tree Protocol on that interface will get completely disabled due to this behavior. If the switches are connected to the ports that are enabled by the BPDU filter accidentally, it will cause damage to the network by forming the Layer 2 switching loop. Read More: https://snabaynetworking.com/