When you want to ensure redundancy of the link towards the default gateway to be part of a series of hosts without making changes on the same there are 2 alternatives
1) HSRP (Hot Standby Routing Protocol): Cisco proprietary protocol defined in RFC 2281-we shall cover in this article
2) VRRP (Virtual Router Redundancy Protocol) standard protocol defined in RFC 2338
As seen nnell'immagine two devices also dissimilar (routers and switches of different models and types as long as capable of routing) share a Layer 3 address
"Virtual" address that is the default gateway and is moved between the group members who are Active or standby.
The holder of IP virtual is that which at that moment is equipped with more high priority.
The prioirtà is influenced by values entered in configuration (numerical values) and tracking the status of an interface (eg if the stream is unavailable pasa isdn router to another)
Configuration
The configuration of HSRP uses the command "standby" with the following variables:
standby [group-number] ip ip-address [secondary] where secondary is the shared address
standby [group-number] mac-address mac-address [mac address opziionale shared when you do not want auto-generated]
standby [group-number] priority priority [preempt [delay delay]] if you want to manage an imbalance of priorities
standby [group-number] track type number [priority] ono outgoing subject headers such as interfaces for monitoring the switch
In practice
interface Ethernet0
ip address 10.10.10.1 255.255.255.0
standby 240 preempt
the number 240 indicates the number shown in the example delgruppo
A router can belong to multiple HSRP groups
For each group can be Active or Standby router depending on the priority that was assigned
the function preemt force the router that has the highest priority to become Active-router instead of the one that is at that moment Active
standby 240 ip 10.10.10.5
standby 240 priority 200
dfault priority is 100 for a router and the router with the highest value is chosen Active
