Chapter 19: First hop redundancy protocols
The introduction explains why redundancy must extend beyond a LAN and identifies FHRPs as a way to provide hosts with a redundant default gateway.
After this chapter you will be able to
- Redundant default gateway role. Describe the purpose of first hop redundancy protocols in terms of routers and hosts in a LAN.
- Redundant first-hop gateway. Explain that an FHRP lets multiple routers work together to provide LAN hosts with a redundant default gateway, reducing downtime if hardware fails. State why the gateway is called the first hop.
- Default gateway failure problem. Explain why having two routers does not by itself keep hosts connected to external destinations when their default-gateway router fails.
- Virtual gateway takeover. Describe how the shared virtual IP address lets R2 take over the default-gateway role after R1 fails.
- FHRP Hello Messages. Explain how periodic multicast hello messages establish and maintain FHRP neighbor relationships, and what a router assumes when a neighbor's hellos stop arriving.
- Active and Standby Roles. Describe how hello messages establish active and standby router roles and what each role does.
- Shared Virtual MAC Address. Trace what happens when a host ARPs for the virtual IP address and then sends traffic toward an external destination.
- Failover and host continuity. Explain what failover is and why LAN hosts do not need to change their default gateway IP address or ARP table when the standby router becomes active.
- Switch MAC table relearning. Trace how gratuitous ARP messages from the new active router allow switches to update their MAC address tables after failover.
- HSRP roles and preemption. Explain which router serves as the default gateway in HSRP, what the standby router does, and what happens when a previously active router recovers with default settings.
- HSRP load balancing across subnets. Explain how assigning different active routers to different subnet-specific HSRP groups distributes gateway traffic.
- Cross-vendor gateway redundancy. Explain why VRRP, rather than HSRP, is needed when routers from different vendors cooperate to provide a redundant default gateway.
- Per-subnet load balancing limit. Describe how VRRP can distribute gateway duties across subnets and distinguish that from load balancing within one subnet.
- Per-host gateway load balancing. Explain that GLBP lets hosts in one subnet use different routers as their default gateway, distributing traffic on a per-host basis.
- AVG and AVF forwarding. Trace how GLBP's Active Virtual Gateway assigns Active Virtual Forwarders and answers ARP requests so hosts using one virtual IP address send frames to different routers.
- Gateway redundancy purpose. Explain why a LAN with two routers can still lose connectivity when hosts use a failed router as their default gateway, and how HSRP reduces that disruption.
- Matching HSRP group settings. Describe the basic settings the two routers must share and the IP addresses they must keep separate to form an HSRP group.
- Active router selection and preemption. Explain how priority and the IP-address tiebreaker select the active router, and how preemption affects the roles after a failure and recovery.
What it builds on
- The need for STP (chapter 14) · The importance of redundancy within a LAN.
- How end hosts send packets (chapter 9) · How end hosts send packets toward destinations outside their network.
- Address Resolution Protocol (chapter 6) · Hosts use ARP to learn a gateway's MAC address.
- Neighbors and adjacencies (chapter 18) · The comparison with OSPF neighbor relationships.
- MAC address learning (chapter 6) · Switches learn source MAC addresses on ports.
Section by section
Introduction
The introduction explains why redundancy must extend beyond a LAN and identifies FHRPs as a way to provide hosts with a redundant default gateway.
- Redundancy beyond the LAN: Explain why network redundancy includes paths to destinations outside the LAN, not just paths within it.
Why it matters: A resilient LAN alone is not enough if hosts have only one way to reach destinations beyond it. - Redundant default gateway role: Describe the purpose of first hop redundancy protocols in terms of routers and hosts in a LAN.
Why it matters: It identifies the host’s default gateway as something that can be made redundant.
Key terms: redundancy, single points of failure, LAN, first hop redundancy protocols, FHRPs, redundant default gateway
19.1 FHRP concepts
Defines FHRPs as protocols that let routers provide a redundant default gateway for LAN hosts and explains the term “first hop.”
- Redundant first-hop gateway: Explain that an FHRP lets multiple routers work together to provide LAN hosts with a redundant default gateway, reducing downtime if hardware fails. State why the gateway is called the first hop.
Why it matters: A redundant gateway helps hosts keep communicating when gateway hardware fails.
Key terms: first hop redundancy protocol (FHRP), default gateway, first hop
19.1.1 Providing a redundant default gateway
This section explains why two routers alone do not provide a redundant default gateway and how an FHRP lets hosts keep using one virtual gateway IP when a router fails.
- Default gateway failure problem: Explain why having two routers does not by itself keep hosts connected to external destinations when their default-gateway router fails.
Why it matters: Extra paths do not help hosts reach other networks if their configured gateway cannot forward traffic. - Virtual gateway takeover: Describe how the shared virtual IP address lets R2 take over the default-gateway role after R1 fails.
Why it matters: The shared address allows the gateway role to move without manually changing each host's gateway setting. - FHRP groups across subnets: State what an FHRP group is and why one router might belong to multiple groups.
Why it matters: Groups let the redundant-gateway idea apply separately to multiple subnets.
Key terms: default gateway, FHRP, ARP, virtual IP (VIP) address, FHRP group, subnet, redundant default gateway IP address
19.1.2 FHRP neighbor relationships
This section explains how FHRP routers maintain neighbor relationships, choose gateway roles, and use a shared virtual MAC address so hosts can reach the active router.
- FHRP Hello Messages: Explain how periodic multicast hello messages establish and maintain FHRP neighbor relationships, and what a router assumes when a neighbor's hellos stop arriving.
Why it matters: The hello messages let redundant routers detect when a gateway may need to be replaced. - Multicast Resource Advantage: Justify why FHRPs send their hello messages by multicast rather than broadcast in this LAN.
Why it matters: Multicast reduces unnecessary work for hosts that are not participating in the FHRP. - Active and Standby Roles: Describe how hello messages establish active and standby router roles and what each role does.
Why it matters: These roles explain which router handles gateway traffic now and which one is prepared to replace it. - Shared Virtual MAC Address: Trace what happens when a host ARPs for the virtual IP address and then sends traffic toward an external destination.
Why it matters: It connects the host's gateway address to the router currently serving as gateway.
Key terms: FHRP neighbor relationship, multicast hello messages, multicast, broadcast, active router, standby router, default gateway, VIP, virtual MAC address, ARP table entry
19.1.3 Failover
This section explains how an FHRP standby router takes over after an active router fails, how switches find the new active router, and what can happen when the original router recovers.
- Failover and host continuity: Explain what failover is and why LAN hosts do not need to change their default gateway IP address or ARP table when the standby router becomes active.
Why it matters: The shared virtual addresses let hosts keep using their gateway through a router failure. - Switch MAC table relearning: Trace how gratuitous ARP messages from the new active router allow switches to update their MAC address tables after failover.
Why it matters: Hosts can retain their gateway information only if switches can deliver their frames to the router now acting as the gateway. - Recovery and preemption outcomes: Distinguish the two possible roles of the original active router after it recovers, and state which outcome is called preemption.
Why it matters: Recovery does not necessarily put the original router back in charge; preemption names the case in which it does.
Key terms: failover, active router, standby router, VIP, virtual MAC address, gratuitous ARP, GARP, broadcast MAC address, MAC address tables, preemption
19.2 Comparing FHRPs
Introduces the three FHRPs supported by Cisco routers and points readers to a comparison table for their characteristics.
19.2.1 Hot Standby Router Protocol
Explains HSRP’s active and standby roles, hot-standby failover, version differences, and load balancing across subnets.
- HSRP roles and preemption: Explain which router serves as the default gateway in HSRP, what the standby router does, and what happens when a previously active router recovers with default settings.
Why it matters: These roles explain both gateway continuity after a failure and why recovery does not necessarily change the active router. - Hot versus cold standby: Distinguish an HSRP hot standby from a cold standby and explain why the hot standby allows a smoother response to failure.
Why it matters: An already-running backup can restore gateway service without the manual work a stored spare would require. - HSRP version differences: Compare HSRPv1 and HSRPv2 by their hello multicast addresses, virtual MAC address formats and group capacity, and IPv6 support.
Why it matters: The version determines which hello address and virtual MAC format appear, how many groups are available, and whether IPv6 is supported. - HSRP load balancing across subnets: Explain how assigning different active routers to different subnet-specific HSRP groups distributes gateway traffic.
Why it matters: Splitting active gateway duties across subnets avoids concentrating normal traffic on a single router or link.
Key terms: HSRP, Cisco-proprietary, active router, standby router, preemption, hot standby, cold standby, HSRPv1, HSRPv2, hello messages
19.2.2 Virtual Router Redundancy Protocol
This section compares VRRP with HSRP, explaining their differences, why VRRP works across vendors, and the limits of their load balancing.
- Cross-vendor gateway redundancy: Explain why VRRP, rather than HSRP, is needed when routers from different vendors cooperate to provide a redundant default gateway.
Why it matters: The protocol choice determines whether routers from different vendors can provide a redundant gateway together. - VRRP roles and preemption: Distinguish VRRP from HSRP by naming VRRP’s router roles and comparing their default preemption settings.
Why it matters: Similar protocols use different role names and have different preemption defaults. - VRRP multicast and MAC addresses: State the multicast address used for VRRP hello messages and describe how a VRRP group’s virtual MAC address is formed.
Why it matters: These addresses are concrete identifiers that distinguish VRRP from HSRP. - Per-subnet load balancing limit: Describe how VRRP can distribute gateway duties across subnets and distinguish that from load balancing within one subnet.
Why it matters: It clarifies what kind of load sharing VRRP can provide and where its limit lies.
Key terms: VRRP, HSRP, industry-standard protocol, redundant default gateway, master, backup, active, standby, preemption, hello messages
19.2.3 Gateway Load Balancing Protocol
This section explains how GLBP balances default-gateway traffic among routers for hosts in a single subnet.
- Per-host gateway load balancing: Explain that GLBP lets hosts in one subnet use different routers as their default gateway, distributing traffic on a per-host basis.
Why it matters: It identifies the benefit that distinguishes GLBP among the FHRPs covered here. - AVG and AVF forwarding: Trace how GLBP's Active Virtual Gateway assigns Active Virtual Forwarders and answers ARP requests so hosts using one virtual IP address send frames to different routers.
Why it matters: The ARP replies explain how one virtual gateway address can direct hosts to different forwarding routers.
Key terms: Gateway Load Balancing Protocol, GLBP, per-host, default gateway, Active Virtual Gateway, AVG, Active Virtual Forwarder, AVF, VIP, virtual MAC address
19.3 Basic HSRP configuration
This section explains why redundant default gateways matter and how to configure and verify a basic two-router HSRP group.
- Gateway redundancy purpose: Explain why a LAN with two routers can still lose connectivity when hosts use a failed router as their default gateway, and how HSRP reduces that disruption.
Why it matters: HSRP helps keep users connected to resources outside their LAN when a router fails. - Matching HSRP group settings: Describe the basic settings the two routers must share and the IP addresses they must keep separate to form an HSRP group.
Why it matters: These settings determine whether the two routers can work together as one redundant default gateway. - Active router selection and preemption: Explain how priority and the IP-address tiebreaker select the active router, and how preemption affects the roles after a failure and recovery.
Why it matters: Priority selects the preferred gateway, while preemption determines whether it regains that role after recovering. - HSRP state verification: Describe how to use show standby brief to identify each router’s role and its peer, and what show standby adds.
Why it matters: Verification confirms which router is currently providing the active gateway role.
Key terms: default gateway, FHRP, HSRP, standby version 2, standby group ip vip, group number, VIP, unique IP address, active router, standby router
19.4 Summary
Concepts explained in this chapter
- first hop redundancy protocolA first hop redundancy protocol is a protocol that provides a resilient default gateway by presenting end hosts with a virtual gateway backed by multiple routers.
- virtual ip addressA virtual IP address is a shared logical IP address presented by a first-hop redundancy group as the default gateway for end hosts.
- active routerAn active router is the router in a first hop redundancy group that currently forwards traffic for the shared default gateway.
- standby routerA standby router is an HSRP router that monitors the active router and is ready to assume packet forwarding if it fails.
- virtual mac addressA virtual MAC address is a logical link-layer address associated with a virtual router or forwarding role rather than a single physical interface.
- Hot Standby Router ProtocolHot Standby Router Protocol is a Cisco-developed first hop redundancy protocol that provides a virtual default gateway through coordinated routers.
- HSRP version 2HSRP version 2 is the second version of the Hot Standby Router Protocol for providing a redundant virtual default gateway.
- HSRP hello messageA hello message is a periodically transmitted control message used by first hop redundancy routers to establish and maintain neighbor relationships.
- HSRP groupAn HSRP group is a set of HSRP-configured routers that share a virtual IP address and elect active and standby roles.
- Gateway Load Balancing ProtocolGateway Load Balancing Protocol is a Cisco first hop redundancy protocol that provides gateway redundancy while distributing traffic across multiple routers.
- HSRP version 1HSRP version 1 is the first HSRP protocol version, using multicast address 224.0.0.2 and a two-hex-digit virtual MAC group identifier.
- Virtual Router Redundancy ProtocolVirtual Router Redundancy Protocol is a standards-based first hop redundancy protocol that provides a virtual default gateway through coordinated routers.
- active virtual forwarderAn active virtual forwarder is a GLBP router role associated with a unique virtual MAC address that forwards traffic for the shared gateway.
- active virtual gatewayAn active virtual gateway is the GLBP router that assigns active virtual forwarders and answers address-resolution requests for the virtual gateway.
- failoverFailover is the transfer of an active network service or role to a standby device after the active device fails.
- gratuitous arpGratuitous ARP is an ARP reply sent without a preceding ARP request, typically to announce or refresh an address-to-MAC mapping.
- HSRP priorityHSRP priority is a numeric value used to rank routers when electing the active router.
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