Cisco CCNA CCNP BCMSN Exam Review Trunking And Trunking Protocols
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Cisco CCNA, CCNP, BCMSN Exam Review: Trunking and Trunking Protocols
Summary
To succeed in your CCNA and CCNP exams, mastering trunking and trunking protocols is essential. This guide, inspired by Chris Bryant, CCIE
12933, covers the vital details you need to know.
Key Topics
- Cisco
- CCNA
- CCNP
- BCMSN
- Trunking
- VLAN
- Native VLAN
- ISL
- IEEE 802.1Q
- Encapsulation
- Header
Understanding Trunking
Gaining a CCNA or CCNP certification demands a solid understanding of trunking fundamentals. This knowledge is crucial not only for the CCNA but also for the BCMSN exam, part of the CCNP certification. Before tackling advanced topics, ensure you have a strong grasp of the basics.
A trunk facilitates inter-VLAN traffic between directly connected switches. By default, a trunk port is part of all VLANs, allowing traffic?"including broadcasts?"to travel across it.
Switch port default modes can vary, so it's important to consult your documentation. On Cisco 2950 switches, ports default to dynamic desirable mode, actively seeking to establish trunks. Simply connect them with a crossover cable, and within seconds, the port light turns green, indicating a successful trunk connection. Use the `show interface trunk` command to verify trunk status.
VLAN Frame Tagging
How does a receiving switch determine the VLAN for a frame? The transmitting switch tags frames with a VLAN ID, identifying the VLAN whose member ports should receive the frame. Upon arrival, the remote switch examines this ID and forwards the frame accordingly.
Trunking Protocols: ISL and IEEE 802.1Q
Two major trunking protocols are essential to understand: ISL and IEEE 802.1Q.
ISL (Inter-Switch Link)
ISL is a Cisco-proprietary protocol, making it unsuitable for a multivendor environment. It encapsulates frames with both a header and a trailer, increasing the trunk line's overhead.
ISL does not recognize the concept of a native VLAN, meaning every frame is encapsulated. A 26-byte header, containing the VLAN ID, and a 4-byte trailer with a Cyclical Redundancy Check (CRC) value are added, which checks the frame's integrity. This encapsulation can lead to frames exceeding the maximum Ethernet frame size of 1518 bytes.
IEEE 802.1Q
In contrast, IEEE 802.1Q (dot1q) does not encapsulate frames. Instead, it adds a 4-byte header, resulting in less overhead compared to ISL. Frames for the native VLAN do not receive this header. Dot1q, therefore, reduces the risk of oversized frames. Untagged frames are understood by the switch to belong to the native VLAN.
Conclusion
Understanding these details can make the difference between passing and failing your CCNA and CCNP exams. Keep studying, practice hands-on, and you'll be on your way to achieving Cisco certification success!
You can find the original non-AI version of this article here: Cisco CCNA CCNP BCMSN Exam Review Trunking And Trunking Protocols.
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