IEEE 802.1ad Access Network

Routing & Switching Switching
1

Hello

It seems like 802.1ad is basically Q-in-Q, which I know how to setup, but on the Cisco switches i get 802.1q packets when doing Q-in-Q. I’m curious if there’s a way to get 802.1ad packets. I’ve had a try at the Chapter: Configuring IEEE 802.1ad on the Cisco configuration guides but I never seem to acquire 802.1ad packets through wireshark when following that guide.

2

Hello Ethan

One of the things that happens with telecom and networking is that some names seem to stick even though they have been officially changed. What I mean by this is that when you applied 802.1ad, and you looked it up in wireshark, you probably saw the 802.1Q Virtual LAN statement in wireshark and figured that it’s still using 802.1q and not 802.1ad. Well, the truth is that even though 802.1ad has superseded 802.1q in 2011, many professionals, and organizations still refer to the feature as 802.1Q. This is especially prevalent in the term QinQ. It just sounds so cool, that people couldn’t stop using it. Try saying ADinAD… it just doesn’t work :stuck_out_tongue:.

So when you tried implementing 802.1ad and “failed” I believe you actually succeeded. Why? Take a look at the following wireshark output. It says 802.1Q but actually, it is 802.1ad. You can verify this by the fact that it indicates a Drop Eligibility Indicator (DEI) field. 802.1ad replaced the Canonical Format Identifier (CFI) with the DEI field, so this is indeed 802.1ad.

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I hope this has been helpful!

Laz

3

I have a question about your statement refering to the DEI and CFI.

While learning about the ethernet frame combined with the dot1q feilds in it, I found out that the mentioned fields inside the dot1q field are:

  1. 802.1p - qos
  2. DEI
  3. VLAN ID

why would they refer it as an DEI if the context is about the normal ethernet header?

4

Hello Nitay

The 802.1Q tag is very specifically defined. In the most current version it is composed of 32 bits separated into the following four fields:

  1. Tag Protocol Identifier (16 bits) which is a value to identify the frame as one that is tagged and has a value of 0x8100. This is in the same position as the EtherType field in untagged frames. When this value is indicated, the host knows it is a tag, as this value is not valid as an EtherType value. This is how a frame is distinguished between one that has and one that doesn’t have a tag.
  2. Priority Code Point (3 bits) this is a field used for quality of service mechanisms. This is synonymous with Class of Service (CoS) used at L2 for QoS.
  3. Drop Eligible Indicator (1 bit) used separately or in conjunction with CoS values to indicate frames eligible to be dropped in case of congestion.
  4. VLAN identifier (12 bit) used to identify the VLAN on which the frame should be placed.

DEI will only appear when the frame is one that has a VLAN tag. It will not be present in a frame that isn’t tagged.

I hope this has been helpful!

Laz

5

that was really a useful answer.
Thanks again

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