Gateway Load Balancing Protocol (GLBP)
- GLBP is Cisco proprietary, and acts like HSRP/VRRP with true load-balancing capability: all routers in a group forward traffic simultaneously.
- GLBP group numbers range from 0 to 1023. Priorities range from 0 to 255 (default is 100).
- GLBP advertisements are multicast to 224.0.0.102
- hello/hold timers (default 3/10 seconds)
- Timers only need to be configured on the AVG; other routers will learn from it.
- Active Virtual Gateway (AVG)
The AVG has the highest priority in the GLBP group (or the highest IP address in the event of a tie); it answers all ARP requests for the group’svirtual IP address. - Active Virtual Forwarder (AVF)
All routers sharing load in GLBP are AVFs.
If an AVF fails, the AVG reassigns its virtual MAC to another router. - Two timers are used to age out the virtual MAC of a failed AVF:
Redirect timer (default 600 seconds) – Determines when the AVG will stop responding to ARP requests with the MAC of the failed AVF
Timeout timer (default 4 hours) – Determines when the failed AVF is no longer expected to return, and its virtual MAC will be flushed from the GLBP group - AVFs are assigned a maximum weight (1-254; default is 100).
IP address(es), router preemption, and hello/hold timers (default 3/10 seconds) can be configured like for HSRP:
glbp 1 ip [virtual_ip]
glbp 1 priority [priority]
glbp 1 preempt
glbp 1 forwarder preempt
Configuring the timers:
glbp 1 timer [hello] [dead]
glbp 1 timer redirect [redirect] [time-out]
Interfaces can be tracked and the AVF’s weight adjusted when interfaces go down
glbp 1 weighting [weight] lower [lower] upper [upper]
glbp 1 weighting track [object] decrement [value]
When the upper or lower threshold is reached, the AVF enters or leaves the group, respectively.
Load Balancing
Up to four virtual MACs can be assigned by the AVG.
Traffic can be distributed among AVFs using one of the following methods:
Round robin (default) – Each new ARP request is answered with the next MAC address available; traffic is distributed evenly among AVFs
Weighted – AVFs are assigned load in proportion to their weight
Host-dependent – Statically maps a requesting client to a single AVF MAC
Configuring load balancing:
glbp 1 load-balacing [method]
Verification
show glbp [brief]
