BPDU - sent/recieve

[IPhoneRZ]

hello,

I have a question about this log, why only few BPDU frames sent versus huge revived ??

Wan-SW123 #sh spanning-tree interface tenGigabitEthernet 1/1/1 detail
Port 29 (TenGigabitEthernet1/1/1) of VLAN322 is root forwarding
   Port path cost 2000, Port priority 128, Port Identifier 128.29.
   Designated root has priority 35990, address 28af.fd9c.4300
   Designated bridge has priority 35990, address 28af.fd9c.4300
   Designated port id is 128.33, designated path cost 0
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default
   BPDU: sent 160, received 1779475

I don't have enabled bpdufilter globally, only bpduguard

Link between this switch and neighbor switch is 10GB link (10 km away) 

Thanks

[MHM Cisco World]

This port is root port' i.e. it point to root bridge' and hence it receive high number of bpdu. 

A few bpdu send is send in time when stp start elect root bridge' after that this port will not send any bpdu (except case that there is some change in STP domain in that case it send TCN)

MHM

[MHM Cisco World]

Check other STP port you will see high send bpdu and low receive bpdu.

Keep in mind that only root SW generate bpdu and send to all SW and other non root SW receive and modify it and resend to other non root SW.

MHM

[MHM Cisco World]

this Lab for you 

Note:- Designated Port SEND the BPDU, Root Port and BLK Port Receive it
NOTE:- Few receive BPDU in Designated as I mention before is receive when SWs elect root SW (or TCN)
NOTE:- few send BPDU in Root Port and BLK port as I mention before is sned when SWs elect root SW (or TCN)
NOTE:- as a Rule One side of interface is send and other receive when you see both send or receive then there is issue in STP domain 

Root SW have two designated Port and you can see it send High number of BPDU to other SW
SW2 have one Root Port and one Des Port one receive and other send 
SW3 have one Root Port and one BLK port both receive 

[David Ruess]

Hello,

If I remember correctly BPDUs are only sent on the root port when a Topology Change Notification is generated and a TCN BPDU is needing to be sent to the Root switch. However, the Root switch will send BPDUs on all its Designated ports downstream every 2 seconds. It looks like that link is a root port towards the root bridge. So its receiving BPDUs fromt eh Root every 2 seconds, hence the large receive amount. Then whenever a change requiring the root bridge to be notified of the downstream switch(s) send a BPDU upstream to the root.

Hope this helps

-David