What is Gateway Load-Balancing protocol (GLBP) and how to implement the configuration?

Gateway Load-Balancing protocol (GLBP)

Let’s take a look of HSRP and VRRP both provide gateway resiliency with per-subnet load balancing, the upstream bandwidth of the standby members

Gateway Load-balancing Protocol GLBP is Cisco proprietary protocol, GLBP allow dynamic selection and simultaneous use of multiple available routers gateways, and also permit automatic failover between those gateways. Multiple routers share the load of frames that, from a client perspective, are sent to a single default gate way address, with GLBP you can fully utilize resources.

(In short)

GLBP Gateway load-balancing protocol is providing redundancy gateway and true load balancing. GLBP allow a mechanism of four routers in each forwarding group. By default, the active router directs the traffic from host to each successive router in the group using Round-Robin algorithm.

GLBP Functions

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      Active Virtual Gateway (AVG): Member of a group elects one gateway to be the AVG for the group, and the other group member provide backup for the AVG in the event of AVG failure. AVG assign a different virtual MAC addresses to each member of the GLBP group.

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      Active Virtual Forwarder Each gateway assume to the responsibility for forwarding packets that are sent to the virtual MAC address assigned to that gateway AVG. these gateways are called AVF for their virtual MAC address.

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      GLBP communication GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address 224.0.0.102, User Datagram Protocol (UDP) port 3222.

GLBP provides upstream load-sharing by utilizing the redundant uplinks simultaneously. It uses link capacity efficiently, thus providing solid peak-load traffic coverage. By making use of multiple available paths upstream from router or layer 3 switch running GLBP, you can reduce output queues.

GLBP Features

·         Load-sharing we can configure GLBP so that traffic from LAN client is shared by multiple routers. Load sharing distributes the traffic load among available routers.

·         Multiple virtual routers GLBP supports up to 1.024 virtual routers as GLBP groups on each routers physical interface and up to four virtual forwarders per group.

·         Preemption GLBP allow us to preempt an AVG router with a higher-priority backup virtual gateway that has become available. Forwarder preemption works in a similar way, except weighting instead of priority and is enabled by default.

·         Efficient resource utilization: GLBP makes it possible for any router in a group to serve as a backup, which eliminates the need for a dedicated backup router because all available routers can support network traffic.

GLBP Balancing host traffic

GLBP balances the traffic using the round –robin algorithm:

• ·         When a host send an ARP message for the gateway IP address, the AVG return the virtual MAC address of one of the AVFS.
• ·         When a second host sends an ARP message, the AVG return the next virtual MAC address from list.

Remember that each GLBP router will be the designated AVF for the specific virtual MAC address that’s been assigned to it.

What is HSRP-(Hot Standby Routing Protocol)?

Hot Standby Routing Protocol (HSRP)  is a cisco proprietary  redundancy protocol that provides  back up to a router in the event of failure.

 HSRP allow you to configure two or more routers into a standby group to present the appearance of single Virtual router on the LAN. The router share the same IP address and MAC address. therefore in the event of failure of one router the host on the LAN are able to continue forwarding packets to a consistent IP and MAC address. HSRP is not provide load-balancing.

Each standby group that you define include the following routers:

• Active router
• Standby router
• Virtual router
• Any other routers that maybe attached to the subnet

Active router one router is active and other two or more routers just sit there in standby mode and won't be active unless a failure occur. 

Standby router the standby group will always have at least two routers that participating in it. The primary players in the group are the active router and one standby router that communicate to each other using multicast hello messages, hello timer in HSRP 3 seconds and hold timer is 10 seconds.

Virtual router in an HSRP group has a virtual IP address and  virtual MAC address. virtual router is not a physical router entity. the physical router that communicates as the virtual router is the current active router.

Other routers just monitor the hello messages sent by the active and standby routers to ensure that an active and standby router exists for the HSRP  group that they belong to.

Interface tracking 

HSRP enable routers can track the interface status of the outside interface. if the outside link of the active router goes down, the standby router will take over and become the active router. There is a default priority of 100 if you raise the priority  its means your router has higher priority to become active router.