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STP Looping Issue - Pls help

Dear Experts,

We are facing issue related to STP.

Find the Topology Diagram:

L2 mac flap.jpg

I am getting MAC FLAP error on Cisco 4510 switch. The effect on network is intermittent Pkt drops in the network. When I checked the specific Ip address I am getting the same with two different

XXXX # show ip arp 172.24.101.115

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  10.10.101.115          0   000c.2903.1625  ARPA   Vlan101

XXXX # show mac add add 000c.2903.162

No entries present.

XXXX# show mac add add 000c.2903.1625

Unicast Entries

vlan   mac address     type        protocols               port

-------+---------------+--------+---------------------+--------------------

  101    000c.2903.1625   dynamic ip                    GigabitEthernet7/20

102    000c.2903.1625   dynamic ip                    GigabitEthernet7/22

When I have check the STP paramerters :

STP config on Cisco 4510 CORE switch :

spanning-tree mode pvst

spanning-tree loopguard default

spanning-tree extend system-id

spanning-tree vlan 100,101,102 priority 0

XXXX# sh spanning-tree

VLAN000100
  Spanning tree enabled protocol ieee
  Root ID    Priority    100

             Address     0023.04c1.b580
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    2      (priority 0 sys-id-ext 100)
             Address     0023.04c1.b580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

VLAN000101
  Spanning tree enabled protocol ieee
  Root ID    Priority    101
             Address     0023.04c1.b580
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    3      (priority 0 sys-id-ext 101)
             Address     0023.04c1.b580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

VLAN0102
  Spanning tree enabled protocol ieee
  Root ID    Priority    101             Address     0023.04c1.b580
             Cost        20020
             Port        406 (GigabitEthernet7/22)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    102    (priority 0 sys-id-ext 102)
             Address     0023.04c1.b580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi7/22              Root FWD 4         128.406  P2p

Vlan is created on CORE switch and assign priority 0 than CORE switch should be the Root. but instead Root port is becoming the port where server is connected. Server at last connected to CORE switch via HP switch via other Vlan to CORE switch and creating a loop as shown in Diag.

SH OUTPUT of 10.10.102.3 switch :

sh spanning-tree interface gi0/5 detail

Port 5 (GigabitEthernet0/5) of VLAN0102 is root forwarding

   Port path cost 4, Port priority 128, Port Identifier 128.5.

   Designated root has priority 3, address 0023.04c1.b580

   Designated bridge has priority 65534, address 0817.f41a.8d00

   Designated port id is 128.23, designated path cost 20012

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

   Number of transitions to forwarding state: 3

   Link type is point-to-point by default

   Loop guard is enabled by default on the port

   BPDU: sent 212, received 3510823

Port 48 (GigabitEthernet0/48) of VLAN0102 is designated forwarding

   Port path cost 4, Port priority 128, Port Identifier 128.48.

   Designated root has priority 3, address 0023.04c1.b580

   Designated bridge has priority 32870, address 0022.0d5d.3d80

   Designated port id is 128.48, designated path cost 20016

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

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   Loop guard is enabled by default on the port

   BPDU: sent 370806, received 1

sh spanning-tree interface gi0/5 rootcost

VLAN0102           20012

sh spanning-tree interface gi0/48 rootcost

VLAN0102           20016

sh spanning-tree interface gi0/9 rootcost

VLAN0102           20016

sh spanning-tree interface gi0/8 rootcost

VLAN0102           20016

Can you help to identify the exact issue? The Priority of Vlan 102 is changed and Root port has been changed due to that. The Bridge ID is the same as CORE switch.

Please help.

Regards,

7 REPLIES

STP Looping Issue - Pls help

Hi Dipesh,

Vlan is created on CORE switch and assign priority 0 than CORE switch should be the Root. but instead Root port is becoming the port where server is connected. Server at last connected to CORE switch via HP switch via other Vlan to CORE switch and creating a loop as shown in Diag.

SH OUTPUT of 10.10.102.3 switch :

sh spanning-tree interface gi0/5 detail

Port 5 (GigabitEthernet0/5) of VLAN0102 is root forwarding

   Port path cost 4, Port priority 128, Port Identifier 128.5.

   Designated root has priority 3, address 0023.04c1.b580

   Designated bridge has priority 65534, address 0817.f41a.8d00

   Designated port id is 128.23, designated path cost 20012

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

   Number of transitions to forwarding state: 3

   Link type is point-to-point by default

   Loop guard is enabled by default on the port

   BPDU: sent 212, received 3510823

Port 48 (GigabitEthernet0/48) of VLAN0102 is designated forwarding

   Port path cost 4, Port priority 128, Port Identifier 128.48.

   Designated root has priority 3, address 0023.04c1.b580

   Designated bridge has priority 32870, address 0022.0d5d.3d80

   Designated port id is 128.48, designated path cost 20016

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

   Number of transitions to forwarding state: 1

   Link type is point-to-point by default

   Loop guard is enabled by default on the port

   BPDU: sent 370806, received 1

The port facing the root bridge will be a root port as you might be aware so I am not surprised at that and the other port is designated forwardign which is the expected behaviour.

You seem to running some VM stuff on the blades. The mac flaps occurs when you are running the VM's in a active/active state or trying to perform load balancing. I would suggest you to check that bit first. STP should work fine unless you tweak it and twist.

HTH

Kishore

New Member

STP Looping Issue - Pls help

Dear Kishore,

Than why the Root port is not teh port connected to CORE switch?

Regards,

New Member

STP Looping Issue - Pls help

The port of the core switch must be all, or also for vlan you have configured a low priority in designated state.

I see the 3 switches connected to the core, so no redundant link exist, but on hpc7000 the link is trunk? The hp procurve is without stp enabled? can crete loop between the switch of blade?

STP Looping Issue - Pls help

The root port is the port on the switch that is connected to the Root switch. The ports on the Root switch will NOT be root ports. They will  be in FWD mode.

Hope this helps otherwise let me know if you want more information

Kishore

STP Looping Issue - Pls help

From looking at your diagram, all vlans that call the 4510 the root switch, will have all ports in DP mode. Remember all ports on the root switch will need to be in designated, because the root switch is the one who sends out BPDU, unless we are talking about RSTP/RPVST in which all switches can do both. The other switches in STP/PVST+ will receive the BPDUs from the root switch and relay them down stream.  The switch at the bottom can form a switching loop, since it has multiple paths to the root switch for the specified vlans. According to the config, One of the ports will be a Root Port, and the others will be in blocking.

So I dont see anything out of the orrdinary with PVST.

As for the VM/Blade servers, I don't have any experience with dealing with those, but I do know that if you are load balancing or perform aggregation/etherchannel that it can cause mac flaps but I believe that is normal in that situation, but don't quote me on that.

New Member

STP Looping Issue - Pls help

How can I made Vlan 102 Priority 102 instead of 101? As per configuration it should 102 but somehow it was changed to 101 why I can't understand. Can anybody tell me?

VLAN0102
  Spanning tree enabled protocol ieee
  Root ID    Priority    101             Address     0023.04c1.b580
             Cost        20020
             Port        406 (GigabitEthernet7/22)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    102    (priority 0 sys-id-ext 102)
             Address     0023.04c1.b580
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi7/22              Root FWD 4         128.406  P2p

Regards,

STP Looping Issue - Pls help

If you give vlan 102 a priority of 0 it should be 102 if you have 'spanning-tree extend system-id' configured.

That should make your priority = 0 + 102 = 102.

If you can following the root ports for vlan 102 back to the root switch and give us the BID and priority of that

root switch.

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