VLAN Gateway Redundancy
Question
A network administrator has designed a network with two multilayer switches on the distribution layer, which act as default gateways for the end hosts.
Which two technologies allow every end host in a VLAN to use both gateways? (Choose two.)
Answers
Explanations
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A. B. C. D. E.AD.
The correct answers are B. GLBP and E. HSRP.
When there are two default gateways (multilayer switches) on the distribution layer, end hosts can use only one gateway at a time. By default, the end hosts use the gateway with the lowest IP address. However, this leads to suboptimal traffic flow, as one gateway remains unused while the other one is overloaded.
To allow every end host in a VLAN to use both gateways, two technologies can be used:
Hot Standby Router Protocol (HSRP): This is a Cisco proprietary protocol that allows two or more routers to work together to present the appearance of a single virtual router to the hosts on a LAN. HSRP routers share a virtual IP address and a virtual MAC address. One router is designated as the active router, and the other router(s) are in standby mode. The active router is responsible for forwarding packets sent to the virtual IP address. If the active router fails, the standby router(s) take over and continue forwarding packets. HSRP is commonly used for redundancy in enterprise networks.
Gateway Load Balancing Protocol (GLBP): This is a Cisco proprietary protocol that allows multiple routers to share the traffic load of a virtual IP address. GLBP routers share a virtual IP address and a virtual MAC address. Each router is responsible for forwarding packets sent to the virtual IP address. However, only one router is designated as the active virtual forwarder (AVF) at any given time, and it is responsible for forwarding the majority of the traffic. The other routers act as backup virtual forwarders (BVFs) and forward a smaller portion of the traffic. GLBP uses a hash algorithm to determine which BVF to use for each packet, so that the traffic is distributed evenly among the available gateways.
Therefore, both HSRP and GLBP provide redundancy and load balancing for the default gateways in a VLAN, allowing every end host in the VLAN to use both gateways. VRRP and MHSRP are similar protocols to HSRP but not supported by Cisco devices. VSS (Virtual Switching System) is a technology that enables two physical Cisco Catalyst switches to act as a single logical switch but does not provide redundancy or load balancing for default gateways.
