Gateway Load Balancing Protocol GLBP Tutorial

HSRP and VRRP share the same basic drawback: only one gateway in the group is ever elected active and actually forwards traffic — every other gateway just sits idle until that active one fails. Gateway Load Balancing Protocol (GLBP) is a Cisco-proprietary protocol built to do the same basic job as HSRP and VRRP, but with real load balancing across every member of the group. This tutorial covers how GLBP actually works.

Note: it’s possible to get a rough approximation of load balancing out of HSRP or VRRP by running multiple groups and splitting hosts across different default gateways — but that means manually assigning different gateways to different hosts, and if one group fails, those hosts’ default gateways have to be manually reconfigured too. That’s a real administrative burden GLBP avoids entirely.

Note: GLBP has been dropped from the current ENCOR syllabus, but it can still turn up in exam questions, so it’s worth knowing.

GLBP election

When a set of gateways is configured into a GLBP group, the first thing that happens is electing one of them as the Active Virtual Gateway (AVG) — based on priority, with the highest priority winning; if priorities tie, the gateway with the highest real IP address wins instead.

The AVG’s job, once elected, is to hand out a virtual MAC address to each member of the group. Any gateway holding one of these virtual MAC addresses is called an Active Virtual Forwarder (AVF). A GLBP group supports at most four AVFs — if a group has more than four gateways, the extras become Standby Virtual Forwarders (SVF), ready to step into an AVF role if one fails. GLBP’s virtual MAC addresses follow the pattern 0007.b400.xxyy, where xx is the GLBP group number and yy distinguishes each gateway (01, 02, 03, and so on).

Note: this tutorial uses “gateway” and “router” interchangeably — GLBP actually runs on both routers and Layer 3 switches, so “gateway” is really the more accurate umbrella term. On the switch side, GLBP is only supported on the Cisco 4500 and 6500 series.

Among the gateways that didn’t win the AVG election, whichever has the highest priority becomes the Standby Virtual Gateway (SVG) — ready to step into the AVG role if the current AVG goes down.

Take the topology above as an example: assuming every gateway has the same priority and GLBP comes up on all of them at the same time (or they’re all configured with the preempt feature), R4 wins the AVG role for having the highest IP address, 10.10.10.4. R3 becomes SVG, having the next-highest IP, 10.10.10.3. The AVFs are then chosen by weight — and since there are only four gateways total here, all four naturally become AVFs. As with HSRP, there’s still a single shared virtual IP address, assigned by the administrator via the glbp ip ... command — for example, glbp 1 ip 10.10.10.100.