1. First Hop Redundancy Protocols (FHRP)
   1. A class of protocols that includes HSRP, VRRP, and GLBP, which allows multiple redundant routers on the same subnet to act as a single default router (first-hop router).
   2. FHRPs allows two or more routers to share a virtual IP address that acts as the default gateway for a connected and shared subnet (VLAN).
   3. Hosts in the subnet (VLAN) use the FHRP virtual IP address as their default gateway.
   4. The routers connected to the same subnet (VLAN) exchange FHRP messages to determine how to divide the workload between them.
   5. When a router fails, FHRP re-assigns traffic responsibilities between all routing devices assigned to the FHRP virtual IP address.
   6. FHRPs:
      1. Hot Standby Router Protocol (HSRP)
         1. A Cisco-proprietary protocol that allows two (or more) routers to share the duties of being the default router on a subnet, with an active/standby model, with one router acting as the default router and the other sitting by waiting to take over that role if the first router fails.
      2. Virtual Router Redundancy Protocol (VRRP)
         1. A TCP/IP RFC protocol that allows two (or more) routers to share the duties of being the default router on a subnet, with an active/standby model, with one router acting as the default router and the other sitting by waiting to take over that role if the first router fails.
         2. VRRP
            1. Virtual Router Redundancy Protocol (VRRP) uses an (active / standby) redundancy approach, and performs load balancing on a per subnet basis.
      3. Gateway Load Balancing Protocol (GLBP)
         1. A Cisco-proprietary protocol that allows two (or more) routers to share the duties of being the default router on a subnet, with an active/active model, with all routers actively forwarding off-subnet traffic for some hosts in the subnet.
         2. GLBP
            1. Gateway Load Balancing Protocol (GLBP) uses an (active / active) redundancy approach, and performs load balancing on a per host basis.
   7. Redundancy:
      1. The inclusion of extra components that are not strictly necessary to functioning, in case of failure in other components.
   8. Availability:
      1. A user's ability to use a network resource at any time.
   9. Single Point of Failure:
      1. In a network, a single device or link which, if it fails, causes an outage for a given population of users.
2. HSRP
   1. Hot Standby Router Protocol (HSRP) uses an (active / standby) redundancy approach, and performs load balancing on a per subnet basis.
   2. When two or more routers are configured to use HSRP, one will be considered the active router, and all others will be considered to be standby routers.
   3. Only the active router actively supports traffic. All traffic sent to the HSRP virtual IP address is sent to the active router.
   4. HSRP Active and Standby State:
      1. HSRP Active State:
         1. A Hot Standby Router Protocol (HSRP) state in which the router actively supports the forwarding of off-subnet packets for hosts in that subnet.
         2. The HSRP Active router implements a virtual IP address and a matching virtual MAC address.
         3. Virtual MAC Address (vMAC):
            1. For any FHRP protocol, a MAC address that the FHRP uses to receive frames from hosts.
      2. HSRP Standby State:
         1. A Hot Standby Router Protocol (HSRP) state in which the router does not currently support the forwarding of off-subnet packets for hosts in that subnet, instead waiting for the currently active router to fail before taking over that role.
      3. Two or more routers with HSRP enabled and with at least one interface in a shared VLAN (subnet) will send HSRP messages to each other to negotiate traffic responsibilities (active and standby states).
   5. HSRP Failover:
      1. When HSRP failover occurs, a standby router becomes the HSRP active router, and all traffic sent to the HSRP virtual IP address is sent to it.
      2. However, any connected LAN switches that forwarded frames to the previous HSRP active router must change their MAC address table to reflect the fact that a new device is performing IP routing as the new HSRP active router.
      3. Switches are signalled to change to a new vMAC when the new HSRP active router sends a broadcast Ethernet frame with the known vMAC as the source MAC address. The switches in the VLAN perform MAC learning normally, and now associate the vMAC with the port of the new HSRP active router.
      4. When HSRP failover occurs, the new HSRP active router sends a broadcast Ethernet frame with new vMAC address information.
         1. This message is an ARP Reply message, also called a Gratuitous ARP (GARP); it is called this because the router replies without receiving a request.
   6. HSRP Load Balancing:
      1. HSRP performs load balancing between multiple VLANs by designating different HSRP enabled routers as the HSRP active router in different VLANs. This is an effective form of load balancing because sites that would deploy a second router for redundancy are also generally large enough to place their traffic into multiple VLANs (subnets).
3. Command Reference
   1. Configuration
      1. Configure a router to participate in an HSRP group
         1. Router(config)# standby <group_number> ip <virtual_ip>
      2. Configure the HSRP priority. 0 to 255. Default 100. Higher wins.
         1. Router(config)# standby <group_number> priority <number>
      3. Configure the device to compete for HSRP active status if the current active router fails (preemption)
         1. Router(config)# standby <group_number> preempt
      4. Name and describe the HSRP group
         1. Router(config)# standby <group_number> <name> <description>
      5. Set the HSRP version (1 or 2)
         1. Router(config)# standby version {1|2}
   2. Troubleshooting
      1. List information on HSRP processes
         1. show standby
      2. List information on HSRP processes, with one line per interface participating in the process
         1. show standby brief
4. Troubleshooting
   1. Configuration Issues
      1. Devices within an HSRP Group must have matching:
         1. HSRP versions
         2. HSRP Group numbers
         3. HSRP Virtual IP Address
      2. ACLs may interfere with HSRP if configured to filter:
         1. UDP port 1985
         2. HSRPv1 Multicast 224.0.0.2
         3. HSRPv2 Multicast 224.0.0.102
   2. Design Issues
      1. The HSRP Virtual IP Address must have these characteristics:
         1. Must be in the same subnet as an interface address
         2. Must not be used by any other device
      2. Layer 2
         1. HSRP router interfaces must exist in the same VLAN