HSRP (Hot Standby Router Protocol)

Definition: Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway, and has been described in detail in RFC 2281.

HSRP use to present the view of a single virtual host on LAN but in real they work as set of routers. This set is known as an HSRP group. Single router is responsible for forwarding the packets that host send to virtual router. This router is called active router.  Another router is selected to work with that which is called standby router. This works as a backup router for active router. When in any network primary gateway become inaccessible and the main host cannot read the IP address of first hop router then this protocol use to develop the framework between the networking routers. Like this the smooth transfer of data in a network become possible and they are always connected with one another.

Working: When HSRP is used then different routers are connected to the same segment of Ethernet. All of these connected segments use to work together so that they can present them self as a single router on LAN. And if routers are connected via any framework of HSRP then all the routers connected to each other in a network will have the same IP or MAC address. In this situation if any of the router fails to work after that as well user can send the data without any inconvenience and this is because of same address that all the routers are using in that network. In case of failure of one router the other one replace that and use to start working in place of that router, but all this process is never in the knowledge of user. This is a transparent procedure for users that use to be followed in HSRP.
This transparent procedure works in the network to control the traffic, active and standby routers periodically use to send HSRP messages. If the active router somehow fails then suddenly standby take over and act as active router.  

HSRP Packet: These packets are used when load has to be split by distributing that among different groups. These groups include MAC header, IP header, UDP header and HSRP packets. Here we are going to discuss the most important one which is HSRP packet. This packet is consisted of Version, Opcode, State, Hellotime, Holdtime, Priority, Group, reserved, Authentication Data and Virtual IP Address.

Version field is the HSRP version number. Opcode fields are of three types hello, coup and resign. State field describe the current state of router sending the messages. State field includes Initial, Learn, Listen, Speak, Standby and Active. Hellotime field is an 8 bit field which take mostly 3 seconds in execution. This field is only meaningful in hello messages. It contains fixed time period between hello messages that router use to send. Holdtime field as well is meaningful in hello message. It take 10 seconds in execution. Between the holdtime received hello message is just considerable. In hello message holdtime must be inserted. Priority field is used to choose between the active and standby routers. These two routers are compared and then the router with high priority is selected for the execution of data. If both the routers are equal in priority then the router with higher IP address use to win. Group field identify the standby groups.
Authentication data field reuse the password. Virtual IP address in not configured by routers, it can be learned by hello message from active router.