Frame format IEEE 802.1Q

insertion of 802.1q tag in ethernet frame

802.1q adds 32-bit field between source mac address , ethertype fields of original frame. minimum frame size left unchanged @ 64 bytes. maximum frame size extended 1,518 bytes 1,522 bytes. 2 bytes used tag protocol identifier (tpid), other 2 bytes tag control information (tci). tci field further divided pcp, dei, , vid.

tag protocol identifier (tpid)
a 16-bit field set value of 0x8100 in order identify frame ieee 802.1q-tagged frame. field located @ same position ethertype field in untagged frames, , used distinguish frame untagged frames.
tag control information (tci)
a 16-bit field containing following sub-fields:

priority code point (pcp)
a 3-bit field refers ieee 802.1p class of service , maps frame priority level. different pcp values can used prioritize different classes of traffic.
drop eligible indicator (dei)
a 1-bit field. (formerly cfi) may used separately or in conjunction pcp indicate frames eligible dropped in presence of congestion.
vlan identifier (vid)
a 12-bit field specifying vlan frame belongs. hexadecimal values of 0x000 , 0xfff reserved. other values may used vlan identifiers, allowing 4,094 vlans. reserved value 0x000 indicates frame not carry vlan id; in case, 802.1q tag specifies priority (in pcp , dei fields) , referred priority tag. on bridges, vid 0x001 (the default vlan id) reserved network management vlan; vendor-specific. vid value 0xfff reserved implementation use; must not configured or transmitted. 0xfff can used indicate wildcard match in management operations or filtering database entries.

for frames using ieee 802.2/snap encapsulation organizationally unique identifier (oui) field of 00-00-00 (so protocol id field in snap header ethertype), case on lans other ethernet, ethertype value in snap header set 0x8100 , aforementioned 4 bytes appended after snap header.

because inserting vlan tag changes frame, 802.1q encapsulation forces recalculation of original frame check sequence field in ethernet trailer.

the ieee 802.3ac standard increased maximum ethernet frame size 1518 bytes 1522 bytes accommodate four-byte vlan tag. network devices not support larger frame size process these frames successfully, may report them baby giant anomalies.

double tagging

with ieee standard 802.1ad, double-tagging can useful internet service providers, allowing them use vlans internally while mixing traffic clients vlan-tagged. outer (next source mac , representing isp vlan) s-tag (service tag) comes first, followed inner c-tag (customer tag). in such cases, 802.1ad specifies tpid of 0x88a8 service-provider outer s-tag.

insertion of 802.1ad double tag in ethernet frame

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