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.
The protocol establishes a framework between network routers in order to achieve default gateway failover if the primary gateway becomes inaccessible,[1] in close association with a rapid-converging routing protocol like EIGRP or OSPF. By multicasting packets, HSRP sends its hello messages to the multicast address 224.0.0.2 (all routers) for version 1, or 224.0.0.102 for version 2[2], using UDP port 1985, to other HSRP-enabled routers, defining priority between the routers. The primary router with the highest configured priority will act as a virtual router with a pre-defined gateway IP address and will respond to the ARP request from machines connected to the LAN with the MAC address 0000.0c07.acXX where XX is the group ID in hex. If the primary router should fail, the router with the next-highest priority would take over the gateway IP address and answer ARP requests with the same mac address, thus achieving transparent default gateway fail-over. A HSRP Basics Simulation visualizes Active/Standby election and link failover with Hello, Coup, ARP Reply packets and timers.
HSRP is not a routing protocol as it does not advertise IP routes or affect the routing table in any way.
HSRP has the ability to trigger a failover if one or more interfaces on the router go down. This can be useful for dual branch routers each with a single serial link back to the head end. If the serial link of the primary router goes down, the backup router would take over the primary functionality and thus retain connectivity to the head end.
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Router 1:
track 1 interface Serial0/0/0.1 ip routing ! Points at the interface that needs to be Prioritized interface FastEthernet0/0 description interface to LAN ip address 10.10.10.1 255.255.255.0 ! Make sure this IP is in the same subnet as your Virtual Gateway1 IP ip address 192.168.1.1 255.255.255.0 Secondary ! Make sure this IP is in the same subnet as your Virtual Gateway2 IP standby 1 ip 10.10.10.25 ! Virtual IP 1 (10.10.10.0 Network Takes Priority) standby 1 priority 105 ! The Higher the # The Higher the Priority standby 1 preempt ! Enables the router with the highest priority to immediately become the active router standby 1 track 1 ! WAN SUBINTERFACE standby 2 ip 192.168.1.25 ! Virtual IP 2 standby 2 priority 100 ! Lower Priority = Backup Route standby 2 preempt ! Enables the router with the highest priority to immediately become the active router standby 2 track 1 ! WAN SUBINTERFACE ! Router bgp <ASN> network 10.10.10.0 mask 255.255.255.0 ! Broadcasts Gateway1 out the WAN through BGP network 192.168.1.0 mask 255.255.255.0 ! Broadcasts Gateway2 out the WAN through BGP
Router 2:
track 1 interface Serial0/0/0.1 ip routing ! Points at the interface that needs to be Prioritized interface FastEthernet0/0 description interface to LAN ip address 192.168.1.2 255.255.255.0 ! Make sure this IP is in the same subnet as your Virtual Gateway2 IP ip address 10.10.10.2 255.255.255.0 Secondary ! Make sure this IP is in the same subnet as your Virtual Gateway1 IP standby 1 ip 10.10.10.25 ! Virtual IP 1 standby 1 priority 100 ! Lower Priority = Backup Router standby 1 preempt ! Enables the router with the highest priority to immediately become the active router standby 1 track 1 ! WAN SUBINTERFACE standby 2 ip 192.168.1.25 ! Virtual IP 2 (192.168.1.0 Network Takes Priority) standby 2 priority 105 ! The Higher the # The Higher the Priority standby 2 preempt ! Enables the router with the highest priority to immediately become the active router standby 2 track 1 ! WAN SUBINTERFACE ! Router bgp <ASN> network 10.10.10.0 mask 255.255.255.0 ! Broadcasts Gateway1 out the WAN through BGP network 192.168.1.0 mask 255.255.255.0 ! Broadcasts Gateway2 out the WAN through BGP
Router 1:
track 1 interface Serial0/0/0.1 ip routing ! Points at the interface that needs to be Prioritized interface FastEthernet0/0 description interface to LAN ip address x.x.x.x 255.255.255.0 standby 1 ip <Gateway> ! Virtual IP standby 1 priority 105 ! Higher Priority = Primary Router standby 1 preempt ! Enables the router with the highest priority to immediately become the active router standby 1 track 1 ! WAN SUBINTERFACE ! Router bgp <ASN> network <Gateway> mask 255.255.255.0 ! Broadcasts Gateway out the WAN through BGP
Router 2:
track 1 interface Serial0/0/0.1 ip routing ! Points at the interface that needs to be Prioritized interface FastEthernet0/0 description interface to LAN ip address x.x.x.x 255.255.255.0 standby 1 ip <Gateway> ! Virtual IP standby 1 priority 100 ! Lower Priority = Backup Router standby 1 preempt ! Enables the router with the highest priority to immediately become the active router standby 1 track 1 ! WAN SUBINTERFACE ! Router bgp <ASN> network <Gateway> mask 255.255.255.0 ! Broadcasts Gateway out the WAN through BGP
This is what you should see in a Primary/Backup situation (notice how Router 1 is active):
Router1# show standby Ethernet0 - Group 1 Local state is Active, priority 105, may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 1.458 Virtual IP address is 171.16.6.100 configured Active router is local Standby router is 171.16.6.6 expires in 8.428 Virtual mac address is 0000.0c07.ac01 2 state changes, last state change 02:09:49 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 1 interface, 1 up: Interface Decrement State Serial0 10 Up Router2# show standby Ethernet0 - Group 1 Local state is Standby, priority 100, may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 1.814 Virtual IP address is 171.16.6.100 Active router is 171.16.6.5, priority 105 expires in 9.896 Standby router is local 3 state changes, last state change 00:10:21 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 1 interface, 1 up: Interface Decrement State Serial1 10 Up
This is what it will look like during a successful fail-over (notice how the Router 2 is active and HSRP Priority was lowered on Router 1 to 95):
R1# show standby Ethernet0 - Group 1 Local state is Standby, priority 95 (confgd 105), may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 2.670 Virtual IP address is 171.16.6.100 configured Active router is 171.16.6.6, priority 100 expires in 8.596 Standby router is local 4 state changes, last state change 00:01:45 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 1 interface, 0 up: Interface Decrement State Serial0 10 Down R2# show standby Ethernet0 - Group 1 Local state is Active, priority 100, may preempt Hellotime 3 sec, holdtime 10 sec Next hello sent in 0.810 Virtual IP address is 171.16.6.100 Active router is local Standby router is 171.16.6.5 expires in 9.028 Virtual mac address is 0000.0c07.ac01 4 state changes, last state change 00:01:38 IP redundancy name is "hsrp-Et0-1" (default) Priority tracking 1 interface, 1 up: Interface Decrement State Serial1 10 Up
Load Sharing will be similar except both routers will have an active and a standby. Just make sure the correct group is active.