• Concept
  • BPDU Frame
  • STP (802.1d) Process Phases
  • STP Port States
  • Timers
  • STP Topology Changes
  • STP Re-convergence Process
  • Optimizing STP Re-convergence
  • Protecting STP Topology
  • RSTP
  • STP and VLANs (CST, PVST+, MST)



Bridging loops forward a single frame around and around between switches forever.

STP prevents bridging loops by forming a spanning-tree topology in a layer 2 network with redundant links.

STP uses a special layer 2 frame called BPDU to operate.

Continue reading STP

First Hop Redundancy Protocols

Cisco Proprietary IETF Standard Cisco Proprietary
Active / Standby Master / Backup Active Virtual Gateway (AVG)
Active Virtual Forwarders (AVF)
·         AVG assigns virtual MAC addresses to each AVF and answers ARP requests.
·         AVFs forward the traffic. 
Active router selection by priority. The higher, the better.

Default=100 (0~255)

Master router selection by priority. The higher, the better.

Default=100 (1~254)

Active Virtual Gateway selection by Priority/IP Address in group. The higher, the better.

Default=100 (1~255)

Group number (0~255) but usually 16 groups are supported.

HSRP group number is locally significant on interfaces.

Group number (1~255) Group number (0~1023)
HSRP Hello packets        

MC: / UDP: 1945
Hello/Dead time= 3/10 s Default

VRRP Hello packets

Hello/Dead time= 1/3.2 s Default

GLBP Hello packets        

MC: / UDP:3222

Hello/Dead  time= 3/10 s Default

No preemption by default. Preemption by default. No preemption by default.
Interface/object tracking;
HSRP decreases the priority (default=10)
Interface/object tracking;
VRRP decreases the priority
Interface/object tracking;
GLBP decreases the weighting
MAC: 000.0c07.acXX
XX= HSRP group in hex
MAC: 000.5e00.01XX

XX= VRRP group in hex

MAC: 0007.b4XX.XXYY


x…x= Group number in bin

YY= GLBP Forwarder Number in hex

Authentication: MD5/Text Authentication: MD5/Text Authentication: MD5/Text
No load balancing No load balancing Load balancing

Continue reading First Hop Redundancy Protocols