350-501 · Question #56
How much must the MTU be increased when configuring the 802.1q VLAN tag?
The correct answer is B. 4 bytes. When configuring 802.1q VLAN tagging, the MTU must be increased by 4 bytes to accommodate the Tag Protocol Identifier (TPID) and Tag Control Information (TCI) fields added to the Ethernet frame.
Question
How much must the MTU be increased when configuring the 802.1q VLAN tag?
Options
- A2 bytes
- B4 bytes
- C8 bytes
- D12 bytes
How the community answered
(44 responses)- B93% (41)
- C5% (2)
- D2% (1)
Why each option
When configuring 802.1q VLAN tagging, the MTU must be increased by 4 bytes to accommodate the Tag Protocol Identifier (TPID) and Tag Control Information (TCI) fields added to the Ethernet frame.
A 2-byte increase would only account for either the TPID or TCI, not the full 802.1Q tag.
The IEEE 802.1Q VLAN tag inserts a 4-byte field into the Ethernet frame header between the source MAC address and the EtherType/Length field. This 4-byte tag includes a 2-byte Tag Protocol Identifier (TPID) and a 2-byte Tag Control Information (TCI) field, increasing the overall frame size by 4 bytes. Therefore, the MTU on the interface must be increased by 4 bytes to prevent fragmentation when transmitting tagged frames.
An 8-byte increase is more than double the required size for a standard 802.1Q tag.
A 12-byte increase is significantly more than the size of a standard 802.1Q tag.
Concept tested: 802.1Q VLAN tag size
Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/multiboot/configuration_guide/b_multiboot_3se_cg/b_multiboot_3se_cg_chapter_0110.html
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