Configuration BPDU at every Hello Time

[shelley_wang]

I am wondering why Root Bridge generates Configuration BPDU at every Hello Timer (default 2 seconds)?

Non-Root Bridges relays the Configuration BPDU and keep the lowest BPDU for each port. Why to keep the lowest BPDU for each port? What happens if a port receives a new BPDU with higher Sender Bridge ID or Root Path Cost?

Thx,

Shelley

[vinod.agrahari · Accepted solution]

Hi Jingyi,

BPDU(Bridge Protocol Data Unit) is a protocol which is a feature of Spaning Tree protocol and enhance the functionality of Manageability,reliability and security of Network Infrastructure.

With the help of BPDU ,we decide the root port in the connected connected and BPDU the network from the loop and it generate in every two second .

If there is any change in the network connectivity (Switch consider as root switch) goes down ,BPDU packet will decide the next root in the network.

Regards

Vinod Agrahari

[shelley_wang]

Thanks for the explanation.

What if a bridge port receives a CBPDU which contains different Root Bridge ID, different Root Path Cost, different Sender BID, or different Sender Port ID?

thx,

jingyi

[shelley_wang]

Hi Vinod,

Good explanation. I would like to provide summarization.

Configuration BPDUs are generated every Hello time (2 seconds) from Root Bridge. A bridge can detect indirect link failure when missing Configuration BPDUs for Max Age time. A bridge can also detect indirect network change like change in any of the 4 important fields in BPDUs: Root BID, Root Path Cost, Sender BID, and Send Port ID.

Each port keeps the lowest Configuration BPDU because a bridge can quickly converge to network change. One example is RP down, the bridge can quickly elect the port with the best Configuration BPDU as new RP. The other example is a port can send superior BPDU of its local copy to suppress the Inferior BPDU received from a neighbor.

Thx,

Jingyi