( certifications, and some of the stupid questions cisco presents ! )
Here's a link to a site that I just did a exam topic refresher & for the 642-813 exam that I'm about to take.
https://learningnetwork.cisco.com/community/certifications/ccnp/switch?tab=practice
Here's the offending question ( #8 )
And my selection
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The above was a question on one of the review sets of questions, and my marked answer "using the Type field in the Ethernet header" was wrong. Really?
Comon cisco, how about providing some better answers.
Okay let's look at the other answers;
using 802.1q ToS bits ( btw this what they suggested was right, and the reason of my rant )
implementing DCSP at layer3 ( definitely right since DSCP is included in layer 3 only )
implementing DCSP at layer 2 ( wrong no DSCP in layer2 )
Okay so answer number #1 was wrong according to the exam, but answer #2 was what they expected & claimed was right. But is that really right?
1st off with the ToS bits, there's no such thing as ToS ( type of service ) or any bits in 802.1q or any thing that says ToS in regards to 802.1q tagging.
With CoS ( class of service ), yes, but with ToS no such beast & it's miss leading and bad wording.
Cisco here's a clue, here's a Ethernet Header ;
Frame 10 (105 bytes on wire, 105 bytes captured)
Arrival Time: May 15, 2013 23:27:30.659288000
[Time delta from previous captured frame: 0.000002000 seconds]
[Time delta from previous displayed frame: 0.000002000 seconds]
[Time since reference or first frame: 0.000067000 seconds]
Frame Number: 10
Frame Length: 105 bytes
Capture Length: 105 bytes
[Frame is marked: False]
[Protocols in frame: eth:vlan:ip:gre:ip:tcp:ssl]
Ethernet II, Src: 40:55:39:2f:11:a9 (40:55:39:2f:11:a9), Dst: Force10N_57:1c:a3 (00:01:e8:57:1c:a3)
Destination: Force10N_57:1c:a3 (00:01:e8:57:1c:a3)
Address: Force10N_57:1c:a3 (00:01:e8:57:1c:a3)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Source: 40:55:39:2f:11:a9 (40:55:39:2f:11:a9)
Address: 40:55:39:2f:11:a9 (40:55:39:2f:11:a9)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Type: 802.1Q Virtual LAN (0x8100)
802.1Q Virtual LAN, PRI: 0, CFI: 0, ID: 203
000. .... .... .... = Priority: 0
...0 .... .... .... = CFI: 0
.... 0000 1100 1011 = ID: 203
Type: IP (0x0800)
Notice, no ToS field in that layer2 802.1q frame. Just ain't happening :)
The 1st answer that I selected, was based on the Ether.Type of a 802.1q tagged framed would be present in the header to indicated it's tagged, and then inspection of the CoS value could be inspected. I bold that type and the priority field for 802.1p ( aka CoS )
This how a switch knows tagging is involved & to determine if any the priority of the frame. Btw that Ether.Type would be 0x8100 where as a regular ipv4 Ether.Type would be 0x0800.
So going by we have no ToS bits in etherytype 0x8100 , or as matter fact even a standard ethernet frame, this should be a wrong answer imho. I'm going to reference wikipedia and what they say about ToS & CoS
http://en.wikipedia.org/wiki/Type_of_service
http://en.wikipedia.org/wiki/IEEE_P802.1p
The question is badly worded & we should really expect more Cisco
Ken Felix
Freelance Network/Security Engineer
kfelix -----at----- hyperfeed ----com----
^ ^
==( O O )==
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