Like HSRP, Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary protocol. GLBP provides high network availability in a manner similar to HSRP and VRRP. However, unlike HSRP and VRRP, in which only a single gateway actively forwards traffic for a particular group at any given time, GLBP allows multiple gateways within the same GLBP group to actively forward network traffic at the same time.
GLBP gateways communicate through Hello messages that are sent every three seconds to the Multicast address 188.8.131.52, using UDP port 3222. This is illustrated below in Figure 34.24:
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