Cisco Switch generating errors

Cisco 2924 is generating the following error messages in the following setup not surewhat is the issue

#: %RTD-1-ADDR_FLAP: FastEthernet0/20 relearning 5 addrs per min


following testing carrried out

1. Shut down port, f0/20 -- the above message appears except it is
reporting FastEthernet 0/21 instead of FastEthernet 0/20.

2. Shut down port, f0/22 (port f0/20 and f0/22 open)-- no message
appears.


On running debug on 2924
#debug ethernet-controller address Ethernet Controller Addresses
debugging is on switch#u all
.Mar 30 17:36:13: Add address 0012.1e17.c821, on port Fa0/22 vlan 2
.Mar 30 17:36:13: 0012.1e17.c821 has moved from port Fa0/20 to port
Fa0/22 in vl
an 2
.Mar 30 17:36:13: Add address 0012.1e17.c821, on port Fa0/20 vlan 2
.Mar 30 17:36:13: 0012.1e17.c821 has moved from port Fa0/22 to port
Fa0/20 in vl an 2

Scenario 2A: port f0/20 was shut down, below is router B's
mac:

.Mar 30 18:54:50: %SYS-5-CONFIG_I: Configured from console by console
.Mar 30 18:56:02: Add address 0012.8029.c854, on port Fa0/22 vlan 2
.Mar 30 18:56:02: 0012.8029.c854 has moved from port Fa0/21 to port
Fa0/22 in vl
an 2
.Mar 30 18:56:02: Add address 0012.8029.c854, on port Fa0/21 vlan 2
.Mar 30 18:56:02: 0012.8029.c854 has moved from port Fa0/22 to port
Fa0/21 in vl
an 2


A.B.C.D/28
RTR A

---

Cisco Swith 2924

---

RTR B A.B.C.D/28
f0/20 | f0/21
f0/22
|
| Ethernet
SP L2 Switch
|
|
|
SP Juniper Rtr A.B.C.D/28


F0/20 connected to Rtr A
F0/21 connected to Rtr B
f0/22 connected to SP L2 ethernet s/w


help appreciated in isolating the problem.

Hi

See the following article on configuring EtherChannel and 802.1Q Trunking Between Catalyst 2900XL/3500XL and Catalyst 2940, 2950/2955, and 2970 Switches.

www.cisco.com/warp/public/473/131.html

Regards

Hi.

simply looking at the problem, it seems it is a spanning tree or ether channel problem??maybe caused by an misconfigured etherchannel in the one side, and spanning tree enabled in the other side, disconvering at a loop, and inmediatly blocking some ports.

I have seen a similar problem with some cisco switches etherchannel capable mixed with no etherchannel capable.