Overview:
Hot Standby Routing Protocol (HSRP) enables high availability by providing multiple default gateways with the same virtual IP address to hosts on a LAN.
Study Notes:
- HSRP is a Cisco proprietary protocol
- HSRP routes IP traffic without relying on the availability of any single router on the LAN
- Multiple router interfaces work together to present a single virtual IP address as the default gateway to LAN hosts
- Those routers share a virtual MAC address and virtual IP address
- The virtual IP can be used by all routers in the HSRP group
- One of the routers is selected as the active router and the other as the standby
- The standby router will assume control of the virtual MAC and virtual IP when the active router fails
- A new standby router is also selected at the time the active fails to the existing standby router
- Devices running HSRP send and receive multicast UDP-based hello packets to detect router failure and designate active and standby routers
- It is possible for switches to run HSRP. To do so, they would need to be configured with some form of Layer 3 interface (Routed port, SVI, Port-channel)
5.5.a Priority
- Assigning a priority allows you to select the active and standby routers
- Priority is a required configuration setting
- The range is 1 to 255
- The default priority is 100
- The highest number represents the highest priority
5.5.b Preemption
- Preemption is an optional configuration setting
- If preemption is enabled, the router with the highest priority becomes the active router
- If priorities are equal, the current active router does not change
- The priority of the device can change dynamically if an interface is configured with the "standby track" command and another interface on the router goes down
5.5.c Version
- HSRPv1—Version 1 of the HSRP (the default) comes with these features:
- Version 1 is the default HSRP version
- The HSRP group number can be from 0 to 255
- HSRPv1 uses the multicast address 224.0.0.2 to send hello packets
- HSRPv1 and CGMP could conflict with each other so only one of the protocols can be enabled. They are mutually exclusive
- HSRPv2—Version 2 of the HSRP has these features:
- HSRPv2 can use a group number from 0 to 4095 and a MAC address from 0000.0C9F.F000 to 0000.0C9F.FFFF
- Recommended to match the HSRP group number to the VLAN ID of a subinterface
- HSRPv2 uses the multicast address 224.0.0.102 to send hello packets.
- HSRPv2 has a different packet format than HRSPv1
- HSRPv2 and CGMP leave processing are no longer mutually exclusive, and both can be enabled at the same time.
- A switch running HSRPv1 cannot identify the physical router that sent a hello packet because the source MAC address of the router is the virtual MAC address.
- HSRPv2 has a different packet format than HSRPv1. A HSRPv2 packet uses the type-length-value (TLV) format and has a 6-byte identifier field with the MAC address of the physical router that sent the packet.
- If an interface running HSRPv1 gets an HSRPv2 packet, the type field is ignored.
- The version of an HSRP group can be changed from HSRPv2 to HSRPv1 only if the group number is less than 256.
- If you change the HSRP version on an interface, each HSRP group resets because it now has a new virtual MAC address.
PacketTracer Lab: CCNA-5.5-Configure-verify-and-troubleshoot-basic-HSRP.pkt
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