Loop on C9500

[lizochkanovichenko]

Hello!

Some time ago I encountered a problem connecting C9500 switches to the network. I have two switches C9500 that use stack virtual  technology. The switch C9500 has a management IP address and a static route to the gateway via VRF. The C9500 switch was connected to two other switches with two cables, that is, in two separate switches (C9300 and C9200). When connecting to the network with two cables, a loop occurred (stp or routing loop). The symptoms were as follows: some of the switches were unavailable for ping, but available via SSH.
I solved this problem by registering the default gateway.
But I have another C9500 switch in another place and it does not have a gateway, there is only a static route to the gateway.
I can't understand what the problem could be.

Please help me figure it out.

[Devaa]

The packet that goes via some switch or cable is affected. You have ECMP or PortChannel configured? 

In case of ECMP, you have to isolate paths / devices one by one and check which is causing drops. (May need downtime)

In case of PortChannel, isolate uplinks one by one and check which link is causing drops. (May need downtime)

Run mtr and check if you observe drops anywhere. 

[marce1000]

      >....When connecting to the network with two cables...a loop occurred
   Kind of a suspicious action in the context of the original question; the question then is why and what is the need for that

 M.

[Rich R]

I have trouble understanding your explanation.

Can you draw a simple diagram explaining your network topology and connections and showing which connection caused the loop?

Generally speaking if you have redundant paths then you need to rely on spanning tree to prevent loops so you cannot use features like portfast on the ports.

[Joshqun Ismayilov]

This issue likely stems from the interaction between VRF routing, STP, and redundancy mechanisms when connecting your C9500 to multiple downstream switches.
Could you please share detail info :

#show ip route vrf <management-vrf>
#show spanning-tree
#ip route vrf <management-vrf> 0.0.0.0 0.0.0.0 <gateway-ip>

Thanks,
Joshqun Ismayilov