Electing The Root Switch Cornell Notes

Key Points

  • Root Bride ID
  • Senders Bridge ID
  • Senders Root Cost
  • Timer Values
  • Better Hello Timer
  • Worse Hello Timer
  • Bridge ID
  • BPDU
  • Root Switch


Hello, Maxage, and forward delay timers are configured on the root switch only

BID is an 8 byte unique value with 2 bytes in the priority and 6 byte system ID

Root Switch is the switch with the lowest BID

Tie Breaker, if the priority matches use the lowest MAC address among the two switches

All switches broadcast their BIDs in the hello BPDUs until they receive a hello message from a switch with a lower BID value. The switch advertises the newly found lowest BID switch in its hello messages from that point on.


Lowest BID value is elected as the root switch and is the advertised switch by others on the network. If a tie occurs with the BID values then the mac portion of the BID is used to break the tie. Since MAC addresses are unique per device no further tie should occur.

Notes form ICDN2 200-101 Cisco Press

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