cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Announcements

346
Views
0
Helpful
0
Replies
Highlighted
Beginner

PVST+ uplinkfast and dummy multicast frames

Hello!

As I am preparing myself for CCNP Switch certification, I have been doing some labs with STP. I have came across PVST+ and uplinkfast functionality. Once reading about dummy mulitcast frames I wanted to see for myself if switch does send that. Unfortuntely I do not own any physical equipment but I do have VIRL access so I am testing everything in GNS3. Now what is my trouble with topology. I cannot see dummy multicast frames once one side fails :(. Here is my topology:topology.PNG

 

Switch A is the root:

 

SwitchA#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0ca8.f920.6500
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0ca8.f920.6500
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Desg FWD 4         128.1    P2p
Gi0/1               Desg FWD 4         128.2    P2p

SWITCH B:

 

SwitchB#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0ca8.f920.6500
             Cost        4
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0ca8.f98a.bc00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 4         128.1    P2p
Gi0/1               Desg FWD 4         128.2    P2p

 

 

SWITCH C:

 

SwitchC#show spanning-tree vlan 1
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     0ca8.f920.6500
             Cost        3004
             Port        1 (GigabitEthernet0/0)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    49153  (priority 49152 sys-id-ext 1)
             Address     0ca8.f9d8.d300
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec
  Uplinkfast enabled
Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0               Root FWD 3004      128.1    P2p
Gi0/1               Altn BLK 3004      128.2    P2p
Gi0/2               Desg FWD 3004      128.3    P2p Edge
Gi0/3               Desg FWD 3004      128.4    P2p Edge

 

 

UplinkFast is only enabled on Switch C as he is the one responsible for spoofing frames

 

SwitchC#show spanning-tree uplinkfast
UplinkFast is enabled

Station update rate set to 150 packets/sec.

UplinkFast statistics
-----------------------
Number of transitions via uplinkFast (all VLANs)            : 1
Number of proxy multicast addresses transmitted (all VLANs) : 0

Name                 Interface List
-------------------- ------------------------------------
VLAN0001             Gi0/0(fwd), Gi0/1

 

So these 2 host populate all mac address-tables:

 

 

SwitchA#show mac address-table
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0050.7966.6800    DYNAMIC     Gi0/1
   1    0050.7966.6801    DYNAMIC     Gi0/1
   1    0ca8.f98a.bc00    DYNAMIC     Gi0/0
   1    0ca8.f9d8.d300    DYNAMIC     Gi0/1

Switch B has learned 2 MAC address correctly on G0/0 which is connected to Switch A. Link from Switch B to Switch C is blocked.

SwitchB#show mac address-table
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0050.7966.6800    DYNAMIC     Gi0/0
   1    0050.7966.6801    DYNAMIC     Gi0/0
   1    0ca8.f920.6500    DYNAMIC     Gi0/0
   1    0ca8.f9d8.d301    DYNAMIC     Gi0/1
SwitchC#show mac address-table
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0050.7966.6800    DYNAMIC     Gi0/2
   1    0050.7966.6801    DYNAMIC     Gi0/3
   1    0ca8.f920.6501    DYNAMIC     Gi0/0

Now when I shutdown port G0/1 on Switch A or G0/0 on Switch C, Switch C should send dummy mulicast frames with source of MACs in CAM table to address 01:00:0C:CD:CD:CD.

 

Uplink fast works here correctly (Switch C debug log):

*May 15 17:05:09.713: STP: VLAN0001 new root port Gi0/1, cost 3008
*May 15 17:05:09.713: STP: VLAN0001 Gi0/0 -> blocking (uplinkfast)
*May 15 17:05:09.714: STP[1]: Generating TC trap for port GigabitEthernet0/0
*May 15 17:05:09.715: %SPANTREE_FAST-7-PORT_FWD_UPLINK: VLAN0001 GigabitEthernet0/1 moved to Forwarding (UplinkFast).
*May 15 17:05:09.715: STP[1]: Generating TC trap for port GigabitEthernet0/1
*May 15 17:05:09.717: STP: VLAN0001 sent Topology Change Notice on Gi0/1
*May 15 17:05:09.720: STP: VLAN0001 Gi0/0 -> listening
*May 15 17:05:24.721: STP: VLAN0001 Gi0/0 -> learning


But while I am sniffing packet with help of wireshark on link from Switch B to Switch C or even to hosts I cannot see that multicast frames :(:

wireshark.PNG

Switch B CAM table only consist:

SwitchB#show mac address-table
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0ca8.f920.6500    DYNAMIC     Gi0/0
   1    0ca8.f9d8.d301    DYNAMIC     Gi0/1

I am 99% sure that I did that correctly but not seeing these packets makes me a bit worried. These MAC addresses MUST be learned on new port (G0/1) on Switch B!

So my questions are:

  • Did anyone had the same issue? If yes, is this some limitation from GNS3 or maybe I did it incorrectly.
  • When switch receives TCN it reduces mac aging time from 300 sec to 15 sec. Does this dummy frames are sent right after 15 sec?
  • Switch C is generating TC BPDU so its CAM table is holding MAC addreses for 300 sec right?

I hope I gave you some insight of my problem. I would appreciate any help on that matter!

 

Everyone's tags (6)
CreatePlease to create content
Content for Community-Ad
July's Community Spotlight Awards