Trying to understand 802.1ag / 4 conceptual questions

All,

Having fetched the draft 6.1 version of IEEE 802.1ag and trying to understand it, I ran into some conceptual questions I cannot answer myself. Maybe there's someone out here able to help?

(1) 802.1ag is set out for end-to-end management and I would assume IEEE to make things as backwards compatible as possible, that is, allow

802.1ag-enabled clouds to be connected via 802.1ag-agnostic older equipment. I would have believed 802.1ag frames just traverse such agnostic equipment transparently. (Maybe this is wrong. Then, my problem had vanished, of course.) Now seeing they use dstMAC==01-80-C2-00-00-0x I have doubts about my guess as I happen to remember such MACs MUST NOT be forwarded at all, per 802.1D. So what about backwards compatibility and 802.1ag-agnostic equipment then?

(2) Again addressing. It appears to me that the ME level shall be signified in the last three bits of the MAC address, with ME level from

0..7. That'd make 01-80-C2-00-00-00 to -00-07, which I believe to conflict with STP, PAUSE/.3x and LACP. What's wrong or missing here (don't believe IEEE to make conflicting specs ;-)?

(3) Let's put 802.1ag and STP together, will that go? If we have a topology with loops (presuming this is supported), will 802.1ag care about it and resolve or will it let STP do the job? My guess is that having both active simultaneously, STP may rearrange topology underneath 802.1ag, which in turn tries to find an alternative path via AIS and LTMs at the same time that STP processes a TC, and that looks strange to me now. Any ideas?

(4) 802.1ag says that it makes use of an extended discovery mechanism that can be found in chapter 12 of 802.1Q -- fine. But given .1Q invented some discovery mechanism prior to .1ag, what did they intend it for, wo used it and what for? I must admit never having seen such .1Q discovery so far.

Any ideas will be greatly appreciated. Thanks, /Thomas.

Reply to
Thomas Bahls
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Let me give it shot. :-)

If you pay close attention, you will see that the addresses used for CFM are not 01-80-C2-00-00-0x, but 01-80-C2-xx-xx-xy. This means that they use the IEEE OUI but are not from the set of reserved addresses that bridges shall not forward. Therefore these frames would pass transparently through legacy equipment.

See above.

Absolutely! It works in the presence or absence of STP. If using STP, one should make sure that the CCM interval is high enough that STP finds the problem and fixes it before CCM detects it. Alternatively, CCM can detect the problem, but it doesn't really fix it, and so the problem will eventually go away when STP reconverges even though a defect may be indicated in the interim.

AIS has been removed from the spec several versions ago. One cannot use "LTMs" or anything else to find paths. If one doesn't use STP, the only way to create loop-free paths in network topology with loops is to pre-provision them. That is certainly an option, but the details of that are out of scope for this spec.

Can you provide a more specific reference on this?

Anoop

Reply to
anoop

Okay, that goes together with my assumption on backwards compatibility. I must admit that I assumed all x in the MAC above be zeros, which doesn't seem to be the case. But then, what are the real values? Does the spec leave them at my discretion? Probably not...

Well, I'd need a closer look to find back what I think I've read about such reference from .1ag to .1Q. However, the point is not so much the formalism but rather the feact that .1Q *does* talk about discovery in chapter 12 while I don't get a clue what it was intended for when drafting

1.Q (and probably not knowing about .1ag to come), or has been used for since. That'd be just interesting to know.

Thanks, /Thomas.

Reply to
Thomas Bahls

Those numbers will be finalized just before the spec is ratified and published.

OK. Now I think I see where the confusion is.

The discovery of specific paths in 802.1ag is related to the use of the link trace protocol which will report all of the maintenance points along the path at a certain maintenance domain level to a given destination.

There is mention of discovery in 802.1Q in the configuration management clause and this refers to providing capability in the bridge so that the bridge, along with all of its configuration, can be discovered by a management station using a protocol like SNMP.

Anoop

Reply to
anoop

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