Importance of Redundancy
High availability is critical in most environments. Even a brief outage due to hardware failure may be considered unacceptable. Consider the following example:
To reach other networks, HostA must utilize a single gateway – SwitchA.The gateway represents a single point of failure on this network. If the gateway fails, hosts will lose access to all resources beyond the gateway.
Using multiple routers or multilayer switches can provide Layer-3 redundancy for hosts:
However, the Layer-3 redundancy must be transparent to each host. Hosts should not be configured with multiple default gateways.
Cisco supports three protocols to provide transparent Layer-3 redundancy:
• Hot Standby Router Protocol (HSRP)
• Virtual Router Redundancy Protocol (VRRP)
• Gateway Load Balancing Protocol (GLBP)
Hot Standby Router Protocol (HSRP)
Cisco developed the proprietary Hot Standby Router Protocol (HSRP) to allow multiple routers or multilayer switches to masquerade as a single gateway. This is accomplished by assigning a virtual IP and MAC address to all routers participating in an HSRP group.
Routers within the same HSRP group must be assigned the same group number, which can range from 0 to 255. However, most Cisco platforms only support 16 configured HSRP groups.
HSRP routers are elected to specific roles:
• Active Router – router currently serving as the gateway.
• Standby Router – backup router to the Active Router.
• Listening Router – all other routers participating in HSRP.
Only one active and one standby router are allowed per HSRP group. Thus, HSRP provides Layer-3 redundancy, but no inherent load balancing.
Hello packets are used to elect HSRP 3 seconds
The role of an HSRP router is dictated by its priority. The priority can range from 0 – 255, with a default of 100. A higher priority is preferred.
HSRP LAB
Virtual Router Redundancy Protocol (VRRP)
The Virtual Router Redundancy Protocol (VRRP) is an industry-standard Layer-3 redundancy protocol, originally defined in RFC 2338. VRRP is nearly identical to HSRP, with some notable exceptions:
• The router with the highest priority becomes the master router.
• All other routers become backup routers.
• The virtual MAC address is the reserved 0000.5e00.01xx, with xx representing the hexadecimal group number.
• Hello packets are sent every 1 second, by default, and sent to multicast address 224.0.0.18.
• VRRP will preempt by default.
• VRRP cannot directly track interfaces – it can track an object which is
tied to an interface, though.
VRRP LAB
Gateway Load Balancing Protocol (GLBP)
To overcome the shortcomings in HSRP and VRRP, Cisco developed the proprietary Gateway Load Balancing Protocol (GLBP).
Routers are added to a GLBP group, numbered 0 to 1023. Unlike HSRP and VRRP, multiple GLBP routers can be active, achieving both redundancy and load balancing.
A priority is assigned to each GLBP interface - 100 by default. The interface with the highest priority becomes the Active Virtual Gateway (AVG). If priorities are equal, the interface with the highest IP will become the AVG.
HSRP LAB
VRRP LAB
High availability is critical in most environments. Even a brief outage due to hardware failure may be considered unacceptable. Consider the following example:
To reach other networks, HostA must utilize a single gateway – SwitchA.The gateway represents a single point of failure on this network. If the gateway fails, hosts will lose access to all resources beyond the gateway.
Using multiple routers or multilayer switches can provide Layer-3 redundancy for hosts:
However, the Layer-3 redundancy must be transparent to each host. Hosts should not be configured with multiple default gateways.
Cisco supports three protocols to provide transparent Layer-3 redundancy:
• Hot Standby Router Protocol (HSRP)
• Virtual Router Redundancy Protocol (VRRP)
• Gateway Load Balancing Protocol (GLBP)
Hot Standby Router Protocol (HSRP)
Cisco developed the proprietary Hot Standby Router Protocol (HSRP) to allow multiple routers or multilayer switches to masquerade as a single gateway. This is accomplished by assigning a virtual IP and MAC address to all routers participating in an HSRP group.
Routers within the same HSRP group must be assigned the same group number, which can range from 0 to 255. However, most Cisco platforms only support 16 configured HSRP groups.
HSRP routers are elected to specific roles:
• Active Router – router currently serving as the gateway.
• Standby Router – backup router to the Active Router.
• Listening Router – all other routers participating in HSRP.
Only one active and one standby router are allowed per HSRP group. Thus, HSRP provides Layer-3 redundancy, but no inherent load balancing.
Hello packets are used to elect HSRP 3 seconds
The role of an HSRP router is dictated by its priority. The priority can range from 0 – 255, with a default of 100. A higher priority is preferred.
HSRP LAB
Virtual Router Redundancy Protocol (VRRP)
The Virtual Router Redundancy Protocol (VRRP) is an industry-standard Layer-3 redundancy protocol, originally defined in RFC 2338. VRRP is nearly identical to HSRP, with some notable exceptions:
• The router with the highest priority becomes the master router.
• All other routers become backup routers.
• The virtual MAC address is the reserved 0000.5e00.01xx, with xx representing the hexadecimal group number.
• Hello packets are sent every 1 second, by default, and sent to multicast address 224.0.0.18.
• VRRP will preempt by default.
• VRRP cannot directly track interfaces – it can track an object which is
tied to an interface, though.
VRRP LAB
Gateway Load Balancing Protocol (GLBP)
To overcome the shortcomings in HSRP and VRRP, Cisco developed the proprietary Gateway Load Balancing Protocol (GLBP).
Routers are added to a GLBP group, numbered 0 to 1023. Unlike HSRP and VRRP, multiple GLBP routers can be active, achieving both redundancy and load balancing.
A priority is assigned to each GLBP interface - 100 by default. The interface with the highest priority becomes the Active Virtual Gateway (AVG). If priorities are equal, the interface with the highest IP will become the AVG.
HSRP LAB
VRRP LAB
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