Notes: GLBP

Feature Overview

• GLBP provides automatic router backup for IP hosts configured with a single default gateway.
• Multiple first-hop routers combine to offer a single virtual router while sharing the IP packet forwarding load.
• GLBP performs a similar, but not identicaly, function to HSRP and VRRP.
• In HSRP, only one member is elected as the active router to forward packets sent to the virtual IP address of the group, the other routers are unused in terms of user data transmission. Load sharing is achieved by configuring multiple HSRP groups and virtual IP addresses, and pointing the end hosts to different default gateways - causing extra administrative burden.
• GLBP provides load balancing over multiple rouers (gateways) by using a single virtual IP address and multiple virtual MAC addresses.
• All end hosts are configured with the same default gateway (the virtual IP address), and all routers in the GLBP group participate in forwarding packets.
• GLBP members communicate between each other through Hello messages every 3 seconds.
• The Hellos are sent to the multicast IP address 224.0.0.102 UDP port 3222 (source and destination).

GLBP Active Virtual Gateway (AVG)

• One member is elected as the AVG for a  GLBP group.
• Other members provide backup for the AVG in the event that the AVG becomes unavailable.
• The AVG assigns a virtual MAC address to each member of the GLBP group.
• Each gateway, an active virtual forwarder (AVF) assumes responsibility for forwarding packets sent to their virtual MAC address (as assigned by the AVG).
• The AVG replies to ARP requests for the virtual IP address.
• Load sharing is achieved by the AVG replying to ARP requests with different virtual MAC addresses.
• If a router goes down, another member will assume responsibility for forwarding packets to the virtual MAC address of the failed router.

GLBP Virtual MAC Address Assignment

• Up to four virtual MAC addresses per group.
• The AVG is responsible for assigning the virtual MAC addresses to each member of the group.
• Members request a virtual MAC address after they discover the AVG through Hello messages.
• Gateways are assigned the next MAC address in sequence.

GLBP Virtual Gateway Redundancy

• Operated in same way as HSRP.
• One gateway is elected as the AVG, another as the standby virtual gateway, and the remaining members 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 routers in the listen state.

GLBP Virtual Forwarder Redundancy

• Similar to virtual gateway redundancy.
• If the AVF fails one of the secondary virtual forwarders in the listen state assumes responsibility for the virtual MAC address.
• The redirect time is the interval during which the AVG continues to redirect hosts to the old virtual forwarder MAC address.
• When the redirect timer expires, the AVG stops redirecting hosts to the virtual forwarder.
• However, 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

• Determines the role that each GLBP gateway plays and what happens if the AVG fails.
• Defines the order of ascendancy to becoming an AVG if the current AVG fails.
• A value of 1 through 255.
• The router with the highest priority is elected as the AVG.
• With same priorities, the higher IP address is preferred.
• By default, the preemption is disabled.
• A backup virtual gateway can become the AVG only if the current AVG fails, regardless of the priorities assigned.
• 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

• A weighting scheme used to determine the forwarding capacity of each router in the GLBP group.
• The weighting determines if the router will forward packets and, if so, the proportion of hosts for which it will forward packets.
• Thresholds can be set to disable forwarding when the weighting falls below a certain value, and when it rises above another threshold, forwarding is automatically re-enabled.
• The GLBP group weighting can be automatically adjusted by tracking the state of an interface.
• 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.

Benefits

• load sharing: traffic from LAN clients can be shared by multiple routers
• multiple virtual routers: supports up to 1024 GLBP groups on each physical interface of a router, and up to four AVFs per group
• preemption: AVG with a higher priority backup virtual gateway; AVF preemption uses weighting instead of priority
• authentication: simple text password authentication scheme to detect configuration errors or the industry-standard MD5 algorithm for improved reliability, security, and protection against GLBP-spoofing software

Configuration Tasks

• Enabling GLBP (required).
• Customizing GLBP (optional).
• Configuring GLBP Weighting (optional).
• Verifying GLBP (optional).

Enabling GLBP

• Easy to configure.
• Each gateway in a GLBP group must be configured with the same group number.
• At least one gateway in the GLBP group must be configured with the virtual IP address of the group.
• All other parameters can be learned.
• If VLANs are in use on an interface, the GLBP group number must be different for each VLAN.

interface GigabitEthernet0/1
  glbp 47 ip 10.1.47.1

Customizing GLBP

• As soon as the GLBP group is enabled, that group is operating.
• If the GLBP group is enabled before customizing is completed, the router could take over control of the group and become the AVG before the full configuration is finished.
• Customize before enabling GLBP.

glbp 47 authentication text GLBPSTRING

• authenticates GLBP packets received from other routers in the group
• if configured, all routers within the GLBP group must use the same authentication string

glbp 47 forwarder preempt [delay minimum 30]

• configures the router to take over as AVF if it has a higher priority than the current AVF
• enabled by default with a delay of 30 seconds

glbp 47 load-balancing {host-dependent | round-robin | weighted}

• specifies the method of load-balancing used by the GLBP AVG

glbp 47 preempt [delay minimum <sec>]

• configures the router to take over as AVG if it has a higher priority than the current AVG
• disabled by default

glbp 47 priority 100

• sets the priority level of the gateway within a GLBP group
• the default is 100

glbp 47 timers [msec] Hellotime [msec] Holdtime

• Hellotime configures the interval between successive Hello packets sent by the AVG in a GLBP group
• Holdtime specifies the interval in seconds before the AVG and AVF information in the Hello packet is considered invalid

Configuring GLBP Weighting

• GLBP Weighting is used to determine whether a router can act as a virtual forwarder.
• Thresholds and tracked interfaces can be set.
• When the GLBP router weighting drops below a specified value, the router will no longer be an active virtual forwarder.
• When the weighting rises above a specified value, the router can resume its role as an active virtual forwarder.

track <obj-num> interface <int-num> {line-protocol | ip routing}

• configures an interface to be tracked where the status change affects the weighting of a GLBP gateway.
• line-protocol keyword tracks whether the interface is up
• ip routing keyword also checks that IP routing is enabled on the interface, and an IP address is configured

glbp 47 weighting <max> [lower <low>] [upper <up>]

• specifies the initial weighting value, and the lower and upper thresholds, for a GLBP gateway

glbp 47 weighting track <obj-num> [decrement <value>]

• specifies an object to be tracked that affects the weighting of a GLBP gateway
• <value> argument specifies a reduction in the weighting of a GLBP gateway when a tracked object fails

Verifying GLBP

dc1-edgertr1#show glbp
GigabitEthernet0/0 - Group 47
  State is Active
    2 state changes, last state change 11w6d
  Virtual IP address is 10.1.47.1
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 1.376 secs
  Redirect time 600 sec, forwarder timeout 14400 sec
  Authentication text, string "GLBPSTRING"
  Preemption disabled
  Active is local
  Standby is 10.42.100.102, priority 101 (expires in 9.440 sec)
  Priority 100 (default)
  Weighting 100 (default 100), thresholds: lower 1, upper 100
  Load balancing: round-robin
  Group members:
    3794.0fc5.3a1b (10.1.47.101)
    ccef.4712.7d1b (10.1.47.102) local
  There are 2 forwarders (1 active)
  Forwarder 1
    State is Active
      1 state change, last state change 19w1d
    MAC address is 0007.b400.0101 (default)
    Owner ID is ccef.4712.7d1b
    Redirection enabled
    Preemption enabled, min delay 30 sec
    Active is local, weighting 100
    Client selection count: 813
  Forwarder 2
    State is Listen
      2 state changes, last state change 11w6d
    MAC address is 0007.b400.0102 (learnt)
    Owner ID is 3794.0fc5.3a1b
    Redirection enabled, 599.456 sec remaining (maximum 600 sec)
    Time to live: 14399.456 sec (maximum 14400 sec)
    Preemption enabled, min delay 30 sec
    Active is 10.42.100.252 (primary), weighting 100 (expires in 9.600 sec)
    Client selection count: 814

References

Gateway Load Balancing Protocol