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What are the advantages and disadvantages of ISL and dot1q protocols?

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VLANs, or virtual LANs, are a way of dividing a physical network into logical segments based on criteria such as function, department, or security level. VLANs can improve network performance, security, and management by reducing broadcast traffic, isolating sensitive data, and simplifying configuration changes. However, to enable communication between VLANs across different switches, a trunking protocol is needed. In this article, we will compare two common trunking protocols: ISL and dot1q.

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CJ O.

Expert blinkinlights engineer. Ninja electron wrangler. Photon motivator extradoinaire.

1 What is ISL?

ISL, or Inter-Switch Link, is a Cisco proprietary protocol that encapsulates the original Ethernet frame with a new header and trailer. The header contains a 10-bit VLAN ID that identifies the VLAN to which the frame belongs. The trailer contains a 4-byte cyclic redundancy check (CRC) that verifies the integrity of the frame. ISL adds 30 bytes of overhead to each frame, which can affect bandwidth and latency.

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CJ O.

Expert blinkinlights engineer. Ninja electron wrangler. Photon motivator extradoinaire.
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ISL has been deprecated by Cisco. It is obsolte. For context sake, there were some neat parts of ISL that made its use appealing 20 years ago. Particularly, you could run ISL between a router and a switch, and certain switches would understand enough information coming back from the router that they could cache the forwarding path and therefore only bother the router with the first packet of a stream. This was a pre-layer-3-switching technology, but this isn't even supported anymore.

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2 What is dot1q?

Dot1q, or IEEE 802.1Q, is an open standard protocol that inserts a 4-byte tag into the original Ethernet frame. The tag contains a 12-bit VLAN ID that identifies the VLAN to which the frame belongs. Dot1q does not add a new CRC, but recalculates the existing one. Dot1q adds only 4 bytes of overhead to each frame, which is less than ISL.

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3 What are the advantages of ISL?

ISL offers some advantages over dot1q, such as supporting more VLANs up to 1024, preserving the original frame format, and providing better error detection. The latter is due to the addition of a new CRC to the encapsulated frame. These benefits can be useful for troubleshooting or compatibility with some protocols.

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4 What are the disadvantages of ISL?

ISL has some drawbacks compared to dot1q, such as incompatibility with non-Cisco devices, increased bandwidth and CPU usage, and lack of support for native VLANs. This can limit interoperability and vendor choice, as well as add more overhead and require more processing. Additionally, native VLANs cannot be used for management or fallback purposes.

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5 What are the advantages of dot1q?

Dot1q has some advantages over ISL, such as compatibility with devices from different vendors, lower bandwidth consumption and CPU resource usage, and the ability to support native VLANs. This can increase flexibility, scalability, and simplify configuration while enhancing security.

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Lu Yu

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Dot1q divides BUM traffic into smaller pieces. In large networks, BUM traffic can consume a lot of bandwidth, making it ideal to eliminate it. However, VLANs can be useful in smaller networks, such as ENNI networks and offices. In an ENNI network, a new circuit can be provisioned by connecting a physical circuit or creating a new VLAN on an existing Ethernet circuit. In an office, using VLANs to separate teams is a good practice. For example, VLAN 20 can be used for accounting, and VLAN 30 for HR. PCs can be connected to different switches in different buildings, and switches can be stacked to share VLAN information.

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6 What are the disadvantages of dot1q?

Dot1q has some disadvantages compared to ISL, such as supporting fewer VLANs, up to 4096 as opposed to 1024 with ISL. Additionally, it modifies the original frame format, which can cause issues with certain protocols or applications. Furthermore, it provides weaker error detection since it does not add a new CRC to the tagged frame.

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What are the advantages and disadvantages of ISL and dot1q protocols?

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1 What is ISL?

2 What is dot1q?

3 What are the advantages of ISL?

4 What are the disadvantages of ISL?

5 What are the advantages of dot1q?

6 What are the disadvantages of dot1q?

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