Inline:
On 9/21/2011 2:58 PM, [email protected] (Jason Boyers) wrote:
Thank you for the clarification. In looking at various documents, there is
a lot of confusion. From what you are stating:
Access Port - sent on the VLAN for which interface is configured
Well, if it's an access port (untagged), then there *is* no VLAN (not
from the perspective of the AP at any rate.) So the AP just sends the
CDP packet out untagged (as it sends *all* packets), in that case.
A *switch* has a notion of what VLAN if any is configured on an access
port, but an *AP* does not.
Trunk Port - sent on VLAN 1, whether or not VLAN 1 is tagged and whether or
not VLAN 1 is allowed and in a spanning-tree forwarding state for that port
Well, an AP doesn't have the notion of "allowed" VLANs. The VLANs (i.e.
subinterfaces of the LAN interface) are either configured or not.
But so anyway - the AP *always* sends CDP in VLAN1, *if* its LAN port is
configured for VLANs.
(Here I am not sure about what if VLAN1 is in a spanning-tree blocked
state - I would assume then that we would *not* send CDP, but would not
wager cash on that point.)
Is that another way of putting it? That is different than my understanding
has been (where CDP is sent untagged on an access or trunk port - period.)
Yep, the notion that CDP is always sent untagged is quite incorrect. (A
notion that is widely held within Cisco as well, in fact by many
developers :)
I would like here to post a reference to the CDP spec but unfortunately
it is confidential. As I reread it for the n'th time, I can now see
that there are two alternate possible interpretations:
One is that, for an 802.1q encapsulated link, it should always be sent
with tagged in VLAN 1, and the other is that, for a link that has both
tagged and untagged frames, it should be sent untagged.
Unfortunately, different implementations have adopted different
interpretations. The AP's interpretation is the VLAN 1 one.
I just did a packet capture on an interface connected to a WLC. That
interface only allows specified VLANs (which don't include VLAN 1) and a
separate native VLAN (which is 999 in this case, which doesn't even exist as
a VLAN on the switch.) In the packet capture, CDP was tagged with VLAN 999
when coming from the WLC. Everything else was tagged with the Management
VLAN (no clients currently on the WLC.)
Well, I was speaking specifically about (WNBU) IOS, *not* about the WLC.
With the WLC, all bets are off.
I don't quite get your scenario here. You say that your native VLAN is
999, and that you see CDP tagged with VLAN 999 coming from the WLC.
Now, on the WLC, you configure a "native" (i.e. *untagged*) VLAN as 0
... so you're saying that you have some interface configured on the WLC
as tagged VLAN 999? Some interface other than the management interface?
I'm skeptical of this ... its sounds more like maybe the WLC just
transmits CDP as untagged.
I appreciate your help in working through this, both for understanding as
well as for proper documentation on Cisco's site.
It sounds like what I *really* need to do is to drive some consensus at
Cisco on this point ... although higher priority (of course) is to study
for my imminent CCIE lab ...
Cheers,
Aaron
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