cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
672
Views
0
Helpful
4
Replies

MAC table entries being deleted, then added, then deleted...

cto_securelogic
Level 1
Level 1

Hi all,

Using the MAC notification feature on a Cisco 2950 I noticed that several MAC table address entries are cycling through delete and add operations:(sample)

History Index 26, Entry Timestamp 172524612, Despatch Timestamp 172524612

Operation: Added Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

History Index 29, Entry Timestamp 172526813, Despatch Timestamp 172526813

Operation: Deleted Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

History Index 30, Entry Timestamp 172527614, Despatch Timestamp 172527614

Operation: Added Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

History Index 32, Entry Timestamp 172550818, Despatch Timestamp 172550818

Operation: Deleted Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

History Index 33, Entry Timestamp 172551719, Despatch Timestamp 172551719

Operation: Added Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

History Index 37, Entry Timestamp 172582435, Despatch Timestamp 172582435

Operation: Deleted Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

History Index 39, Entry Timestamp 172584739, Despatch Timestamp 172584739

Operation: Added Vlan: 1 MAC Addr: 0000.c054.7eda Dot1dBasePort: 1

etc.

These adds and deletes are occuring about every 8-12 seconds. The MAC table entry aging timer is set to 300 seconds, the default. Any idea what is causing these MAC addresses to be added and deleted this way?

My FastEthernet ports are connected to NICs.

4 Replies 4

Prashanth Krishnappa
Cisco Employee
Cisco Employee

Do a "sh mac-address-table aging-time vlan 1" when this is happening and see if the MAC aging time is 15 seconds when the addresses are being deleted and relearnt. If it is, probably the switch is getting STP TCNs

http://www.cisco.com/warp/public/473/17.html

If you are getting TCNs, check to see if there is any link flaps. Make sure you have portfast enabled on user ports so that TCNs are not generated when a user reboots his workstation.

Thanks for your response. I cleared the mac address table then did the "sh mac-address-table aging-time vlan 1", but it always showed 300. Here is a look at the VLAN STP info.

sh spanning vlan 1

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 4097

Address 0011.93b9.d900

Cost 3008

Port 25 (GigabitEthernet0/1)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 49153 (priority 49152 sys-id-ext 1)

Address 0011.bbb9.2ec0

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 15

Uplinkfast enabled

Interface Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/5 Desg FWD 3100 128.5 Shr

Fa0/6 Desg FWD 3100 128.6 Shr

Fa0/8 Desg FWD 3019 128.8 P2p

Fa0/10 Desg FWD 3019 128.10 P2p

Gi0/1 Root FWD 3004 128.25 P2p

Gi0/2 Altn BLK 3004 128.26 P2p

Portfast is not enabled.

Seems as though something is causing my mac addresses to be deleted before they age out, then readds them. Any idea what would cause this? Any thing else I can show that would help?

Thanks for the help.

Issue "sh spanning-tree vlan 1 detail" on the STP root switch and post the first few lines. Looks like your root switch is on Gig0/1

Here is an example

2950#sh spanning-tree vlan 1 detail

VLAN0001 is executing the ieee compatible Spanning Tree protocol

Bridge Identifier has priority 32768, sysid 1, address 0005.7428.1c40

Configured hello time 2, max age 20, forward delay 15

We are the root of the spanning tree

Topology change flag not set, detected flag not set

Number of topology changes 7 last change occurred 22:43:35 ago <<<<-------

from FastEthernet0/1

Times: hold 1, topology change 35, notification 2

hello 2, max age 20, forward delay 15

Timers: hello 0, topology change 0, notification 0, aging 300

sh spanning-tree det

VLAN0001 is executing the ieee compatible Spanning Tree protocol

Bridge Identifier has priority 49152, sysid 1, address 0011.bbb9.2ec0

Configured hello time 2, max age 20, forward delay 15

Current root has priority 4097, address 0011.93b9.d900

Root port is 25 (GigabitEthernet0/1), cost of root path is 3008

Topology change flag not set, detected flag not set

Number of topology changes 236 last change occurred 01:12:42 ago

from FastEthernet0/1

Times: hold 1, topology change 35, notification 2

hello 2, max age 20, forward delay 15

Timers: hello 0, topology change 0, notification 0, aging 300

Uplinkfast enabled

Port 5 (FastEthernet0/5) of VLAN0001 is forwarding

Port path cost 3100, Port priority 128, Port Identifier 128.5.

Designated root has priority 4097, address 0011.93b9.d900

Designated bridge has priority 49153, address 0011.bbb9.2ec0

Designated port id is 128.5, designated path cost 3008

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is shared by default

BPDU: sent 18226, received 0

Port 6 (FastEthernet0/6) of VLAN0001 is forwarding

Port path cost 3100, Port priority 128, Port Identifier 128.6.

Designated root has priority 4097, address 0011.93b9.d900

Designated bridge has priority 49153, address 0011.bbb9.2ec0

Designated port id is 128.6, designated path cost 3008

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is shared by default

BPDU: sent 18224, received 0

Port 8 (FastEthernet0/8) of VLAN0001 is forwarding

Port path cost 3019, Port priority 128, Port Identifier 128.8.

Designated root has priority 4097, address 0011.93b9.d900

Designated bridge has priority 49153, address 0011.bbb9.2ec0

Designated port id is 128.8, designated path cost 3008

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 3724, received 0

Port 10 (FastEthernet0/10) of VLAN0001 is forwarding

Port path cost 3019, Port priority 128, Port Identifier 128.10.

Designated root has priority 4097, address 0011.93b9.d900

Designated bridge has priority 49153, address 0011.bbb9.2ec0

Designated port id is 128.10, designated path cost 3008

Timers: message age 0, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 173777, received 0

Port 25 (GigabitEthernet0/1) of VLAN0001 is forwarding

Port path cost 3004, Port priority 128, Port Identifier 128.25.

Designated root has priority 4097, address 0011.93b9.d900

Designated bridge has priority 32769, address 0011.bb3d.4f00

Designated port id is 128.8, designated path cost 4

Timers: message age 3, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 215, received 726863

Port 26 (GigabitEthernet0/2) of VLAN0001 is blocking

Port path cost 3004, Port priority 128, Port Identifier 128.26.

Designated root has priority 4097, address 0011.93b9.d900

Designated bridge has priority 16385, address 0011.93b9.e200

Designated port id is 128.3, designated path cost 12

Timers: message age 5, forward delay 0, hold 0

Number of transitions to forwarding state: 0

Link type is point-to-point by default

BPDU: sent 2, received 727415