6.3.a Implement and troubleshoot first-hop redundancy protocols

6.3.a [i] HSRP, GLBP, VRRP

HSRP, GLBP, and VRRP are examples of First Hop Redundancy Protocols (FHRP).

HSRP is the Cisco standard method of providing high network availability by providing first-hop redundancy for IP hosts on an IEEE 802.3 LAN configured with a default gateway IP address. HSRP routes IP traffic without relying on the availability of any single router. It enables a set of router interfaces to work together to present the appearance of a single virtual router or default gateway to the hosts on a LAN. When HSRP is configured on a network or segment, it provides a virtual Media Access Control (MAC) address and an IP address that is shared among a group of configured routers. HSRP allows two or more HSRP-configured routers to use the MAC address and IP network address of a virtual router. The virtual router does not exist; it represents the common target for routers that are configured to provide backup to each other. One of the routers is selected to be the active router and another to be the standby router, which assumes control of the group MAC address and IP address should the designated active router fail.

VRRP is an election protocol that dynamically assigns responsibility for one or more virtual routers to the VRRP routers on a LAN, allowing several routers on a multi-access link to use the same virtual IP address. A VRRP router is configured to run VRRP in conjunction with one or more other routers attached to a LAN. In a VRRP configuration, one router is elected as the virtual router master, and the other routers act as backups in case it fails.

GLBP provides automatic router backup for IP hosts configured with a single default gateway on a LAN. Multiple first-hop routers on the LAN combine to offer a single virtual first-hop IP router while sharing the IP packet forwarding load. Other routers on the LAN act as redundant GLBP routers that become active if any of the existing forwarding routers fail. GLBP provides load balancing over multiple routers by using a single virtual IP address and multiple virtual MAC addresses. Each host is configured with the same virtual IP address, and all routers in the virtual router group forward packets.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 5820-5829).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/switches/metro/catalyst3750m/software/release/12-2_58_se/configuration/guide/3750metro_scg/swhsrp.html