question about vlan tagging

[Dr.X]

hi ,

i want to ask a question about the tagging in switcing .

assume i have switch and f0/1 is connected to pc   , f0/2 is connected to another switch

f0/1 is access with vlan 10

f0/2 is trunk

i want to ask , when will the frames of f0/1 be tagged with vlan 10

is it tagged as in entered the interface f0/1 ???

or

it is tagged when it leave the trunk interface to another switch ??

=======================================================

aother question :

now we have two identification methods

isl

or

.1q

i want to ask , how we can control the access port of switch to choose either isl or dot1q ??

as i know , i can control this method to the trunk ports ?

regards

[Reza Sharifi]

It is tagged on its way out to the other switch (in your case f0/2).

BTW, if you just have one vlan, you don't need to trunk the uplink.

HTH

[Dr.X]

so , the encapsulation is done on the trunk lins not ont he access links ??

lets talk about the encapsulation , i mean assume i have vlan 20  on f0/4  and incomaing frame from the trunk is tagged with vlan 20

when the switch remove the tag ,

as it reach the trunk

or

as it reach the access link vefore reaching the destination

regards

[mahmoodmkl]

It removes while forwarding to the access port

Sent from Cisco Technical Support iPhone App

[Dr.X]

hi , thanks alot .

does all the times  the frames are encapsulated ???

i mean that in ISL  it differs than dot1q  , not always it encapsualte the frames ,

i mean that isl sometimes dont encapsulate the frames ... when that occurs ??

i dont  mean here the native vlan .

regards

[Reza Sharifi]

A trunk is a point-to-point link between the device and another  networking device. Trunks carry the traffic of multiple VLANs over a  single link and allow you to extend VLANs across an entire network.

To correctly deliver the traffic on a trunk port with several VLANs, the  device uses the IEEE 802.1Q encapsulation (tagging) method that uses a  tag that is inserted into the frame header. This tag carries information  about the specific VLAN to which the frame and packet belong. This  method allows packets that are encapsulated for several different VLANs  to traverse the same port and maintain traffic separation between the  VLANs. The encapsulated VLAN tag also allows the trunk to move traffic  end-to-end through the network on the same VLAN.

have a look:

http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli/AccessTrunk.html#wp1206629

HTH

[jimmysands73_2]

Let me try a different approach...

The vlan 10 question, as it exits the PC and enters the switch, I believe the sw then associates that ports mac address (actuallly the MAC recv'd on that port) with vlan 10, such as this sh mac-address table dy:

*  111  0012.f234.1acf   dynamic  Yes   --  Gi3/3

*  119  9084.0dcc.ddc8   dynamic  Yes   --  Gi2/5

*  119  d49a.20ad.c424   dynamic  Yes   --  Gi2/5

*   10  f04d.a236.8255   dynamic  Yes   --  Fa5/23

*  111  0012.f234.1acf   dynamic  Yes   --  Gi3/3
*  119  9084.0dcc.ddc8   dynamic  Yes   --  Gi2/5
*  119  d49a.20ad.c424   dynamic  Yes   --  Gi2/5
*   10  f04d.a236.8255   dynamic  Yes   --  Fa5/23

These packets arent tagged per se on this sw, but if they went over a trunk link, they would need tagged.

If the packet is destined for a local interface on the same sw, no tagging takes place (but since the mac address recv'd on  a port, in your case fa0/1, it associates that MAC address via the mac address table with vlan 10 since all untagged packets entering fa0/1 would be in vlan10).  If the packets goes out a trunk link (Inot throwing any native statements in right now) and both sides are trunked, then a tag is placed as it exits the trunk from sw1 and when it enter sw2 it removes the tag, and from the tag knows that that mac address is associated with vlan10.  This is what I believe to be true

For the isl/dot1q, I believe that is only used on trunk ports, since most are dot1q, but you can see, and must know a tad ity bity bit about isl for ccna/np exams.

Message was edited by: Jimmy Sands