Table Of Contents
GLBP Virtual MAC Address Assignment
GLBP Virtual Gateway Redundancy
GLBP Virtual Forwarder Redundancy
GLBP Load Balancing and Tracking
Licensing Requirements for GLBP
Configuring GLBP Authentication
Configuring GLBP Load Balancing
Configuring GLBP Weighting and Tracking
Configuring Gateway Preemption
GLBP: Group Details Tab: Group Details Section
GLBP: Group Details Tab: Authentication, Gateway Preemption Section
GLBP: Group Details Tab: Weighting and Object Tracking Section
GLBP: Group Details Tab: Virtual Forwarder Setting Section
GLBP: Group Details Tab: Timers Section
GLBP: Virtual Gateways and Forwarders Tab: Forwarder Details Section
GLBP: Virtual Gateways and Forwarders Tab: GLBP Group Member Details Section
Configuring GLBP
This chapter describes how to configure the Gateway Load Balancing Protocol (GLBP).
This chapter includes the following sections:
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Licensing Requirements for GLBP
Information About GLBP
GLBP provides path redundancy for IP by sharing protocol and Media Access Control (MAC) addresses between redundant gateways. Additionally, GLBP allows a group of Layer 3 routers to share the load of the default gateway on a LAN. A GLBP router can automatically assume the forwarding function of another router in the group if the other router fails.
This section includes the following topics:
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GLBP Overview
GLBP provides automatic gateway backup for IP hosts configured with a single default gateway on an IEEE 802.3 LAN. Multiple routers on the LAN combine to offer a single virtual first-hop IP gateway while sharing the IP packet forwarding load. Other routers on the LAN may act as redundant GLBP gateways that become active if any of the existing forwarding gateways fail.
GLBP performs a similar function to the Hot Standby Redundancy Protocol (HSRP) and the Virtual Router Redundancy Protocol (VRRP). HSRP and VRRP allow multiple routers to participate in a virtual group configured with a virtual IP address. These protocols elect one member as the active router to forward packets to the virtual IP address for the group. The other routers in the group are redundant until the active router fails.
GLBP performs an additional load balancing function that the other protocols do not provide. GLBP load balances over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. GLBP shares the forwarding load among all routers in a GLBP group instead of allowing a single router to handle the whole load while the other routers remain idle. You configure each host with the same virtual IP address, and all routers in the virtual group participate in forwarding packets. GLBP members communicate between each other using periodic hello messages.
GLBP Active Virtual Gateway
GLBP prioritizes gateways to elect an active virtual gateway (AVG). If multiple gateways have the same priority, the gateway with the highest real IP address becomes the AVG. The AVG assigns a virtual MAC address to each member of the GLBP group. Each member is the active virtual forwarder (AVF) for its assigned virtual MAC address, forwarding packets sent to its assigned virtual MAC address.
The AVG also answers Address Resolution Protocol (ARP) requests for the virtual IP address. Load sharing is achieved when the AVG replies to the ARP requests with different virtual MAC addresses.
GLBP Virtual MAC Address Assignment
The AVG assigns the virtual MAC addresses to each member of the group. The group members request a virtual MAC address after they discover the AVG through hello messages. The AVG assigns the next MAC address based on the load-balancing algorithm selected (see the "GLBP Load Balancing and Tracking" section). A gateway that is assigned with a virtual MAC address by the AVG is the primary virtual forwarder. The other members of the GLBP group that learn the virtual MAC addresses from hello messages are secondary virtual forwarders.
GLBP Virtual Gateway Redundancy
GLBP provides virtual gateway redundancy. A member in a group can be in the active, standby, or listen state. GLBP uses a priority algorithm to elect one gateway as the AVG and elect another gateway as the standby virtual gateway. The remaining gateways go into the listen state. You can configure the GLBP priority on each gateway. If the GLBP priority is identical on multiple gateways, GLBP uses the gateway with the highest IP address as the AVG.
If an AVG fails, the standby virtual gateway assumes responsibility for the virtual IP address. GLBP elects a new standby virtual gateway from the gateways in the listen state.
GLBP Virtual Forwarder Redundancy
GLBP provides virtual forwarder redundancy. Virtual forwarder redundancy is similar to virtual gateway redundancy with an active virtual forwarder (AVF). If the AVF fails, a secondary virtual forwarder in the listen state assumes responsibility for the virtual MAC address. This secondary virtual forwarder is also a primary virtual forwarder for a different virtual MAC address. GLBP migrates hosts away from the old virtual MAC address of the failed AVF, using the following two timers.
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GLBP uses hello messages to communicate the current state of the timers.
In Figure 4-1, router A is the AVG for a GLBP group and is responsible for the virtual IP address 192.0.2.1. 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 192.0.2.1, the virtual IP address, and a gateway MAC address of 0007.b400.0101 that points to router A. 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.
Figure 4-1 GLBP Topology
If router A becomes unavailable, client 1 does not lose access to the WAN because router B assumes 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 also assumes 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 Authentication
GLBP has three authentication types:
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MD5 authentication provides greater security than plain text authentication. 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. At the receiving end, a keyed hash of an incoming packet is generated. 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.
You can also choose to use a simple password in plain text to authenticate GLBP packets, or choose no authentication for GLBP.
GLBP rejects packets in any of the following cases:
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GLBP Load Balancing and Tracking
You can configure the following load-balancing methods for GLBP:
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The default for IPv4 networks is round-robin. You can disable all load balancing for GLBP on an interface. If you do not configure load balancing, the AVG handles all traffic for the hosts while the other GLBP group members are in standby or listen mode.
You can configure GLBP to track an interface or routes and enable the secondary virtual forwarder to take over if a tracked link goes down. GLBP tracking uses weighted load-balancing to determine whether a GLBP group member acts as an AVF. You must configure the initial weighting values and optional thresholds to enable or disable this group member as an AVF. You can also configure the interface to track and the value that will reduce the interface's weighting if the interface goes down. When the GLBP group weighting drops below the lower threshold, the member is no longer an AVF and a secondary virtual forwarder takes over. When the weighting rises above the upper threshold, the member can resume its role as an AVF.
Figure 4-2 shows an example of GLBP tracking and weighting.
Figure 4-2 GLBP Object Tracking and Weighting
In Figure 4-2, the Ethernet 1/2 interface on router 1 is the gateway for host 1 (the AVF for virtual MAC address, vMAC1), while Ethernet 2/2 on router 2 acts as a secondary virtual forwarder for Host 1. Ethernet 1/2 tracks Ethernet 3/1, which is the network connection for router 1. If Ethernet 3/1 goes down, the weighting for Ethernet 1/2 drops to 90. Ethernet 2/2 on router 2 preempts Ethernet 1/2 and takes over as AVF because it has the default weighting of 100 and is configured to preempt the AVF.
See the "Configuring GLBP Weighting and Tracking" section for details about configuring weighting and tracking.
High Availability
GLBP supports stateful restarts and stateful switchover. A stateful restart occurs when the GLBP process fails and is restarted. Stateful switchover occurs when the active supervisor switches to the standby supervisor. Cisco NX-OS applies the run-time configuration after the switchover.
Virtualization Support
GLBP supports Virtual Routing and Forwarding instances (VRFs). VRFs exist within virtual device contexts (VDCs). By default, Cisco NX-OS places you in the default VDC and default VRF unless you specifically configure another VDC and VRF.
If you change the VRF membership of an interface, Cisco NX-OS removes all layer 3 configuration, including GLBP.
For more information, see the Cisco DCNM Virtual Device Context Configuration Guide.
Licensing Requirements for GLBP
The following table shows the licensing requirements for this feature:
Prerequisites for GLBP
GLBP has the following prerequisites:
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logging event link-status defaultlogging level glbp 6logging logfile messages 6•
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Guidelines and Limitations
GLBP has the following guidelines and limitations:
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Configuring GLBP
You can access GLBP from the Routing feature selection. Figure 4-3 shows GLBP.
Figure 4-3 Configuring GLBP
For more information about the Data Center Network Manager features, see the Cisco Data Center Network Manager Fundamentals Guide
This section includes the following topics:
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Enabling the GLBP Feature
You must enable the GLBP feature before you can configure and enable any GLBP groups.
BEFORE YOU BEGIN
Ensure that you configure the logging level for GLBP to 6 (Informational) or a higher level. To configure the device with the minimal required logging level, log into the command-line interface of the device and use the following commands:
logging event link-status defaultlogging level glbp 6logging logfile messages 6DETAILED STEPS
To enable the GLBP feature, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Creating a GLBP Group
You can create a GLBP group on an interface.
BEFORE YOU BEGIN
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
DETAILED STEPS
To create a GLBP group on an interface, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
The system highlights the new GLBP row in the Summary pane, and tabs update in the Details pane.
Step 4
Step 5
The range is from 0 to 1023.
The system creates the new group on the device and highlights the new GLBP group in the Summary pane, and tabs update in the Details pane.
Step 6
The Group Details tab appears.
Step 7
The basic group information appears in the Details pane.
Step 8
The range is from 1 to 255.
Step 9
Step 10
Configuring GLBP Authentication
You can configure GLBP to authenticate the protocol using cleartext or an MD5 digest. MD5 authentication uses a key chain (see the Cisco DCNM Security Configuration Guide).
BEFORE YOU BEGIN
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
Note
DETAILED STEPS
To configure authentication on a GLBP group, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Step 4
The Group Details tab appears.
Step 5
The authentication information appears in the Details pane.
Step 6
Step 7
Step 8
Step 9
Step 10
Step 11
Configuring GLBP Load Balancing
You can configure GLBP to use load balancing based on round-robin, weighted, or host-dependent methods (see the "GLBP Load Balancing and Tracking" section).
BEFORE YOU BEGIN
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
DETAILED STEPS
To configure GLBP load balancing on an interface, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Step 4
The Group Details tab appears.
Step 5
The basic group information appears in the Details pane.
Step 6
Step 7
Configuring GLBP Weighting and Tracking
You can configure GLBP weighting values and object tracking to work with the GLBP weighted load-balancing method.
You can optionally configure the interface to preempt an AVF if the interface was originally assigned with the virtual MAC address or if this interface has a higher weight than the AVF.
BEFORE YOU BEGIN
Ensure that you have configured the object tracking entry that you want to use to modify GLBP weighting (see the "Configuring Object Tracking" section on page 5-1).
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
DETAILED STEPS
To configure GLBP weighting and tracking on an interface, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Step 4
The Group Details tab appears.
Step 5
The weighting and object tracking information appears in the Details pane.
Step 6
The range is from 1 to 254.
Step 7
Step 8
Step 9
Step 10
The virtual forwarders information appears in the Details pane.
Step 11
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Configuring Gateway Preemption
You can configure gateway preemption in a GLBP group.
BEFORE YOU BEGIN
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
DETAILED STEPS
To configure GLBP preemption, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Step 4
The Group Details tab appears.
Step 5
The preemption information appears in the Details pane.
Step 6
Step 7
The default is 3600 seconds.
From the Sync field, enter the sync delay to wait before preemption occurs.
The default is 30 seconds.
Step 8
Customizing GLBP
Customizing the behavior of GLBP is optional. Be aware that as soon as you enable a GLBP group by configuring a virtual IP address, that group is operational. If you enable a GLBP group before you customize GLBP, the router could take over control of the group and become the AVG before you finish customizing the feature. If you plan to customize GLBP, you should do so before enabling GLBP.
BEFORE YOU BEGIN
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
DETAILED STEPS
To customize GLBP timers, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Step 4
The Group Details tab appears.
Step 5
The timers information appears in the Details pane.
Step 6
Step 7
Step 8
Step 9
Step 10
Enabling a GLBP Group
You can configure the virtual IP address on an interface to enable the GLBP group. You must configure each gateway in the GLBP group with the same group number. The GLBP member can learn all other required parameters from another GLBP member.
BEFORE YOU BEGIN
Enable the GLBP feature (see the "Enabling the GLBP Feature" section).
DETAILED STEPS
To enable a GLBP group, follow these steps:
Step 1
The available devices appear in the Summary pane.
Step 2
Step 3
Step 4
The Group Details tab appears.
Step 5
The general information appears in the Details pane.
Step 6
Step 7
Step 8
Step 9
Step 10
Step 11
Field Descriptions for GLBP
This section includes the following field descriptions for GLBP:
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GLBP: Group Details Tab: Group Details Section
GLBP: Group Details Tab: Authentication, Gateway Preemption Section
GLBP: Group Details Tab: Weighting and Object Tracking Section
GLBP: Group Details Tab: Virtual Forwarder Setting Section
GLBP: Group Details Tab: Timers Section
GLBP: Virtual Gateways and Forwarders Tab: Forwarder Details Section
GLBP: Virtual Gateways and Forwarders Tab: GLBP Group Member Details Section
Additional References
For additional information related to implementing GLBP, see the following sections:
Related Documents
GLBP CLI commands
Cisco NX-OS Unicast Routing Command Reference, Release 4.0
Configuring high availability
Cisco NX-OS High Availability and Redundancy Guide, Release 4.0
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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