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hosts per vlan with juniper switches

Posted on 2014-11-14
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Last Modified: 2014-12-01
I inherited a network will all Juniper switches EX series. I notice that there are about 40 vlans and the subnets are /16. Isn't it too many hosts per vlan? What could go wrong in this case? From my understanding rstp is the default spanning tree on the Juniper. Now if there is convergence with rstp, it will affect the whole network and not just the vlan in per vlan stp. Correct? If yes, should I redesigned my rstp to per vlan rstp? Thanks in advance for your inputs.
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Question by:leblanc
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Don Johnston earned 500 total points
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I inherited a network will all Juniper switches EX series. I notice that there are about 40 vlans and the subnets are /16. Isn't it too many hosts per vlan? What could go wrong in this case?
The mask size only defines how many hosts could exist on a network. There could only be 50 hosts of the network. But with that mask, you could have 64,000.  The primary issue these days with large broadcast domains is excessive broadcast traffic.  In that case, you would see reduced bandwidth available and all hosts processing large volumes of broadcast traffic.

From my understanding rstp is the default spanning tree on the Juniper. Now if there is convergence with rstp, it will affect the whole network and not just the vlan in per vlan stp. Correct?
Yes. In a Common (or mono) Spanning Tree environment, there is only one instance of STP.  So if the network re-converges, it will affect traffic in all VLANs.

If yes, should I redesigned my rstp to per vlan rstp? Thanks in advance for your inputs.
I wouldn't say "should".  A Per-VLAN STP  (VSTP in Juniper parlance) environment is typically used to allow you to define different root bridges and path costs on a per-VLAN basis. That way you can control which ports are forwarding/blocking to provide better paths for the hosts.  Since in most multi-VLAN environments the VLANs cross common trunks, a failure causing re-convergence will most likely affect all VLANs.
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