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understanding native VLAN

Hi all,

Before I approach our carrier with this, I want to make sure I've got everything straight here...

The scenario is as follows. We have a 100Mb FE link to ConEdison Communications in NYC. This is what they call a "hubbed" connection (carryover from telco ds3 "hubbing" I guess). In this case it means that on that connection we can order multiple metro ethernet circuits to other locations and have them all appear as distinct VLANs. Pretty straightforward, right? Our "hub" connection should send/receive tagged ethernet frames. ConEd specifies what VLAN ID each remote ethernet circuit will have.

So we brought up our first circuit (to our office). This circuit is untagged. They told me it would appear on the other end as VLAN 3264. So I put something similar to this in the router at the "hub" end:

in fa5/0 no ip address duplex full

in fa5/0.3264 encapsulation dot1Q 3264 ip address 10.0.0.1 255.255.255.252

Then put my laptop at 10.0.0.2/30 and tried pinging 10.0.0.1. Link on both sides was up. No ping response, no arp entries on the router.

After some head-scratching I added "native" to the subinterface above and everything worked.

After reading a bit in the archives here, it sounds like I do need a native vlan, no matter whether I want one or not. But the fact that it works with the "native" tag really isn't making sense to me. If ConEd is expecting tagged packets (with ID 3264) on their side of the 100Mb connection and is adding tags on the other end (with ID 3264) why would this work?

Of course now we're up and running on the link, so I'm a little hesitant to add a "dummy" subinterface/vlan at the 100Mb end, make it native and then remove the "native" directive from subif 3264.

Any ideas? I'm thinking that perhaps our order got screwed up and our

100Mb end is not really vlan-enabled. But I'm also a bit stumped on what exactly happens with a config such as I've posted above. Am I actually sending ANY tagged frames? If tagged frames come in, would I see them?

Thanks,

Charles

Reply to
spork.sporkman
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Hi Charles,

Using only a native vlan is like using no trunking/tagging at all. You can put the ip address on the physical interface and things would work as well. What I think they did is configure a trunk on their side of the remote end and issue vlan 3264 as their first and native vlan. Adding a second "circuit" to the already existing hub connection now only means adding a second sub-interface. This is when they actually start tagging frames. If tagged frames do come in (with unknown tags, for unknown subinterfaces), I guess you'll see them as input errors on your interface. If they promised a 802.1q trunk I would leave the config as you have it right now. It's a correct configuration and allows for easy additions of new vlans.

Erik

Reply to
Erik Tamminga

Excellent, thanks. I wish I had a better idea of how things work on their side. It seems fairly simple - they have about 6 buildings per ring with a tiny little Cisco 35xx in each building. They pull the fiber to the switch in each building and the GigE (or 10GigE?) loop actually goes into then out of each switch. But I'm not sure what gear they use to aggregate everything. They do allow me to get a vlan out to any port in any building, so I guess we'll just see what happens when the next vlan gets turned up.

Reply to
spork.sporkman

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