Topic: Describe the purpose and functions of first hop redundancy protocols

Did you ever think about how important it is to reach the Internet? Such reachability is more important today than it’s ever been as we are relying more and more on cloud-based resources. The first hop when reaching these resources is the default gateway. How can we help ensure that a default gateway is always available? One method is to use a first hop redundancy protocol (FHRP). A FHRP ensures that at least one of multiple default gateways can be reached by clients that rely on them for access beyond their local network—even in the presence of some failures.

There are three FHRPs you should be knowledgeable about as you prepare to take the CCNA 200-301 exam:

• ▸ Hot Standby Router Protocol (HSRP): This is the Cisco invention that basically gave rise to the world of FHRPs. With HSRP, you configure the default gateway on hosts with a “phantom” IP address that the healthy default gateway in the HSRP group can respond to.

• ▸ Virtual Router Redundancy Protocol (VRRP): This is the open standard version of HSRP. There are more similarities between VRRP and HSRP than there are differences.

• ▸ Gateway Load Balancing Protocol (GLBP): This Cisco invention is super clever. GLBP not only allows a healthy default gateway to respond to clients but also permits dynamic load balancing between the multiple healthy default gateways that might exist. Load balancing is technically possible in HSRP and VRRP, but the configuration of such load balancing is laborious and unnecessarily complex, especially compared to the simplicity of GLBP.

As I am sure you know, HSRP allows multiple default gateways to respond to clients and route traffic onto and off their LAN segment.

With HSRP, each router presents a virtual IP address to the LAN segment. The active router can respond to ARP requests to this virtual IP address and forward traffic. The device that forwards traffic is called the active router, and the others in the group are called standby routers.

The HSRP routers communicate with each other every three seconds by default to ensure that they are up. There is also a dead timer, which is by default 10 seconds.

As mentioned earlier, VRRP is almost identical to HSRP. The biggest difference between them is that VRRP is not a Cisco-proprietary invention but an open standard. Like HSRP, VRRP dynamically allows several routers on a multiaccess link to utilize the same virtual IP address. In a VRRP configuration, one router is elected as the virtual router master, and the other routers act as backups in case the virtual router master fails.

Realizing that improvements were needed in the HSRP approach to first hop reachability, Cisco Systems worked hard on GLBP and succeeded in delivering a much more flexible and clever protocol. Once again, multiple first hop routers on the LAN combine to offer a single virtual first hop IP router; however, with GLBP, load balancing with other healthy routers is possible.

GLBP provides load balancing over multiple routers using a single virtual IP address and multiple virtual MAC addresses. These multiple virtual MAC addresses are the magic behind the scenes that permit all of the healthy routers to help share the forwarding load.

Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group. Conveniently, the other group members provide backup for the AVG in the event that the AVG becomes unavailable.

The AVG assigns a virtual MAC address to each member of the GLBP group. Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. Each of these gateways is known as the active virtual forwarder (AVF) for its virtual MAC address.

The AVG is responsible for answering Address Resolution Protocol (ARP) requests for the virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests with different virtual MAC addresses.

For more information on the exciting GLBP protocol, be sure to see the GLBP video suggested in the “Additional Resources” section at the end of this chapter.

Review Questions

1. What is the name for the router that is responsible for assigning traffic to routers in GLBP?

A. AVF

B. AVR

C. AVG

D. AVA

2. How is the load sharing of GLBP ultimately accomplished?

A. Using a round robin approach based on the physical IP addresses of gateways

B. Using virtual MAC addresses

C. Using multiple virtual IP addresses

D. Using the routing table of the IGP

3. In HSRP, what is the term for routers that are not forwarding traffic for the LAN but are waiting to see if they are needed?

A. Distinct routers

B. Backup routers

C. Standby routers

D. Secondary routers

4. In VRRP, what is the name of the router that is forwarding traffic for the clients?

A. Virtual router master

B. Main router

C. Active router

D. Primary router

Answers to Review Questions

1. C is correct. The active virtual gateway (AVG) responds to ARP requests with the different virtual MAC addresses of the healthy gateways (AVFs). This creates the load balancing.

2. B is correct. Virtual MAC addresses are used to distribute the load from clients among healthy routers in the infrastructure.

3. C is correct. HSRP uses the concept of active routers and standby routers.

4. A is correct. The forwarding device in VRRP is termed the virtual router master.

Additional Resources

Understanding (and Configuring) HSRP

https://youtu.be/-IaUa4-6ZeI

What Is Administrative Distance?

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html

GLBP Operation

https://youtu.be/ujApoqozzsE