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Intervlan switching on access link

amit.vashishtha
Level 1
Level 1

Hi ,

Just wanted to know how Vlan tagged frames flow on the access link .

For example : I have 2 switches A and B which are connected by a link configured as access vlan 5 on switch A and access Vlan 8 on switch B . Switch A has 2 hosts , 1 in Vlan 5 and other in Vlan 8 and same in switch B . Will Host in Vlan 5 on switch A communicate with host on Vlan 5 in switch B .

The incoming frames from switch A will be checked for Vlan tagging on the port itself and dropped or will the switch B process the frames on basis of Vlan database and then return frames will be dropped .

Regards

Amit

6 Replies 6

Jon Marshall
Hall of Fame
Hall of Fame

amit.vashishtha wrote:

Hi ,

Just wanted to know how Vlan tagged frames flow on the access link .

For example : I have 2 switches A and B which are connected by a link configured as access vlan 5 on switch A and access Vlan 8 on switch B . Switch A has 2 hosts , 1 in Vlan 5 and other in Vlan 8 and same in switch B . Will Host in Vlan 5 on switch A communicate with host on Vlan 5 in switch B .

The incoming frames from switch A will be checked for Vlan tagging on the port itself and dropped or will the switch B process the frames on basis of Vlan database and then return frames will be dropped .

Regards

Amit

Amit

If the link is an access link then the frames will not be tagged at all. Frames are only tagged on 802.1q trunk links and not access port links.

What you have done is "join" 2 vlans together because there is no tag in the frames each switch simply assumes the vlan id is the one configured on the port ie. switchport access vlan xxx.

This is not a recommended thing to do.

Jon

Jon ,

A lot of websites say that frames are tagged on 802.1q access links but not on ISL access links .

Amit

Amit

Are we talking about the same thing.

When we refer to an access link i am assuming this means that each port on either side of the link between the switches is configured to be in only one vlan eg.

int gi0/1

switchport mode access

switchport access vlan

is this what you mean by access links ?

Can you point me to some of the websites, i'm always happy to be proved wrong as it helps me learn but as far as i know only a link configured as an 802.1q tags the frames.

Jon

Hi John ,

Yes , I am referring to the same config . So does that mean that frames will not be tagged and will not contain Vlan Identifiers et all . So how does communication happen between the hosts on 2 switches . Thanks to elaborate .

Amit

amit.vashishtha wrote:

Hi John ,

Yes , I am referring to the same config . So does that mean that frames will not be tagged and will not contain Vlan Identifiers et all . So how does communication happen between the hosts on 2 switches . Thanks to elaborate .

Amit

Amit

H1 (vlan 5)  -> SW1 (vlan 5) -> (vlan 5) SW2 -> (vlan 5) H2

H1 sends a packet to H2. They are in the same subnet and the switches are connected via an access link in vlan 5.  You don't need vlan identifiers within the frame because the frame leaves SW1 in vlan 5 and arrives at SW2 in vlan 5 and is then forwarded to H2 in vlan 5.

H1 (vlan 5)  -> SW1 (vlan 5) -> (vlan 8) SW2 ->  (vlan 8) H2

If H1 and H2 use the same IP subnet this would still work but as i say you have joined two vlans together which is something that should be avoided.

Jon

Hi Jon ,

Thanks for the explanation .

Regards

Amit

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