Re: native vlan question

Bod43@xxxxxxxxxxxxx wrote:
On 16 Apr, 05:31, aaabb...@xxxxxxxxxxx wrote:
On 4月15日, 下午1时13分, "stephen" <stephen_h...@xxxxxxxxxxxx> wrote:





<aaabb...@xxxxxxxxxxx> wrote in message
news:123637df-5ec8-499a-a32c-cef68e1e21b4@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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?
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?-

I have read something somewhere that stated that the Tags are
stripped off when frames enter the switch however clearly equivalent
information is carried 'with' the frame 'through' the switch.

So as far as we the Users are concerned I think that
you would have a good way of thinking about it if
you assumed that such a tag was used.



When you configure a port-based VLAN on a switch, you are telling the
switch which ports are to be grouped into a common broadcast domain
(VLAN). Therefore when a switch receives a tagged frame on a trunk port,
it knows which ports it is permitted to forward too based on the
separation of broadcast domains (VLANs). Of course, if the switch has
learned the port that the destination host resides on, it will only
forward it out that port within the appropriate VLAN (assuming a unicast
packet). Broadcast packets may be flooded to all ports within a VLAN.

Best Regards,
News Reader
.

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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