First Hop Redundancy Protocols Configuration Guide, Cisco IOS Release 12.4T
Last Updated: December 20, 2011
Gateway Load Balancing Protocol (GLBP) protects data traffic from a failed router or circuit, like Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP), while allowing packet load sharing between a group of redundant routers.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Restrictions for GLBP
Enhanced Object Tracking (EOT) is not stateful switchover (SSO)-aware and cannot be used with GLBP in SSO mode.
Prerequisites for GLBP
Before configuring GLBP, ensure that the routers can support multiple MAC addresses on the physical interfaces. For each GLBP forwarder to be configured, an additional MAC address is used.
Information About GLBP
GLBP provides automatic router backup for IP hosts configured with a single default gateway on an IEEE 802.3 LAN. Multiple first-hop routers on the LAN combine to offer a single virtual first-hop IP router while sharing the IP packet forwarding load. Other routers on the LAN may act as redundant GLBP routers that will become active if any of the existing forwarding routers fail.
GLBP performs a similar function for the user as HSRP and VRRP. HSRP and VRRP allow multiple routers to participate in a virtual router group configured with a virtual IP address. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. The other routers in the group are redundant until the active router fails. These standby routers have unused bandwidth that the protocol is not using. Although multiple virtual router groups can be configured for the same set of routers, the hosts must be configured for different default gateways, which results in an extra administrative burden. The advantage of GLBP is that it additionally provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. The forwarding load is shared among all routers in a GLBP group rather than being handled by a single router while the other routers stand idle. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets. GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address 188.8.131.52, UDP port 3222 (source and destination).
GLBP Active Virtual Gateway
Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group. Other group members provide backup for the AVG if the AVG becomes unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group. Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. These gateways are known as active virtual forwarders (AVFs) for their virtual MAC address.
The AVG is also responsible for answering Address Resolution Protocol (ARP) requests for the virtual IP address. Load sharing is achieved by the AVG replying to the ARP requests with different virtual MAC addresses.
Prior to Cisco IOS Release 15.0(1)M1, 12.4(24)T2, 15.1(2)T, and later releases, when the no glbp load-balancing command is configured, the AVG always responds to ARP requests with the MAC address of its AVF.
In Cisco IOS Release 15.0(1)M1, 12.4(24)T2, 15.1(2)T, and later releases, when the no glbp load-balancing command is configured, if the AVG does not have an AVF, it preferentially responds to ARP requests with the MAC address of the first listening virtual forwarder (VF), which will causes traffic to route via another gateway until that VF migrates back to being the current AVG.
In the figure below, Router A is the AVG for a GLBP group, and is responsible for the virtual IP address 10.21.8.10. Router A is also an AVF for the virtual MAC address 0007.b400.0101. Router B is a member of the same GLBP group and is designated as the AVF for the virtual MAC address 0007.b400.0102. Client 1 has a default gateway IP address of 10.21.8.10 and a gateway MAC address of 0007.b400.0101. Client 2 shares the same default gateway IP address but receives the gateway MAC address 0007.b400.0102 because Router B is sharing the traffic load with Router A.
If Router A becomes unavailable, Client 1 will not lose access to the WAN because Router B will assume responsibility for forwarding packets sent to the virtual MAC address of Router A, and for responding to packets sent to its own virtual MAC address. Router B will also assume the role of the AVG for the entire GLBP group. Communication for the GLBP members continues despite the failure of a router in the GLBP group.
GLBP Virtual MAC Address Assignment
A GLBP group allows up to four virtual MAC addresses per group. The AVG is responsible for assigning the virtual MAC addresses to each member of the group. Other group members request a virtual MAC address after they discover the AVG through hello messages. Gateways are assigned the next MAC address in sequence. A virtual forwarder that is assigned a virtual MAC address by the AVG is known as a primary virtual forwarder. Other members of the GLBP group learn the virtual MAC addresses from hello messages. A virtual forwarder that has learned the virtual MAC address is referred to as a secondary virtual forwarder.
GLBP Virtual Gateway Redundancy
GLBP operates virtual gateway redundancy in the same way as HSRP. One gateway is elected as the AVG, another gateway is elected as the standby virtual gateway, and the remaining gateways are placed in a listen state.
If an AVG fails, the standby virtual gateway will assume responsibility for the virtual IP address. A new standby virtual gateway is then elected from the gateways in the listen state.
GLBP Virtual Forwarder Redundancy
Virtual forwarder redundancy is similar to virtual gateway redundancy with an AVF. If the AVF fails, one of the secondary virtual forwarders in the listen state assumes responsibility for the virtual MAC address.
The new AVF is also a primary virtual forwarder for a different forwarder number. GLBP migrates hosts away from the old forwarder number using two timers that start as soon as the gateway changes to the active virtual forwarder state. GLBP uses the hello messages to communicate the current state of the timers.
The redirect time is the interval during which the AVG continues to redirect hosts to the old virtual forwarder MAC address. When the redirect time expires, the AVG stops using the old virtual forwarder MAC address in ARP replies, although the virtual forwarder will continue to forward packets that were sent to the old virtual forwarder MAC address.
The secondary holdtime is the interval during which the virtual forwarder is valid. When the secondary holdtime expires, the virtual forwarder is removed from all gateways in the GLBP group. The expired virtual forwarder number becomes eligible for reassignment by the AVG.
GLBP Gateway Priority
GLBP gateway priority determines the role that each GLBP gateway plays and what happens if the AVG fails.
Priority also determines if a GLBP router functions as a backup virtual gateway and the order of ascendancy to becoming an AVG if the current AVG fails. You can configure the priority of each backup virtual gateway with a value of 1 through 255 using the glbp priority command.
In the "GLBP Topology" figure, if Router A--the AVG in a LAN topology--fails, an election process takes place to determine which backup virtual gateway should take over. In this example, Router B is the only other member in the group so it will automatically become the new AVG. If another router existed in the same GLBP group with a higher priority, then the router with the higher priority would be elected. If both routers have the same priority, the backup virtual gateway with the higher IP address would be elected to become the active virtual gateway.
By default, the GLBP virtual gateway preemptive scheme is disabled. A backup virtual gateway can become the AVG only if the current AVG fails, regardless of the priorities assigned to the virtual gateways. You can enable the GLBP virtual gateway preemptive scheme using the glbp preempt command. Preemption allows a backup virtual gateway to become the AVG, if the backup virtual gateway is assigned a higher priority than the current AVG.
GLBP Gateway Weighting and Tracking
GLBP uses a weighting scheme to determine the forwarding capacity of each router in the GLBP group. The weighting assigned to a router in the GLBP group can be used to determine whether it will forward packets and, if so, the proportion of hosts in the LAN for which it will forward packets. Thresholds can be set to disable forwarding when the weighting for a GLBP group falls below a certain value, and when it rises above another threshold, forwarding is automatically reenabled.
The GLBP group weighting can be automatically adjusted by tracking the state of an interface within the router. If a tracked interface goes down, the GLBP group weighting is reduced by a specified value. Different interfaces can be tracked to decrement the GLBP weighting by varying amounts.
By default, the GLBP virtual forwarder preemptive scheme is enabled with a delay of 30 seconds. A backup virtual forwarder can become the AVF if the current AVF weighting falls below the low weighting threshold for 30 seconds. You can disable the GLBP forwarder preemptive scheme using the no glbp forwarder preempt command or change the delay using the glbp forwarder preempt delay minimum command.
GLBP Client Cache
The GLBP client cache contains information about network hosts that are using a GLBP group as the default gateway.
When an IPv4 Address Resolution Protocol (ARP) request or an IPv6 Neighbor Discovery (ND) request for a GLBP virtual IP address is received from a network host by a GLBP group's active virtual gateway (AVG), a new entry is created in the GLBP client cache. The cache entry contains information about the host that sent the ARP or ND request and which forwarder the AVG has assigned to it.
The GLBP client cache stores the MAC address of each host that is using a particular GLBP group, the number of the GLBP forwarder that each network host has been assigned to and the total number of network hosts currently assigned to each forwarder in a GLBP group. The GLBP client cache also stores the protocol address used by each network host and the time elapsed since the host-to-forwarder assignment was last updated.
The GLBP client cache can store information on up to 2000 network hosts for a GLBP group. The expected normal maximum configuration is 1000 network hosts. You can configure a lower maximum number of network hosts that will be cached for each GLBP group independently based on the number of network hosts that are using each GLBP group by using the glbp client-cache maximum command. This command enables you to limit the amount of memory used by the cache per GLBP group. If the GLBP client cache has reached the maximum configured number of clients and a new client is added, the least recently updated client entry will be discarded. Reaching this condition indicates that the configured maximum limit is too small.
The amount of memory that is used by the GLBP client cache depends on the number of network hosts using GLBP groups for which the client cache is enabled. For each host at least 20 bytes is required, with an additional 3200 bytes per GLBP group.
You can display the contents of the GLBP client cache using the show glbp detail command on the router that is currently the AVG for a GLBP group. If you issue the show glbp detail command on any other router in a GLBP group, you will be directed to reissue the command on the AVG to view client cache information. The show glbp detail command also displays statistics about the GLBP client cache usage and the distribution of clients among forwarders. These statistics are accurate as long as the cache timeout and client limit parameters have been set appropriately. Appropriate values would be where the number of end hosts on the network does not exceed the configured limit and where the maximum end host ARP cache timeout does not exceed the configured GLBP client cache timeout.
You can enable or disable the GLBP client cache independently for each GLBP group by using the glbp client-cache command. The GLBP client cache is disabled by default. There is no limit on the number of groups for which the GLBP client cache can be enabled.
You can configure GLBP cache entries to time out after a specified time by using the timeout keyword option with the glbp client-cache maximum command.
GLBP MD5 Authentication
GLBP MD5 authentication uses the industry-standard MD5 algorithm for improved reliability and security. MD5 authentication provides greater security than the alternative plain text authentication scheme and protects against spoofing software.
MD5 authentication allows each GLBP group member to use a secret key to generate a keyed MD5 hash that is part of the outgoing packet. A keyed hash of an incoming packet is generated and, if the hash within the incoming packet does not match the generated hash, the packet is ignored.
The key for the MD5 hash can either be given directly in the configuration using a key string or supplied indirectly through a key chain. The key string cannot exceed 100 characters in length.
A router will ignore incoming GLBP packets from routers that do not have the same authentication configuration for a GLBP group. GLBP has three authentication schemes:
GLBP packets will be rejected in any of the following cases:
GLBP supports In Service Software Upgrade (ISSU). In Service Software Upgrade (ISSU) allows a high-availability (HA) system to run in Stateful Switchover (SSO) mode even when different versions of Cisco IOS software are running on the active and standby Route Processors (RPs) or line cards.
ISSU provides the ability to upgrade or downgrade from one supported Cisco IOS release to another while continuing to forward packets and maintain sessions, thereby reducing planned outage time. The ability to upgrade or downgrade is achieved by running different software versions on the active RP and standby RP for a short period of time to maintain state information between RPs. This feature allows the system to switch over to a secondary RP running upgraded (or downgraded) software and continue forwarding packets without session loss and with minimal or no packet loss. This feature is enabled by default.
For detailed information about ISSU, see the Cisco IOS In Service Software Upgrade Process in the Cisco IOS High Availability Configuration Guide
For detailed information about ISSU on the 7600 series routers, see the ISSU and eFSU on Cisco 7600 Series Routers document at the following URL:
With the introduction of the GLBP SSO feature, GLBP is stateful switchover (SSO) aware. GLBP can detect when a router is failing over to the secondary router processor (RP) and continue in its current group state.
SSO functions in networking devices (usually edge devices) that support dua RPs. SSO provides RP redundancy by establishing one of the RPs as the active processor and the other RP as the standby processor. SSO also synchronizes critical state information between the RPs so that network state information is dynamically maintained between RPs.
Without SSO-awareness, if GLBP is deployed on a router with redundant RPs, a switchover of roles between the active RP and the standby RP results in the router relinquishing its activity as a GLBP group member and then rejoining the group as if it had been reloaded. The GLBP SSO feature enables GLBP to continue its activities as a group member during a switchover. GLBP state information between redundant RPs is maintained so that the standby RP can continue the router's activities within the GLBP during and after a switchover.
This feature is enabled by default. To disable this feature, use the no glbp sso command in global configuration mode.
For more information, see the Stateful Swithover document in the Cisco IOS High Availability Configuration Guide.
You can configure GLBP in such a way that traffic from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers.
Multiple Virtual Routers
GLBP supports up to 1024 virtual routers (GLBP groups) on each physical interface of a router and up to four virtual forwarders per group.
The redundancy scheme of GLBP enables you to preempt an active virtual gateway with a higher priority backup virtual gateway that has become available. Forwarder preemption works in a similar way, except that forwarder preemption uses weighting instead of priority and is enabled by default.
GLBP supports the industry-standard message digest 5 (MD5) algorithm for improved reliability, security, and protection against GLBP-spoofing software. A router within a GLBP group with a different authentication string than other routers will be ignored by other group members. You can alternatively use a simple text password authentication scheme between GLBP group members to detect configuration errors.
How to Configure GLBP
Enabling and Verifying GLBP
Perform this task to enable GLBP on an interface and verify its configuration and operation. GLBP is designed to be easy to configure. Each gateway in a GLBP group must be configured with the same group number, and at least one gateway in the GLBP group must be configured with the virtual IP address to be used by the group. All other required parameters can be learned.
Before You BeginSUMMARY STEPS
If VLANs are in use on an interface, the GLBP group number must be different for each VLAN.
In the following example, sample output is displayed about the status of the GLBP group, named 10, on the router:
Router# show glbp 10 GigabitEthernet0/0/0 - Group 10 State is Active 2 state changes, last state change 23:50:33 Virtual IP address is 10.21.8.10 Hello time 5 sec, hold time 18 sec Next hello sent in 4.300 secs Redirect time 600 sec, forwarder time-out 7200 sec Authentication text "stringabc" Preemption enabled, min delay 60 sec Active is local Standby is unknown Priority 254 (configured) Weighting 105 (configured 110), thresholds: lower 95, upper 105 Track object 2 state Down decrement 5 Load balancing: host-dependent There is 1 forwarder (1 active) Forwarder 1 State is Active 1 state change, last state change 23:50:15 MAC address is 0007.b400.0101 (default) Owner ID is 0005.0050.6c08 Redirection enabled Preemption enabled, min delay 60 sec Active is local, weighting 105
Customizing the behavior of GLBP is optional. Be aware that as soon as you enable a GLBP group, that group is operating. It is possible that if you first enable a GLBP group before customizing GLBP, the router could take over control of the group and become the AVG before you have finished customizing the feature. Therefore, if you plan to customize GLBP, it is a good idea to do so before enabling GLBP.
Configuring GLBP MD5 Authentication Using a Key String
Configuring GLBP MD5 Authentication Using a Key Chain
Perform this task to configure GLBP MD5 authentication using a key chain. Key chains allow a different key string to be used at different times according to the key chain configuration. GLBP will query the appropriate key chain to obtain the current live key and key ID for the specified key chain.
Configuring GLBP Text Authentication
Configuring GLBP Weighting Values and Object Tracking
GLBP weighting is used to determine whether a GLBP group can act as a virtual forwarder. Initial weighting values can be set and optional thresholds specified. Interface states can be tracked and a decrement value set to reduce the weighting value if the interface goes down. When the GLBP group weighting drops below a specified value, the group will no longer be an active virtual forwarder. When the weighting rises above a specified value, the group can resume its role as an active virtual forwarder.
GLBP introduces five privileged EXEC mode commands to enable display of diagnostic output concerning various events relating to the operation of GLBP. The debug condition glbp,debug glbp errors, debug glbp events, debug glbp packets, and debug glbp terse commands are intended only for troubleshooting purposes because the volume of output generated by the software can result in severe performance degradation on the router. Perform this task to minimize the impact of using the debug glbp commands.
This procedure will minimize the load on the router created by the debug condition glbpor debug glbp command because the console port is no longer generating character-by-character processor interrupts. If you cannot connect to a console directly, you can run this procedure via a terminal server. If you must break the Telnet connection, however, you may not be able to reconnect because the router may be unable to respond due to the processor load of generating the debugging output.
Configuration Examples for GLBP
Example: Customizing GLBP Configuration
Router(config)# interface fastethernet 0/0 Router(config-if)# ip address 10.21.8.32 255.255.255.0 Router(config-if)# glbp 10 timers 5 18 Router(config-if)# glbp 10 timers redirect 1800 28800 Router(config-if)# glbp 10 load-balancing host-dependent Router(config-if)# glbp 10 priority 254 Router(config-if)# glbp 10 preempt delay minimum 60 Router(config-if)# glbp 10 client-cache maximum 1200 timeout 245
Example: Configuring GLBP MD5 Authentication Using Key Strings
The following example shows how to configure GLBP MD5 authentication using a key string:
Router(config)# interface Ethernet 0/1 Router(config-if)# ip address 10.0.0.1 255.255.255.0 Router(config-if)# glbp 2 authentication md5 key-string ThisStringIsTheSecretKey Router(config-if)# glbp 2 ip 10.0.0.10
Example: Configuring GLBP MD5 Authentication Using Key Chains
In the following example, GLBP queries the key chain "AuthenticateGLBP" to obtain the current live key and key ID for the specified key chain:
Router(config)# key chain AuthenticateGLBP Router(config-keychain)# key 1 Router(config-keychain-key)# key-string ThisIsASecretKey Router(config-keychain-key)# exit Router(config-keychain)# exit Router(config)# interface Ethernet 0/1 Router(config-if)# ip address 10.0.0.1 255.255.255.0 Router(config-if)# glbp 2 authentication md5 key-chain AuthenticateGLBP Router(config-if)# glbp 2 ip 10.0.0.10
Example: Configuring GLBP Text Authentication
Example: Configuring GLBP Weighting
In the following example, Router A is configured to track the IP routing state of the POS interface 5/0/0 and 6/0/0, an initial GLBP weighting with upper and lower thresholds is set, and a weighting decrement value of 10 is set. If POS interface 5/0/0 and 6/0/0 goes down, the weighting value of the router is reduced.
Router(config)# track 1 interface POS 5/0/0 ip routing Router(config)# track 2 interface POS 6/0/0 ip routing Router(config)# interface fastethernet 0/0/0 Router(config-if)# glbp 10 weighting 110 lower 95 upper 105 Router(config-if)# glbp 10 weighting track 1 decrement 10 Router(config-if)# glbp 10 weighting track 2 decrement 10 Router(config-if)# glbp 10 forwarder preempt delay minimum 60
Example: Enabling GLBP Configuration
Feature Information for GLBP
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
active RP--The Route Processor (RP) controls the system, provides network services, runs routing protocols and presents the system management interface.
AVF--active virtual forwarder. One virtual forwarder within a GLBP group is elected as active virtual forwarder for a specified virtual MAC address, and it is responsible for forwarding packets sent to that MAC address. Multiple active virtual forwarders can exist for each GLBP group.
AVG--active virtual gateway. One virtual gateway within a GLBP group is elected as the active virtual gateway, and is responsible for the operation of the protocol.
GLBP gateway--Gateway Load Balancing Protocol gateway. A router or gateway running GLBP. Each GLBP gateway may participate in one or more GLBP groups.
GLBP group--Gateway Load Balancing Protocol group. One or more GLBP gateways configured with the same GLBP group number on connected Ethernet interfaces.
ISSU--In Service Software Upgrade. A process that allows Cisco IOS XE software to be updated or otherwise modified while packet forwarding continues. In most networks, planned software upgrades are a significant cause of downtime. ISSU allows Cisco IOS software to be modified while packet forwarding continues, which increases network availability and reduces downtime caused by planned software upgrades.
NSF--nonstop forwarding. The ability of a router to continue to forward traffic to a router that may be recovering from a failure. Also, the ability of a router recovering from a failure to continue to correctly forward traffic sent to it by a peer.
RP--Route Processor. A generic term for the centralized control unit in a chassis. Platforms usually use a platform-specific term, such as RSP on the Cisco 7500, the PRE on the Cisco 10000, or the SUP+MSFC on the Cisco 7600.
RPR--Route Processor Redundancy. RPR provides an alternative to the High System Availability (HSA) feature. HSA enables a system to reset and use a standby Route Processor (RP) if the active RP fails. Using RPR, you can reduce unplanned downtime because RPR enables a quicker switchover between an active and standby RP if the active RP experiences a fatal error.
RPR+--An enhancement to RPR in which the standby RP is fully initialized.
SSO--Stateful Switchover. Enables applications and features to maintain state information between an active and standby unit.
standby RP--An RP that has been fully initialized and is ready to assume control from the active RP should a manual or fault-induced switchover occur.
switchover--An event in which system control and routing protocol execution are transferred from the active RP to the standby RP. Switchover may be a manual operation or may be induced by a hardware or software fault. Switchover may include transfer of the packet forwarding function in systems that combine system control and packet forwarding in an indivisible unit.
vIP--virtual IP address. An IPv4 address. There must be only one virtual IP address for each configured GLBP group. The virtual IP address must be configured on at least one GLBP group member. Other GLBP group members can learn the virtual IP address from hello messages.
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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