VSP Fabric VIST settings for multiple clusters

  • 8 October 2020
  • 4 replies

Hi Community,


When configuring multiple VIST/SMLT clustered pairs of VSP VOSS switches do we need to create a unique VLAN and I-SID for each pair?




4 replies

Userlevel 5
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You can reuse the VLAN but the i-sid must be unique


Thank you Miguel.

Userlevel 4

@Miguel-Angel RODRIGUEZ-GARCIAis correct.

 VLANs have local significance on the switch, I-Sid’s have global significance in the entire fabric.


The VSPs use the i-SID I.D. to create the vIST peer association. 


So if your IST VLAN is 100 and the VLAN I-SID is 10,000 the VSPs use the i-SID ID to bond the viST pair.  But if that i-sid shows up somewhere else in your fabric the VSP won’t know who its true vIST peer is. 


This is important to know because with SPBm fabric vIST pairs do not need to be local or physically connected to each other.  So the VSPs will search the entire fabric for their viST peer so its imperative the i-sid is unique and matches.


As a guiding rule I always encourage my customers to still use a unique VLAN, IPs and I-SID for each vIST pair.   VLANs and IPs are cheap.  But if you use the same IPs for each viST cluster just make sure you are suppressing the IST IP routing.    

Userlevel 5
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To be added in your check list for vIST:

  • use a different /30 subnet in a /24 scope for your different clusters
  • disable the redistribution of the vIST subnet in the ISIS routing table
  • use a template for the isid like:
    • VLAN=4000, personally I use the same VLAN ID
    • i-sid’s= 10014000 for cluster 1, 10024000 for cluster 2
    • with this you can identify quickly a vIST i-sid
    • Here an example of naming convention: