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How is the fabric-virtual-gateway ARP responder elected in a VCS fabric?

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TitleHow is the fabric-virtual-gateway ARP responder elected in a VCS fabric?
Question
 How is the fabric-virtual-gateway ARP responder elected in a VCS fabric?
Environment
  • VDX Series
  • Network OS 5.0.1 or later
  • fabric-virtual-gateway
Answer
Engineering do advise from the code analysis that the ARP responder is typically based on the lowest Rbridge-id values. This is usually the case in a single platform VCS fabric i.e. only VDX6740 or VDX8770.

In a mixed platform VCS fabric which is a combination of VDX6740 and VDX8770. The VDX8770 will have more priority over the VDX6740 in the ARP responder election process. There is no preemption once the ARP responder has been elected. The only way the election can be triggered is to issue the clear ip fabric-vritual-gateway all command.

Operational scenarios based on the mixed VCS Fabric below
  • First Scenario: Rbridge-Id 2 will typically be the ARP responder because it is a VDX8770 and has the lower Rbridge-Id out of the two 8770's. If Rbridge-Id 2 is rebooted, Rbridge-Id 4 will become the new ARP responder. 
  • Second Scenario: If Rbridge-Id 2 & 4 are rebooted, Rbridge-Id 1 will be elected as the new ARP responder.
  • Third Scenario: If the ISL link for Rbridge-Id 2 is disconnected from the VCS fabric, Rbridge-Id 2 will be segmented out of the cluster and become a single node cluster, still maintaining the role as the ARP responder. Rbridge-Id 4 will become the new ARP responder for the remaining nodes in the cluster. Once the ISL link is enabled, Rbridge-Id 2 will become the ARP responder for the whole of the VCS cluster and Rbridge-id 4 will cease being the ARP responder. In this third scenario, it is not a preemption but a contention since both Rbridge-Id 2 and Rbridge-Id 4 were ARP responder(s) at the time Rbridge-Id 2 rejoined.
  • Fourth Scenario: If all the switches in the VCS fabric are reloaded, Rbridge-Id 1 will become the ARP responder because of the quicker bootup time and its has the lower Rbridge-Id out of the two VDX6740's.


    ​​​​​
    sw2# show vcs
    Config Mode    : Distributed
    VCS Mode       : Logical Chassis
    VCS ID         : 1
    VCS GUID       : d732300f-eaae-4d44-beb1-834f98b0cf7a
    Total Number of Nodes           : 4
    Rbridge-Id       WWN                            Management IP   VCS Status       Fabric Status        HostName
    --------------------------------------------------------------------------------------------------------------
    1                10:00:50:EB:1A:3A:D7:6D        10.26.141.171   Online           Online               sw1    <<<<<<<<<<<<<< VDX 6740
    2               >10:00:00:27:F8:5C:CD:94*       10.26.138.17    Online           Online               sw2    <<<<<<<<<<<<<< VDX 8770
    3                10:00:50:EB:1A:2C:32:C6        10.26.141.239   Online           Online               sw3    <<<<<<<<<<<<<< VDX 6740
    4                10:00:00:27:F8:5C:C6:88        10.26.142.223   Online           Online               sw4    <<<<<<<<<<<<<< VDX 8770
    sw2# show ip fabric-virtual-gateway 
    
    ============Rbridge-id:2============
    Total number of IPv4 Fabric Virtual Gateway sessions   : 12
    Total number of sessions in Active state   : 12
    Total number of sessions in InActive state : 0
    Total number of sessions in Init state     : 0
    Gateway MAC address: 02e0.5200.01ff 
    
    Interface  Admin     State        Gateway        ARP        Load       Threshold  Track
               State                  IP Address     Responder  Balancing  Priority   Priority
    =========  =====     =====    ================== =========  =========  =========  ==========
    Ve 35      Enabled   Active   20.50.4.1/29      Rbr-id 2   Enabled     unset      0         
    Ve 100     Enabled   Active   20.50.0.1/26      Rbr-id 2   Enabled     unset      0         
    Ve 101     Enabled   Active   20.50.0.65/26     Rbr-id 2   Enabled     unset      0         
    Ve 102     Enabled   Active   20.50.0.129/25    Rbr-id 2   Enabled     unset      0         
    Ve 103     Enabled   Active   20.50.1.1/26      Rbr-id 2   Enabled     unset      0         
    Ve 110     Enabled   Active   20.50.2.4/29      Rbr-id 2   Enabled     unset      0         
    Ve 112     Enabled   Active   20.50.2.17/30     Rbr-id 2   Enabled     unset      0         
    Ve 113     Enabled   Active   20.50.2.21/30     Rbr-id 2   Enabled     unset      0         
    Ve 120     Enabled   Active   20.50.3.1/24      Rbr-id 2   Enabled     unset      0         
    Ve 142     Enabled   Active   20.50.5.129/26    Rbr-id 2   Enabled     unset      0         
    Ve 143     Enabled   Active   20.50.5.193/26    Rbr-id 2   Enabled     unset      0         
    Ve 150     Enabled   Active   20.50.6.1/24      Rbr-id 2   Enabled     unset      0
Additional notes
Rbridge-Id 2 & 4 ==== VDX 8770
Rbridge-Id 1 & 3 ==== VDX 6740

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