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 roles and to ensure all routers are
functional. If the current active router fails, the standby router will
immediately take over as active, and a new standby is elected. By default,
hello packets are sent every 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.
Thus, the router with the highest priority is elected the active router –
SwitchB in the above example. The router with the second highest priority
becomes the standby router – SwitchA in the example. If all priorities are
equal, whichever router has the highest IP Address on its HSRP interface is
elected the active router.
HSRP States
A router interface participating in HSRP must progress through several
states before settling into a role:
• Disabled
• Initial
• Learn
• Listen
• Speak
• Standby
• Active
A disabled state indicates that the interface is either not configured for
HSRP, or is administratively shutdown.
An interface begins in an initial state when first configured with HSRP, or
taken out of an administratively shutdown state.
An interface enters a learn state if it does not know the HSRP virtual IP
address. Normally the virtual IP is manually configured on the interface –
otherwise, it will be learned from the current Active router via hello packets.
An interface in a listen state knows the virtual IP address, but was not
elected as either the Active or Standby Router.
Interfaces in a speak state are currently participating in the election of an
active or standby router. Elections are performed using hello packets, which
are sent out every 3 seconds by default.
A standby state indicates that the interface is acting as a backup to the
active router. The standby router continuously exchanges hello packets with
the active router, and will take over if the active router fails.
An interface in an active state is the live gateway, and will forward traffic
sent to the virtual IP address. Hosts will use the virtual IP address as their
default gateway. The active router will respond to ARP requests for the
virtual IP with the virtual MAC address.
Note that hello packets are only exchanged in three HSRP states:
• Speak
• Standby
• Active
Interfaces in a listen state will only listen for hello packets. If an active or
standby router fails, a listen interface will transition to a speak state to
participate in a new election.
HSRP Basic Configuration HSRP is configured on the interface that is accepting traffic from hosts. Recall that the interface with the highest priority is elected the active router. To configure the priority of a router from its default of 100:
Router(config)# interface gi0/3
Router(config-if)# standby 1 priority 150
The standby 1 command specifies the HSRP group the interface belongs to. HSRP can also be configured on a VLAN interface on a multilayer switch:
SwitchB(config)# interface vlan 100
SwitchB(config-if)# standby 1 priority 150
Each interface in the HSRP group retains its local IP address. The HSRP group itself is assigned a virtual IP address, which hosts will use as their default gateway:
HSRP Basic Configuration (continued) HSRP supports using multiple virtual IP addresses:
SwitchB(config-if)# standby 1 ip 10.1.1.1
SwitchB(config-if)# standby 1 ip 10.1.1.5 secondary
The active router will respond to ARP requests for the virtual IP with the virtual MAC address.
The virtual MAC is a reserved address in the following format: 0000.0c07.acxx …with xx representing the HSRP group number in hexadecimal. For example, if the HSRP group number is 8, the resulting virtual MAC address would be 0000.0c07.ac08. The HSRP virtual MAC address can be manually changed:
Switch(config-if)# standby 1 mac-address 0000.00ab.12ef
HSRP authentication prevents an unauthorized router from joining the HSRP group. All routers in the HSRP group must be configured with an identical authentication string. To specify a clear-text authentication string:
Switch(config-if)# standby 1 authentication STAYOUT
To specify an MD5-hashed authentication string:
Switch(config-if)# standby 1 authentication md5 key-string 7 STAYOUT
HSRP Preempt If a new router is added to an HSRP group, it will not preemptively assume the role of the active router, even if it has the best priority. In fact, the router that is first powered on will become the active router, even if it has the lowest priority!
Consider the above example: 1. If SwitchB was powered on first, it would become the active router. 2. SwitchA would be elected the standby router. 3. If SwitchB fails, SwitchA would take over as the active router. 4. Once SwitchB recovers, it will not retake its role of active router, despite having a higher priority. The preempt parameter will allow a router to forcibly assume the role of active router, if it has the highest priority. The preempt feature is disabled by default: SwitchB(config-if)# standby 1 preempt The optional delay parameter will force a router to wait before preempting as the active router. The delay is measured in seconds: Switch(config-if)# standby 1 preempt delay 10 The router can also be forced to wait a specified number of seconds after a reload before preempting the active role: Switch(config-if)# standby 1 preempt reload 20
This allows routing protocols to converge before the router becomes active.
HSRP Timers Hello packets are used to elect the active and standby router, and to detect if there is a failure. By default, hello packets are exchanged every 3 seconds. HSRP Hello packets are sent to the multicast address 224.0.0.2 over UDP port 1985. If no elections are occurring, only the active and standby routers exchange hello packets. If no hello packets are received from the active router within the holddown timer duration, the standby router will assume it failed and take over as active. By default, the holddown timer is three times the hello timer, or 10 seconds. Cisco’s math, not mine. To manually adjust the HSRP timers, measured in seconds:
SwitchB(config-if)# standby 1 timers 4 12
The first timer value represents the hello timer, while the second represents the holddown timer. The timers can also be specified in milliseconds:
SwitchB(config-if)# standby 1 timers msec 800 msec 2400
Troubleshooting HSRP To view the status of each HSRP group: SwitchB# show standby VLAN 100 - Group 1 State is Active 1 state changes, last state change 00:02:19 Virtual IP address is 10.1.1.1 Active virtual MAC address is 0000.0c07.ac01 Local virtual MAC address is 0000.0c07.ac01 (bia) Hello time 3 sec, hold time 10 sec Next hello sent in 1.412 secs Preemption enabled, min delay 50 sec, sync delay 40 sec Active router is local Standby router is 10.1.1.2, priority 100 (expires in 6.158 sec) Priority 150 (configured 150) Tracking 0 objects, 0 up To view a more abbreviated version of this output:
SwitchB# show standby brie
f P indicates configured to preempt. | Interface Grp Prio P State Active addr Standby addr Group addr Vlan100 1 150 P Active local 10.1.1.2 10.1.1.1
Taken from packetlife.net
VRRP
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. Configuration of VRRP is also very similar to HSRP: SwitchB(config)# interface vlan 100 SwitchB(config-if)# ip address 10.1.1.3 255.255.255.0 SwitchB(config-if)# vrrp 1 priority 150 SwitchB(config-if)# vrrp 1 authentication STAYOUT SwitchB(config-if)# vrrp 1 ip 10.1.1.1 As with HSRP, the default VRRP priority is 100, and a higher priority is preferred. Unlike HSRP, preemption is enabled by default. To manually disable preempt: Switch(config-if)# no vrrp 1 preempt To view the status of each VRRP group: Switch# show vrrp Vlan 100 - Group 1 State is Master Virtual IP address is 10.1.1.1 Virtual MAC address is 0000.5e00.0101 Advertisement interval is 3.000 sec Preemption is enabled min delay is 0.000 sec Priority 150 Master Router is 10.1.1.3 (local), priority is 150 Master Advertisement interval is 3.000 sec Master Down interval is 9.711 sec
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GLBP
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.
Routers in the GLBP group are assigned a single virtual IP address. Hosts will use this virtual address as their default gateway. The AVG will respond to ARP requests for the virtual IP with the virtual MAC address of an Active Virtual Forwarder (AVF). Up to three routers can be elected as AVFs. The AVG assigns a virtual MAC address to each AVF, and to itself, for a maximum total of 4 virtual MAC addresses. Only the AVG and AVFs can forward traffic for hosts. Any router not elected as an AVF or AVG will become a Secondary Virtual Forwarder (SVF), and will wait in standby until an AVF fails
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Gateway Load Balancing Protocol (GLBP) (continued) Basic GLBP configuration is nearly identical to HSRP: SwitchB(config)# interface gi2/22 SwitchB(config-if)# glbp 1 priority 150 SwitchB(config-if)# glbp 1 preempt SwitchB(config-if)# glbp 1 ip 10.1.1.1 Remember that the interface with the highest priority is elected as the AVG. Preemption is disabled by default with GLBP. What determines whether a router becomes an AVF or SVF? Each router is assigned a weight, and the default weight is 100. A higher weight is preferred. Weight can be configured two ways: • Statically • Dynamically, by tracking an interface To manually specify the weight of a router: SwitchB(config)# interface gi2/22 SwitchB(config-if)# glbp 1 weighting 150 Like VRRP, GLBP cannot directly track an interface. Instead, an interface must be specified as a tracked object: SwitchB(config)# track 10 interface gi2/23 The tracked object can then be referenced as part of a weighting command: SwitchB(config)# interface gi2/22 SwitchB(config-if)# glbp 1 weighting track 10 decrement 65 Note that the tracking object-number must match. By default, the weight will decrement by 10. Weight thresholds can be configured, forcing a router to relinquish its AVF role if it falls below the minimum threshold: SwitchB(config)# interface gi2/22 SwitchB(config-if)# glbp 1 weighting 150 lower 90 upper 125 If the weight falls below the lower threshold, the router must stop functioning as an AVF. The router will become an AVF again once its weight reaches the upper threshold, as long as preempt is configured.
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Gateway Load Balancing Protocol (GLBP) (continued) The AVG assigns a virtual MAC address to itself and up to three AVFs. The AVG will respond to ARP requests for the virtual IP address with one of these virtual MAC addresses. This allows GLBP to provide load balancing in addition to redundancy. GLBP supports three load balancing methods: • Round Robin • Weighted • Host-dependent The default load balancing method is per-host round robin. Traffic from hosts is distributed equally across all routers in the GLBP group. The AVG will respond to the first host ARP request with the first virtual MAC address. The second ARP request will receive the second virtual MAC address, etc. The weighted load balancing method will distribute traffic proportionally, based on a router’s weight. Routers with a higher weight will receive a proportionally higher percentage of traffic. Host-dependent load balancing will provide a host device with the same virtual MAC address every time it performs an ARP request. The load balancing method should be configured on the AVG: SwitchB(config-if)# glbp 1 load-balancing round-robin SwitchB(config-if)# glbp 1 load-balancing weighted SwitchB(config-if)# glbp 1 load-balancing host-dependent Other fun facts about GLBP: • Hello packets are sent every 3 seconds. • Hello packets are sent to multicast address 224.0.0.102. • The default holddown time is 10 seconds. • The virtual MAC address is the reserved 0007.b4xx.xxyy, with xxxx representing the GLBP group number, and yy representing the AVF number. To view the status of each GLBP group: SwitchB# show glbp
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