Firsthop Redundancy Protocol

As the name suggests it is a redundancy protocol that manages multiple gateway routers, i.e., it automatically replaces a failed gateway router with a functional backup (redundant) gateway router. FHRP is a not single protocol in general, but a class of protocols that provide similar functions:

• HSRP – Hot Standby Router Protocol
• VRRP – Virtual Router Redundancy Protocol
• GLBP – Gateway Load Balancing Protocol.  

Working of FHRP:

• FHRP uses a virtual IP address and a virtual MAC address to be assigned on the routers which are to be used in FHRP as active/standby routers. 
• These virtual IP addresses and virtual MAC addresses are used as the default gateway IP address and default gateway MAC address on the end hosts.
• By default, the active router is responsible for forwarding the data (coming from the end hosts) across the different networks, and if the active router goes down then the standby router takes its place and responsibilities to be the gateway router for the end hosts. 

Hot Standby Router Protocol (HSRP):

• CISCO proprietary protocol
• Uses port no. 1985
• Allows several routers (or MLS) to appear as a single gateway using virtual IP address.
• HSRP multicasts its hello messages to 224.0.0.2 at regular intervals (default – 3 seconds).

Working:

• Routers or Multi-Layer Switches (MLS) that are supposed to provide redundancy for a given gateway address are assigned to a common HSRP group.
• One router is elected as the Active HSRP router, while another router is elected as a Standby HSRP router and all the other routers remain in the Listen state.
• Routers exchange HSRP hello messages at regular intervals so that they can be aware of each other’s existence and who is the current Active router.

HSRP Router Election Criteria:

1. Highest Priority (range 1 – 255, default – 100)
2. The highest IP address on the interface (which is part of the HSRP group)

Devices participating in HSRP must progress their interfaces through the following states: 

1. Disabled
2. Init
3. Listen
4. Speak
5. Standby
6. Act

Preemption is a feature in FHRP which enables all the routers in the group to participate in the Active Router election based on the highest priority (at any point in time). By default, Preemption is disabled in HSRP, and therefore once the Active router is elected it does not change its role until preemption is enabled. Whereas, Elections between other routers for the role of Standby can take place any time based on the eligibility criteria.

Virtual Router Redundancy Protocol (VRRP):

1. Open Standard protocol
2. IP protocol no. 112
3. Uses Multicast address 224.0.0.18 to send its advertisements.
4. Preemption is enabled by default. 
5. Versions: 
   o VRRP v2 supports IPv4 only.
   o VRRP v3 supports IPv4 and IPv6 (whereas both versions of IP addresses cannot be used simultaneously).

Working:

1. Routers or Multi-Layer Switches (MLS) that are supposed to provide redundancy for a given gateway address are assigned to common VRRP group.
2. One router is elected as the Master Router, while all the remaining routers are Backup Routers.
3. Master Router sends Advertisements at regular intervals (default 1 second) while all the other Backup Routers learn this advertisement.

VRRP Router Election Criteria: 

1. Highest Priority (range 1 – 255, default – 100)
2. The highest IP address on the interface (which is part of the VRRP group)

Gateway Load Balancing Protocol (GLBP):

1. Cisco proprietary protocol
2. Uses port no. 3222
3. Uses multicast address 224.0.0.102 to send its hello messages (default – 3 seconds).
4. Preemption is disabled by default.
5. Provides load-balancing in such a way that all the routers in a GLBP group can forward traffic coming from the end hosts.

 Components of GLBP:

• Active Virtual Gateway (AVG): There is only a single AVG in a GLBP group responsible for answering all the ARP requests coming for the Virtual MAC address or the Virtual IP address (generated by end-hosts). The AVG then replies with the Virtual MAC address of the AVF (Active Virtual Forwarder) assigned to that host.
• Active Virtual Forwarder (AVF): There can be at most 4 AVF’s in a GLBP group which are responsible for forwarding data.

How GLBP provides Load-Balancing: 

• When multiple routers are assigned to a GLBP Group, one router is elected as an AVG and one router is elected as a standby AVG, and including these 2 routers, there can be a maximum of 4 AVF’s in a single GLBP group.
• Since all the routers in the group have the same virtual IP address, all routers can offer load-balancing by forwarding a portion of overall traffic.
• This load-balancing is provided completely by the use of a Virtual MAC address which is assigned by AVG to all the AVF’s (unique in the group) and these Virtual MAC addresses are sent to the host as their Gateway MAC address in the ARP responses.