Specifications

CHAPTER 6

First Hop Redundancy

Chapter 6

First Hop Redundancy

Specifying a default gateway leads to a single point of failure. Proxy Address Resolution Protocol (ARP) is one method

for hosts to dynamically discover gateways, but it has issues in a highly available environment. With Proxy ARP:

n Hosts ARP for all destinations, even remote.

n Router responds with its MAC.

n Problem: Slow failover because ARP entries take minutes to timeout.

Instead of making the host responsible for choosing a new gateway, router redundancy protocols enable two or more

routers to support a shared MAC address. If the primary router is lost, the backup router assumes control of traffic

forwarded to that MAC. This section refers to routers but includes those multilayer switches that can also implement

Layer 3 redundancy.

Hot Standby Router Protocol

Hot Standby Router Protocol (HSRP) is a Cisco proprietary protocol.

With HSRP, two or more devices support a virtual router with a fictitious MAC address and unique IP address. Hosts use

this IP address as their default gateway and the MAC address for the Layer 2 header. The virtual router’s MAC address is

0000.0c07.ACxx, in which xx is the HSRP group. Multiple groups (virtual routers) are allowed.

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

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