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VLANs Design

Posted on 2016-08-16
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Last Modified: 2016-08-20
If I understand VLANs are Layer 2 concept , and they are the Broadcast boundary, which means the Broadcast on one Vlan does not propagate to another VLAN. However I would like an Expert to clarify some dark areas about VLANs.

-- are VLANs the boundary of STP. I mean if there is a Switching Loop on one VLAN, the Loop will stay just on that VLAN and it does go to other VLANs ?

--If we have VLAN10 on a Switch in Building1 and 10 miles away in Building 2 we have another Switch that has VLAN10 and both VLANs are in the same VTP domain. is that considered Bad practice ?

--if we need to make Vlan2050 with 510 or 508 hosts  has subnets  192.168.20.0 to 192.168.20.254 and 192.168.50.0 to 192.168.50.254 , to make it one VLAN for all wireless Access Points.

in other words, is it possible to have one VLAN for 2 subnets ?

Thank you
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Question by:jskfan
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Expert Comment

by:Predrag Jovic
ID: 41758861
are VLANs the boundary of STP. I mean if there is a Switching Loop on one VLAN, the Loop will stay just on that VLAN and it does go to other VLANs ?
Yes, but it will most likely skyrocket CPU of the switch and all VLANs will be affected in the end.
If we have VLAN10 on a Switch in Building1 and 10 miles away in Building 2 we have another Switch that has VLAN10 and both VLANs are in the same VTP domain. is that considered Bad practice ?
Typically it is considered bad practice. Having the same VLAN number is not a problem, but VTP typically should be stay local. Max STP diameter is 7 "hops".  That looks like:
Switch1 - Switch2 - Switch3 - VTP server switch - Switch4 - Switch5 - Switch6
BPDUs can be too slowly propagated from end to end (10 miles distance does not help at all).
Please read article.
in other words, is it possible to have one VLAN for 2 subnets ?
Yes. You can use secondary IP address for for other range in the same VLAN (and not even just one secondary ip address range)
interface vlan 10
ip address 192.168.20.1 255.255.255.0
ip address 192.168.50.1 255.255.255.0 secondary
ip helper-address x.x.x.x

Addresses from secondary IP address range will be assigned by DHCP when primary IP address pool is wasted.
But, on the other hand it is not recommended to have 1000 hosts in one VLAN.
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by:jskfan
ID: 41759831
are VLANs the boundary of STP. I mean if there is a Switching Loop on one VLAN, the Loop will stay just on that VLAN and it does go to other VLANs ?
Yes, but it will most likely skyrocket CPU of the switch and all VLANs will be affected in the end.


But the problem will impact only  hosts connected to that single switch ? other switches will be OK ?
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by:Predrag Jovic
ID: 41760096
Most likely all switches configured with that VLAN will be affected.
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by:jskfan
ID: 41760364
So if it impacts only switches where that one VLAN exist, there is nothing you can do to prevent the Loop to happen.
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by:jskfan
ID: 41760395
Also if I am not wrong, it is better to not spread one VLAN across several switches.
for instance if you can have VLAN 10 on Switch 1 only, if there is a Loop it will impact jut that Switch is better than having VLAN 10 span 2 or more Switches.
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Predrag Jovic earned 500 total points
ID: 41760509
So if it impacts only switches where that one VLAN exist, there is nothing you can do to prevent the Loop to happen.
You need to separate 2 things - preventing loop and effects of a loop. You can prevent loop for sure. On newer devices you can control effects of a loop by using :
 storm-control broadcast level <level>
 storm-control multicast level <level>
 storm-control unicast level <level>
 But, it is better to prevent loop than using network equipment to control it, Typically loops can be prevented by choosing proper network design and using STP, RSTP or MSTP with enhancements that are needed.
Also if I am not wrong, it is better to not spread one VLAN across several switches.
Everything is moving into that direction, but if I remember correctly Cisco's current recommendation is that VLAN should stay inside rack (I still do not see that reflected in productions, typically VLANs are spreading through building).
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Author Closing Comment

by:jskfan
ID: 41763648
Thank you Jovic
I will do more readings about the Design
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