Re: native vlan question

<aaabbb16@xxxxxxxxxxx> wrote in message
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On 4ÔÂ15ÈÕ, ÏÂÎç1ʱ13·Ö, "stephen" <stephen_h...@xxxxxxxxxxxx> wrote:
<aaabb...@xxxxxxxxxxx> wrote in message

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A lot of people ask native vlan question.
I think using "vlan dot1q tag native" should eliminate this question.
at least for sw---sw connection. (all tagged just like isl)
I may not fully understand what's purpose why cisco make "native
vlan".

cisco didnt invent this - it is part of 802.1Q.

Some article say because of backward compatible with 802.3.

i suggest you try to find the standard and read around that,
maybe start atwww.ieee.org

AFAIR some of the standards docs are without charge for Ethernet.

Can anyone give an example?

there are 2 Qs to think about.

1. set a port to be tagged
- what do you do with a packet that arrives with no tag?

the 2 common answers are to throw it away, or to put it into some sort of
"default VLAN" - that is what untagged means for incoming packets.

not putting a tag on outbound packets form that VLAN on that port allows 2
way comms.

this sounds silly - but it is what often needs to happen when you hook up
an
unconfigured device to set it up.

2. what happens when you want to split up 2 streams of packets on a port?

sometimes you have a device that will add its own stream of packets to a
set
it gets from elsewhere
- the classic case is an IP phone where there is a plug on the phone to
connect a PC.
- Pcs dont normally send tagged frames, and the 3 port bridge in the phone
doesnt have the horsepower to wrap a tag around every packet.
- but you want the phone traffic kept separate from PC (security, QoS and
so on).

So - pass the PC packet thru untagged, and tag the phone traffic.
At the switch the PC "stuff" is untagged and goes into the native VLAN,
phone traffic is tagged and goes into a different VLAN.

TIA,
st

--
Regards

stephen_h...@xxxxxxxxxxxx - replace xyz with ntl

Thanks,
For "Some article say because of backward compatible with 802.3"
they may think far end is Hub or switch/bridge which does not support
802.3,
right?

SPH - possibly, depends on where you found this.

One more question, when a access port receive a untag frame, does it
add
a 802.1q tag or some other tag to make sure it can go same vlan inside
of
this switch?

SH - the standard doesnt care.

802.x is all about what happens on network links between devices - what a
manufacturer chooses to do inside the box is private / implementation
dependent.

Now - on a cisco switch it any untagged frame is either thrown away or
associated with a VLAN (the VLAN is configured against the port, or you
inherit the default VLAN 1). Once "in" a VLAN it stays there through the
box, and may actually get wrapped in a tag once it goes onto a link to the
outside world.
--
Regards

stephen_hope@xxxxxxxxxxxx - replace xyz with ntl



.

Relevant Pages

  • Re: native vlan question
    ... I think using "vlan dot1q tag native" should eliminate this question. ... not putting a tag on outbound packets form that VLAN on that port allows 2 ... At the switch the PC "stuff" is untagged and goes into the native VLAN, ...
    (comp.dcom.sys.cisco)
  • Re: Linux, tcpdump and vlan
    ... stripping the tag early on RX and adding it late on TX. ... That means tcpdump would see packets with exactly one tag removed ... Tcpdump would need other means to get VLAN id... ...
    (Linux-Kernel)
  • Re: native vlan question
    ... I think using "vlan dot1q tag native" should eliminate this question. ... not putting a tag on outbound packets form that VLAN on that port allows 2 ... At the switch the PC "stuff" is untagged and goes into the native VLAN, ...
    (comp.dcom.sys.cisco)
  • Re: native vlan question
    ... I think using "vlan dot1q tag native" should eliminate this question. ... "default VLAN" - that is what untagged means for incoming packets. ... not putting a tag on outbound packets form that VLAN on that port allows 2 ...
    (comp.dcom.sys.cisco)
  • Re: native vlan question
    ... I think using "vlan dot1q tag native" should eliminate this question. ... "default VLAN" - that is what untagged means for incoming packets. ... not putting a tag on outbound packets form that VLAN on that port allows 2 ...
    (comp.dcom.sys.cisco)
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