How is multi-VLAN frame tagged in ISL?
johnifanx98
Member Posts: 329
in CCNA & CCENT
An ISL header is added for each VLAN, or, only one ISL header is added, but the VLAN field incorporates all VLAN ids? Both schemes sound odd to me...
Comments
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Roguetadhg
Member Posts: 2,489 ■■■■■■■■□□
ISL encapsulates the existing ethernet frame. Within the encapsulation, they have a VLAN ID which is set to whichever the packet belongs with.
I'm not sure where you're seeing the vlan field has all the VLAN ids.In order to succeed, your desire for success should be greater than your fear of failure.
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johnifanx98
Member Posts: 329
A frame can be tagged with multiple VLAN in ISL. I just don't know how this is implemented...
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networker050184
Mod Posts: 11,962 Mod
An ISL header has one field that holds a single VLAN ID. So you would need multiple headers for multiple VLANs. To put multiple tags you would need something like QinQ but I'm not sure if there is an equivalent feature for ISL.An expert is a man who has made all the mistakes which can be made. -
SubnetZero
Member Posts: 124
I know 802.1q frames can be double tagged (QinQ) but this is completely different than double tagging with ISL.
With QinQ you have an outer tag and an inner tag. The outer tag is used to identify and segregate traffic from different customers (since this is mainly used in service provider networks), and the inner tag is preserved from the original frame.
On the other hand ISL adds a 26-byte header and a 4-byte trailer to the frame which some refer to as ISL double tagging.
They are totally different.
If there is a way to do this with ISL I'm not sure what it is...
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