I have a peculiar issue since Monday Jul.5,2010. There was network loop which slowly freezing my network & intermittent disconnections to the server sessions from the workstations. When i logged in the core & access switch, i saw the log as
Jul 6 15:11:11 IST: %SW_MATM-4-MACFLAP_NOTIF: Host 0009.0fa3.7168 in vlan 3 is flapping between port Po1 and port Gi0/3
Jul 6 15:11:12 IST: %SW_MATM-4-MACFLAP_NOTIF: Host 0009.0fa3.6fe8 in vlan 3 is flapping between port Gi0/22 and port Po1
Jul 6 15:12:02 IST: %SW_MATM-4-MACFLAP_NOTIF: Host 0009.0fa3.7168 in vlan 3 is flapping between port Po1 and port Gi0/3
Jul 6 15:12:07 IST: %SW_MATM-4-MACFLAP_NOTIF: Host 001f.d0bb.69a4 in vlan 7 is flapping between port Gi0/19 and port Po1
The network setup is a HSRP having 2 core switches & 5 access switches. Each switch is connected one to CORE-1 & the other to CORE-2. CORE-1 & CORE-2 are ether-channeled. This setup is running since June.2009 having no issue. But all of sudden since Jul.5,2010 as mentioned earlier i was getting logs on the switches console. when i ran "sh cdp neighbors" on access switches, it listed me only CORE-1 switches & not CORE-2. But on CORE-1 & CORE-2 when i ran "sh cdp neighbors" it displayed all the access switches. So then digged in to identify the issue. Initially i disconnected all the access switches uplink cables of CORE-1. Network was completely down, even then "sh cdp neighbors" did not display the CORE-2 switch though the interface is UP. Then i connected back the uplink of CORE-1 on all access switches & disconnected CORE-2 uplink on all access switches, there were no loop & network was healthy. Log did not show anything. Then i decided to connect the CORE-2 uplinks on all access to a different port from
Gi 0/20,Gi 0/21,Gi 0/22,Gi 0/23,Gi 0/24 to Gi 0/15,Gi 0/16,Gi 0/17,Gi 0/18,Gi 0/19 no issues where there. Network was healthy no loops & was able to view CORE-1 & CORE-2 when i ran "sh cdp neighbors" on the access switches. Wondering what went wrong on the ports on the CORE-2 switches, when the configuration was same on the other connected ports. Any ideas?