When it comes to load balancing, which is the distribution of network traffic across many existing paths between servers to allow more efficient use of network bandwidth, it’s important to understand the difference between HSRP (Hot Standby Router Protocol) and GLBP (Gateway Load Balancing Protocol).
FHRP (First Hop Redundancy Protocol) is a category of protocols that provide seamless, fault-tolerant default gateways for hosts and includes HSRP, VRRP, and GLBP. HSRP and VRRP work similarly, with the most significant difference being HSRP is proprietary to Cisco. Like HSRP, GLBP is also Cisco proprietary and the most significant difference between these two protocols is how they handle load balancing.
The basic HSRP configuration will provide one active router, which does all the forwarding for all its hosts, and the standby router is simply used only if the active router fails. Load balancing with HSRP requires the creation of two HSRP groups as shown in the configuration below.
In the above configuration, Router 1 is to be the active default gateway at the address of 172.16.6.200 and Router 2 will be the active default gateway at the address of 172.16.6.100. But to utilize both gateways, you will need to configure multiple DHCP (Dynamic Host Configuration Protocol) scopes to assign some clients to each of the two different default gateway addresses for a given subnet. And even then, the load balancing will not be optimal.
GLBP has the redundant gateways share multiple, dynamically-assigned virtual MAC addresses and both routers are considered active for any given subnet.
In the above example, all clients in the subnet will be assigned the default gateway of 172.16.6.100. By default, if no load-balance algorithm is specified (as in the above example) then all traffic will be directed to only one router with no occurrence of load balancing.
With the use of the command below, load balancing can be configured one of three ways:
glbp group# load-balancing [host-dependent | round-robin | weighted]
- Round-robin: GLBP responds to each of the clients’ gateway ARP requests with the virtual MAC addresses of the routers in an alternating fashion, load balancing the traffic across all the routers.
- Weighted: The primary router uses the advertised weight for secondary routers to decide the load that will be directed to them. (Requires the weight of each router to be configured)
- Host dependent: GLBP uses the MAC address of the host to determine which virtual MAC address to direct the host to use. This algorithm guarantees that a host gets the same virtual MAC address if the number of virtual forwarders does not change.
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