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Posted by on December 18, 2008, 6:45 am
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the books i've read it's explained horribly incomplete. is it the management vlan ? kind regards, bm | ||||||||||||||||||||||
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Posted by Sam Wilson on December 18, 2008, 7:21 am
Please log in for more thread options <http://www.avaya.com/master-usa/en-us/resource/assets/applicationnotes/v lan-tutorial.pdf> has a very understandable explanation and, starting at
slide 29, the most succinct description of VLANs that I've come across.
The term "native VLAN" is most usually the same as port VLAN id or PVID. > is it the management vlan ?
Not generally. Some early kit (not Cisco, I think) used to have the management on what it thought of as VLAN 1, which was generally the native vlan, but on all the kit we deal with these days the management VLAN can be configured. Sam | ||||||||||||||||||||||
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Posted by bod43 on December 18, 2008, 8:59 am
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> =A0alefv...@versatel.nl wrote: > > Can anyone maybe explain to me the concept of native vlans ? In all
> > the books i've read it's explained horribly incomplete. >
=A0
> <http://www.avaya.com/master-usa/en-us/resource/assets/applicationnotes/v > lan-tutorial.pdf> has a very understandable explanation and, starting at > slide 29, the most succinct description of VLANs that I've come across. = > The term "native VLAN" is most usually the same as port VLAN id or PVID.
> > > is it the management vlan ?
>
> Not generally. =A0Some early kit (not Cisco, I think) used to have the > management on what it thought of as VLAN 1, which was generally the > native vlan, but on all the kit we deal with these days the management > VLAN can be configured. I agree. If you understand 802.1Q VLAN tagging then the native VLAN is easy. It is the VLAN on an 802.1Q trunk which uses the null tag. No 802.1Q header is applied to native VLAN traffic on a trunk. The VLAN to which a particular frame 'belongs' is identified in the receiving switch by the absence of the 802.1Q header. For all other configured VLANS the VLAN to which a particular frame 'belongs' is identified by the contents of the 802.1Q header. There is no native VLAN with ISL since all packets are tagged (encapsulated - may be the preferred ISL term.) Have you looked at any cisco documents? www.cisco.com. Most things are described pretty clearly now. A lot of technology books are shoddily written (I presume partly due to time to market considerations) but there are good ones around too. I have not actually read this (since I already knew how the stuff worked before it was published - just too slow Rich:) however the author has been posting on comp.dcom.lans.ethernet for decades and his writing there has been truely marvelous. He *wrote* the IEEE 802.3 Ethernet standard so knows his stuff. I would be pretty surprised if it was not very good. The switch Book - Rich Seifert (Author) http://www.amazon.com/Switch-Book-Complete-Switching-Technology/dp/04713458= 65 Read the reviews there. They are *really* positive. Note that it is not Cisco specific so no CDP, cisco commands, etc - I assume. When you get to routing, get TCP/IP Routing - Jeff Doyle. On Second Edition now. The "entry level" books just add confusion by missing stuff out and are often poorly written too. I have several:( | ||||||||||||||||||||||
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Posted by Sam Wilson on December 18, 2008, 12:05 pm
Please log in for more thread options In article
> >
> > alefv...@versatel.nl wrote: > > > Can anyone maybe explain to me the concept of native vlans ? In all
> > > the books i've read it's explained horribly incomplete. > >
> > <http://www.avaya.com/master-usa/en-us/resource/assets/applicationnotes/v > > lan-tutorial.pdf> has a very understandable explanation and, starting at > > slide 29, the most succinct description of VLANs that I've come across. > > The term "native VLAN" is most usually the same as port VLAN id or PVID. > > > [snip]
> > If you understand 802.1Q VLAN tagging then > the native VLAN is easy. > > It is the VLAN on an 802.1Q trunk which > uses the null tag. No 802.1Q header > is applied to native VLAN traffic on > a trunk. > > The VLAN to which a particular frame 'belongs' > is identified in the receiving switch by the absence > of the 802.1Q header. For all other configured VLANS > the VLAN to which a particular frame 'belongs' > is identified by the contents of the 802.1Q > header. Nitpick: According to the Avaya presentation linked above then a link with untagged frames is NOT a trunk port, it is a hybrid port. A trunk port in 802.1Q parlance has all frames tagged (it says, and I feel I should bow to the author's greater knowledge of IEEE standards). I hestitate to go into too much more detail, but I do recommend the presentation above *again*. :-) > There is no native VLAN with ISL since
> all packets are tagged (encapsulated > - may be the preferred ISL term.) It is the preferred ISL term because it's actually a different technique. 802.1Q adds 4 octets into the frame header, the tag, but it retains its original source and destination MAC addresses. ISL adds a new MAC header with different, Cisco-specific MAC addresses, to the front of the whole frame, encapsulating the original frame. | ||||||||||||||||||||||
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Posted by Uli Link on December 18, 2008, 10:05 am
Please log in for more thread options alefveld@versatel.nl schrieb:
> Can anyone maybe explain to me the concept of native vlans ? In all
> the books i've read it's explained horribly incomplete. > > is it the management vlan ? It is often used for management protocols, but the management VLAN is something different. The native VLAN has to seen from a single switchport's point of view: Technically general: When the switch forwards a packet from the switching engine to a port with the matching VLAN configured as native, the dot1q header is removed (untagged). Vice versa an untagged packet coming into this port will be tagged with the native VLAN internally. Any switchport can belong to many VLANs tagged, but only one VLAN untagged (with mostly weird exceptions). native <==> "untagged" as a good rule of thumb.
SNMP, RADIUS, SSH to and from the switch.
management <==> source and destination for management protocols like Traditional defaults: If you don't explicitly configure what VLAN is used as native, VLAN 1 is used for untagged packets. For a trunk with all VLANs tagged there is no native VLAN, or you call VLAN 1 native. This is a common practice. Simple rule: 1.) configuring weird but possible things without wholly understanding things can lead to unpredictable behavoiur. 2.) Keep it as simple as possible. 3.) Some dynamic configuration protocols (CDP for e.g.) rely on using the "native" VLAN. So if this is not the same on both sides (which is possible and sometimes needed!). This may the reason why native and management are tied together in some way. But they are different things. -- Uli Link | ||||||||||||||||||||||
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> the books i've read it's explained horribly incomplete.