L2 Loops

Hi Experts,

Between multiple pairs of  AGG layer switches, each connected to upstream L3  for routing, should not have same VLANs amongst themselves?

i.e to say, if there are 2 AGG pairs, AGGP1 and AGGP2, say, vlan10 should NOT be ccommon to both the pairs to avoid loops.

WHY?

Thanks
genseek
genseekAsked:
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Don JohnstonConnect With a Mentor InstructorCommented:
That's just clarity. They absolutely do NOT have to be the same.
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Istvan KalmarHead of IT Security Division Commented:
Hi,

You need to enable rpvst ont AGG ports, and you need to finetuning which VLAN-s blocked!

Best regards,
Istvan
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genseekAuthor Commented:
Hi,

Would appreciate, if you could first affirm, whether loop would be created, Yes or No. If yes, then why.

genseek
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Don JohnstonInstructorCommented:
Between multiple pairs of  AGG layer switches, each connected to upstream L3  for routing, should not have same VLANs amongst themselves?

Says who? Design can be a funny thing. The only absolute is that it meets the requirements.

i.e to say, if there are 2 AGG pairs, AGGP1 and AGGP2, say, vlan10 should NOT be ccommon to both the pairs to avoid loops.

If a VLAN is specific to one switch and redundant links are not required, then that would be correct.
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genseekAuthor Commented:
donjohnston:

Can you please elaborate you answer in little more detail?

Thank you,

genseek
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Don JohnstonInstructorCommented:
Elaborate on which answer?
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genseekAuthor Commented:
I read in a Data Center architecture document, that

" A VLAN is to be only available across all the switches connected to the same L2Aggregation pair. This limits the scope of a VLAN to a single collocation "

Does the above mean that if, say, vla 10 is created on 2 AGG pairs, it creates scope for switching loops by any means?
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Don JohnstonInstructorCommented:
Context would be helpful. In this case being able to read the document that this came from.

But absent that, it would appear that they are specifying that a VLAN be localized to a pair of switches. Since it's from a Data Center document, it's likely that that redundancy is a requirement. Which means this is not done to avoid loops but instead to limit the reach of a particular VLAN.
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genseekAuthor Commented:
I agree, donjohnston.

But just to reconfirm, is there any "best practice"  or rule for a data center design that says...same VLANs should not be created on different AGG pairs?
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Don JohnstonInstructorCommented:
No.
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genseekAuthor Commented:
Thanks. Another one.

For a single Core pair, how many AGG pairs can be connected? How is this decided?

Is there any desig rule for number of Core to AGG pairs?

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Don JohnstonInstructorCommented:
The number of ports will limit the number of connected switches.
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genseekAuthor Commented:
Thanks donjohnston. One more.

Normally, core and agg pairs are connected via portchannels/trunks.

And, if it is portchannel, then the portchannel number shud be SAME on both the Core and the Agg pair.

If the portchannle numbers are different, will the portchannel work? Example, as below.

Cs7k_Core
 TenG3/1 (portch1004)
     |
     |
CS65K_AGG
 TenG1/1 (portch1001)



     

   
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Don JohnstonInstructorCommented:
The channel group numbers do not have to match. It makes life easier though. :-)
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genseekAuthor Commented:
But normally in all standard Cisco or any vendor portchannel configuration document, thay ALWAYS stipulate that channel group number have to be SIMILAR.

Never, i have seen it been said that they CAN be different or do not have to match.

So, what is then the standard practise?
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genseekAuthor Commented:
THANKS a LOT, donjohnston for the clarity.

I shall aware the points now.
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genseekAuthor Commented:
Got the answer as i expected in the time i wanted.
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