We had a technician move one of two port channel links from an old switch to a new switch before telling anyone. This left the new and old switch both connected to different links of the same channel group on the distribution switch. However, the new switch did not respond to ping or SSH until I removed the link to the old switch from the channel group. This fixed it before I could investigate more.
I understand that this is an incorrect configuration, but what was blocking traffic to the new switch? Is this part of the port channel or spanning tree or something else? What is the risk of a new switch coming up and existing link going down?
EDIT: adding switch info.
Old switch - Cisco Catalyst WS-C3750-48P
Port channel Po2
Gi1/0/1 & Gi1/0/2 members
channel-group 2 mode on switchport trunk encapsulation dot1q switchport trunk allowed vlan 10,20,30 switchport mode trunk ip dhcp snooping trust
New switch - Aruba 3810M-48G-PoE+
1/A1, 1/A2 members
dhcp-snooping trust tagged vlan 10,20,30 untagged vlan 1 spanning-tree priority 4
Distribution switch - Cisco Catalyst WS-C3750-12S
Port channel Po2 Gi1/0/3, Gi1/0/4 members
switchport trunk encapsulation dot1q switchport trunk allowed vlan 10,20,30 switchport mode trunk channel-group 2 mode on spanning-tree mode rapid-pvst spanning-tree portfast bpduguard default no spanning-tree optimize bpdu transmission spanning-tree etherchannel guard misconfig spanning-tree extend system-id