VRRP and GLBP

VRRP (Virtual Router Redundancy Protocol) is an open standard FHRP defined in RFC 5798. It allows a group of routers to share a virtual IP and MAC address, with one router acting as the Master (active forwarder) and the others as Backups. If the Master fails, a Backup takes over with minimal disruption. GLBP (Gateway Load Balancing Protocol) is a Cisco-proprietary FHRP that not only provides redundancy but also load balances traffic across multiple routers. GLBP uses a single virtual IP address but multiple virtual MAC addresses, each assigned to a different router in the group.

Without an FHRP, if a host's default gateway router fails, the host must use a different gateway IP (if configured) or rely on dynamic routing — neither of which is practical for end hosts. VRRP and GLBP provide a virtual IP that hosts use as their default gateway. The physical routers in the group cooperate to ensure that at least one router is always available to forward traffic for that virtual IP. GLBP adds the benefit of load sharing, which can improve throughput and reduce latency by distributing traffic across multiple routers.

VRRP elects a Master router based on priority (1-255, default 100). The Master owns the virtual IP and responds to ARP requests with the virtual MAC (0000.5e00.01xx, where xx is the VRRP group number in hex). It sends VRRP advertisements (protocol number 112) every 1 second (default) to the multicast address 224.0.0.18. Backups listen for advertisements; if they miss three consecutive advertisements (3-second hold timer), they declare the Master dead and initiate an election. The Backup with the highest priority becomes the new Master. Preemption is enabled by default, meaning a higher-priority router that comes online later will take over from the current Master. VRRP supports up to 255 group numbers per interface.

GLBP introduces two roles: Active Virtual Gateway (AVG) and Active Virtual Forwarder (AVF). The AVG is elected (highest priority) and is responsible for assigning virtual MAC addresses to other routers. The AVG responds to ARP requests for the virtual IP, handing out different virtual MACs to different hosts, thereby distributing traffic. Each router in the group (including the AVG) can be an AVF, forwarding traffic for one or more virtual MACs. GLBP uses a single virtual IP, but each AVF is assigned a unique virtual MAC (0007.b400.xxyy, where xx is the group number and yy is the forwarder number). GLBP sends hello messages every 1 second to 224.0.0.102 (protocol number 112). The hold time is 3 seconds by default. GLBP supports load balancing methods: round-robin (default), weighted, or host-dependent. Preemption is disabled by default for AVF but enabled for AVG.

interface GigabitEthernet0/0
 ip address 192.168.1.2 255.255.255.0
 vrrp 10 ip 192.168.1.1
 vrrp 10 priority 120
 vrrp 10 preempt

On another router:

interface GigabitEthernet0/0
 ip address 192.168.1.3 255.255.255.0
 vrrp 10 ip 192.168.1.1
 vrrp 10 priority 100
 vrrp 10 preempt

interface GigabitEthernet0/0
 ip address 192.168.1.2 255.255.255.0
 glbp 10 ip 192.168.1.1
 glbp 10 priority 120
 glbp 10 preempt

On another router:

interface GigabitEthernet0/0
 ip address 192.168.1.3 255.255.255.0
 glbp 10 ip 192.168.1.1
 glbp 10 priority 100
 glbp 10 preempt

show vrrp brief
show vrrp
show glbp brief
show glbp

Example output for show vrrp brief:

Interface          Grp Pri Time  Own Pre State   Master addr     Group addr
Gi0/0              10  120 3214       Y  Master  192.168.1.2     192.168.1.1

Example output for show glbp brief:

Interface   Grp  Fwd Pri State    Address         Active router   Standby router
Gi0/0       10   -   120 Active   192.168.1.1     local          192.168.1.3
Gi0/0       10   1   7   Active   0007.b400.0a01  local          -
Gi0/0       10   2   7   Standby  0007.b400.0a02  192.168.1.3    -

Both VRRP and GLBP operate independently of dynamic routing protocols like OSPF or EIGRP. They provide a default gateway for hosts; the routers themselves still run a routing protocol to learn routes to remote networks. VRRP and GLBP do not affect routing protocol adjacencies, but they do rely on the underlying IP connectivity. Spanning Tree Protocol (STP) can interact with FHRPs if the virtual IP is on a VLAN interface; ensure the STP root bridge aligns with the FHRP master to avoid suboptimal forwarding.

In enterprise networks, VRRP is often chosen for multi-vendor environments where interoperability is required. For example, a company with a mix of Cisco and Juniper routers might use VRRP because it is an open standard. A typical deployment involves two routers connected to the same VLAN, each with a link to the upstream network. The virtual IP is configured as the default gateway for all hosts. The network engineer sets priorities to ensure the preferred router is Master, and enables preemption so that if the Master recovers, it resumes forwarding. This is common in data center access layers or campus distribution blocks.

GLBP is preferred when load sharing across multiple routers is beneficial, such as in a high-traffic environment where a single router might become a bottleneck. For instance, a large office with hundreds of users might deploy GLBP across two routers, each handling roughly half the traffic. The network engineer can adjust the load balancing method — weighted, based on router capacity, or host-dependent for session persistence. One challenge is that GLBP is Cisco-proprietary, so it only works with Cisco devices. In a mixed-vendor environment, VRRP is the only choice.

Scale considerations: Both protocols support up to 255 groups per interface, but in practice, you rarely need more than a few. Performance is generally excellent, but the advertisement interval and hold timers can be tuned for faster convergence (e.g., 200 ms hello, 700 ms hold) at the cost of increased CPU usage. Misconfiguration often involves mismatched virtual IP addresses or group numbers between routers, causing split-brain scenarios where both routers think they are the Master. This can lead to intermittent connectivity. Another common mistake is forgetting to enable preemption, causing a recovered router to stay in backup while the lower-priority router remains Master. Always verify with show vrrp or show glbp after configuration.