Caption script:

Overview： HSRP basic

Goal: Visualize the effect of HSRP election and link failover procesa. Including HSRP states, timers.
Topology: A LAN includes a host, a switch, and two routers. Routers act as redundant gateway.
Steps: 1) At init, R1 and R2 both think they are Active. After receiving HSRP packet (Hello), R2 changs to Speak.
           2) When Standby timer timeout, R2 becoems Standby.
           3) H1 ping H2. It succeeds. Ping is switched to the Active router R1.
           4) R1's interface shutdown. H1 ping H2 again and fails.
           5) When R2's Active timer expires. It elects itself as Active.
           6) H1 ping H2 again and succeeds.

HSRP in a nutshell

- R1 and R2 are routers running HSRP to form a virtual gateway router with address (192.168.1.1, MAC.virt).
- HSRP elects one router as the virtual gateway (Active) by comparing priority between R1 and R2.
- Switch S1 connects to routers and hosts. When receiving a packet from a host, S1 lookup its MAC table for 
  MAC.virt and forwards the packet to the Active router.
- HSRP uses timers to control state changes. When a router's Standby timer expires, it becomes Standby. When 
  a router's Active timer expires, it becomes Active.

Elect an Active router (priority)

- After power on, R2 sends a HSRP packet to declare: I am Active. My priority is 100. Switch S1 adds an entry 
  (MAC.virt, F0/2) when receiving R2's HSRP packet.
- R1 does the same when powered on. It declares: I am Active. My priority is 105. Switch S1 updates it virtual 
  MAC entry to (MAC.virt, F0/1) when receiving R1's HSRP packet.
- When R2 receives R1's HSRP packet, it withdraws itself from election and enters Speak state. since 
   its proiority is smaller than R1.
- When R1 receives R2's HSRP packet, it elects itself as Active since its proiority is hugher than R12.

R2: Standby timeout > Standby

When R2's Standby timer expires, R2 becoems Standby and foods a HSRP packet.
When R1 receives this HSRP packet, it learns that R2 is Standby and updates its HSRP status.

R1 link down

When H1 receives echo from H2, it sends the second ping..
At tick 30, R22;s E0/0 is donw. R1 becomes offline. Its HSRP Status table refllects the following changes:  
    Actvie router:     from (Active, local) to (Init, unknown).  
    Standby router: from R2 to unknown.
At tick 33, R1 recevies  ping forwarded by S1. R1 drops ping.
At this time, R2 is unaware of the Active router's problem. R2 is still in Standby state.

R2: Active timeout > Active

When R2's active timer expires, it becomes Active and floods a HSRP packet. When S1 receives this pakcet, it updates its MAC table. The virtual MAC entry is changed to (MAC.virt, F0/2.)

Ping succeeds. Gateway is up

H1 ping H2 for the third time. When S1 receives ping from H1, it switches ping to R2 according to its MAC table.
R2 then forwards ping to H2. As far as H1 is concerned, gateway works fine. H1 does not know that R1 is down.
This is demonstrates the capability of hot standby protocol: When the gateway router (R1) is down, the backup router (R2) can take over in seconds. The traffic flow disruption is minimum.