HSRP are one of commonly used First Hop Redundancy Protocols (FHRP). We also have the (VRRP) and (GLBP).
HSRP are Cisco proprietary redundancy protocol that’s allows failover of next-hop IP devices. It’s operates with the Active-Standby Model where are only one device is supporting end-user traffic at any time, and other device are on standby waiting to the takeover should be the active device fail.
One major factor’s that network’s architects or engineers consider when the designing network is high availability (HA) and avoiding are single point of the failure (SPOF). It important are especially for the critical operations like healthcare, emergency, government, & banking services.
How Does HSRP Work?
In the example above, we have the two Layer 3 Cisco Switches in our Distribution Layer, MLS1 & MLS2. Behind these are the Layer 3 Switches is our Access Layer Switch, where in our end-user connects to. If either L3 Devices or the physical link fails, we need are dynamic way to the failover the traffic from MLS1 to the MLS2, and HSRP will be take care of it.
Once we are configuring HSRP on L3 devices, it’s selects Active Router based on higher priority. If the both routers have default priority, the router with higher HSRP interface IP Address becomes Active Router.
For configuration we need to HSRP Virtual IP address (VIP), which should on same subnet as HSRP interfaces. You cannot set physical IP as VIP. After are selection, the Active Router implements the VIP and the virtual MAC Address and starts responding to the ARP requests from hosts.
HSRP Messages
With HSRP, there are three types of the multicast messages sent between are devices:
Hello – sent between Active & Standby devices every 3 seconds by the default. If MLS2 does not hear from the MLS1 in the10 seconds, MLS2 will be take over active role.
Resign – sent by active device when it going the offline or ready to the give up active role for some other’s reason. This message agree tells MLS2 to be ready & take over the active roles.
Coup – used when the standby router wants to the assume the active role (pre-empt).
The command to the configure a pre-empt delay in the HSRP are:
(config-if)# standby <HSRP group> preempt delay minimum <0-3600>
HSRP States
Listed below the different are:
Initial – when link come up.
Learn – HSRP are device is trying to the learn VIP.
Listen – the device are knows VIP and listens for the hello messages from other HSRP devices.
Speak – the device sends the hello messages and participates in selection to the become the active router.
Standby – the device are patiently waiting to the takeover should be the active router fails.
Active – the device receives are the data intended for the VIP.
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