Multilayer Switching: VRRP and GLBP

continued… VVRP has preemption enabled by default.


The primary purpose of the Gateway Load Balancing Protocol (GLBP) is just that – load balancing!

Cisco proprietary

GLBP uses a round robin format to distribute traffic to all group routers.

Multiple gateways in use at the same time.

GLBP also allows standard configuration of the hosts, who will all have their gateway address set to the virtual router’s address.

Key Concept:

The key to GLBP is that when a host sends an ARP request for the MAC of the virtual router, one of the physical routers will answer with its own MAC address. The host will then have the IP address of the GLBP virtual router and the MAC address of a physical router in the group.

 The Active Virtual Gateway (AVG) will be the router with the highest GLBP priority, and this router will send back ARP responses containing different virtual MAC addresses.

The three hosts will have the same Layer 3 address for their gateway, but a different L2 address.

The routers receiving and forwarding traffic received on this virtual MAC address are Active Virtual Forwarders (AVFs).

GLBP routers use Hellos to detect whether other routers in their group are available or not.

GLBP groups can have up to four members.

If the AVG fails, the router serving as the standby AVG will take over.

GLBP offers three different forms of MAC address assignment, the default being round-robin. With round-robin assignments, a host that sends an ARP request will receive a response containing the next virtual MAC address in line.

If a host or hosts need the same MAC gateway address every time it sends an ARP request, host-dependent load balancing is the way to go.

Weighted MAC assignments affect the percentage of traffic that will be sent to a given AVF. The higher the assigned weight, the more often that particular router’s virtual MAC will be sent to a requesting host.


MLS(config-if)# glbp 5 priority 150
MLS(config-if)# glbp 5 preempt

GLBP Weighting

A router can be configured to give up its role as the AVF if its overall weight drops below a configured value. The default weight of a GLBP AVF is 100. The router is configured with upper and lower weight thresholds, and should the router’s weight fall below its lower threshold, it gives up the role of AVF. When the router’s GLBP weight exceeds the higher threshold, it resumes the role of GLBP AVF. Before configuring the GLBP-specific commands, we configure track statements to number and name the interfaces being tracked.


Multilayer Switching: HSRP Interface Tracking + Troubleshooting + Intro to VRRP

HSRP Interface Tracking

The default decrement in the priority when the tracked interface goes down is 10.

‘show standby’ will show the interface being tracked, the state and the decrement value.

A state change can be caused by either a Layer 1 or Layer 2 failure on the tracked interface.

You can change the default decrement at the end of the standby interface command.

You can track more than 1 interface, be aware of combined decrement values when doing this.

If you see HSRP states regularly transitioning, particularly between Speak and Standby, check your cabling – you’d be surprised how often that happens, especially in labs.

Frankly, most HSRP issues you run into fall into these categories:

  1. The secondary router didn’t become the Active router when it should have.
  2. The former Active router didn’t take back over when it came back online.

If either of those happens to you, check these values:

  1. Is the preempt command properly configured?
  2. What are the priority values of each HSRP speaker?
  3. Watch your decrement values with HSRP interface tracking. Don’t get cute with these. If you’re having a problem with interface tracking and you see decrements that don’t end in 0 or 5, I can practically guarantee they’re misconfigured.

Virtual Router Redundancy Protocol

Defined in RFC 2338, VRRP is the open-standard equivalent of the Cisco proprietary HSRP. VRRP works very much like HSRP, and is suited to a multivendor environment.

Very similar operation to HSRP.

There are some differences, a few of which are:

  • VRRP’s equivalent to HSRP’s Active router is the Master router. (Some VRRP documentation refers to this router as the IP Address Owner.) This is the router that has the virtual router’s IP address as a real IP address on the interface it will receive packets on.
  • The physical routers in a VRRP Group combine to form a Virtual Router. Note that the VRRP Virtual Router uses an IP address already configured on a router in its group, as opposed to how the HSRP router is assigned a separate virtual IP address.
  • VRRP Advertisements are multicast to
  • VRRP’s equivalent to HSRP’s Standby router state is the Backup state.
  • The MAC address of VRRP virtual routers is 00-00-5e-00-01-xx, xx is the group number in hexadecimal.
  • “preempt” is a default setting for VRRP routers.
  • As of IOS Version 12.3(2)T, VRRP now has an Object Tracking feature. Similar to HSRP’s Interface Tracking feature, a WAN interface can be tracked and a router’s VRRP priority dropped when that interface goes down.