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GLBP gateway load balancing protocol principle

GLBP( the Gateway Load Balancing Protocol), is Cisco’s proprietary protocol.LBP not only provides redundant gateways, but also provides load balancing between gateways.He compensates for the limitations of existing redundant router protocols. The goal of designing GLBP is to automatically select and use multiple available gateways simultaneously.Unlike HSRP and VRRP, GLBP can make full use of resources without having to configure multiple groups and manage multiple default gateway configurations.

Up to 4 routers in the GLBP group can be used as the IP default gateway. These gateways are called AVF (Active Virtual Forwarder).GLBP automatically manages the allocation of virtual MAC addresses and decides who is responsible for handling the forwarding work (this is the key to distinguishing between HSRP and VRRP, which has a virtual IP in GLBP but corresponds to multiple virtual MACs).

GLBP provides the following features:

GLBP AVG (Active Virtual Gateway): A member of a GLBP group will elect a gateway to become the AVG for that group.Other group members act as backup gateways for AVG in case AVG fails. AVG assigns a virtual MAC address to each member of the GLBP group.

GLBP AVF (Active Virtual Forwarder): AVG will assign virtual MAC addresses to the gateway, and each gateway has the responsibility to forward those packets sent to this virtual MAC address.These gateways are AVFs of virtual MAC addresses.

GLBP Communication :The GLBP members communicate with each other by sending a Hello packet to the multicast address 224.0.0.102 and UDP3222 every 3 seconds.

Active Gateway Election:Elect the active gateway using a mechanism similar to HSRP.

Election comparison

  • Priority (1-255 default 100)

  • ip address

The router with the highest priority becomes the active router. If the priorities are the same, the router with the highest IP address becomes the active router. Called Active Virtual Gateway, other non-AVGs provide redundancy.

After a router is elected as AVG, a different job from HSRP begins, and AVG assigns a virtual MAC address to other GLBP group members.

All routers in the GLBP group forward packets, but each router is only responsible for forwarding packets related to its own virtual MAC address.

GLBP allows automatic selection and uses all available gateways in the group at the same time. Members of the GLBP group will elect a gateway to serve as the AVG for the group.Other group members provide backups for AVG to prevent AVG from malfunctioning. AVG assigns a virtual MAC address to each member of the GLBP group.All routers will forward as data frames sent by the AVF to the virtual MAC address. When the client sends an ARP request to get the address of the default gateway, AVG will send these virtual MAC addresses in the ARP response.A maximum of 4 group members can be in a GLBP group. The practice is to provide users with a single IP, each router provides users with different MAC addresses, so that the packets sent by the user to the group IP are successfully shared on each router.

GLBP supports the use of multiple default routers to provide traffic load balancing for the same default gateway IP address. In this environment, GLBP supports the following load sharing modes.

The GLBP protocol supports three load balancing methods:

First: according to the source mac address of different hosts
Second: poll based on arp request
Third: According to the weight distribution of the router, the higher the weight, the more likely it is to be assigned.

bluefox(config-subif)#GLbp 1 load-balancing ?

  host-dependent Load balance equally, source MAC determines forwarder choice

  round-robin Load balance equally using each forwarder in turn

  weighted Load balance in proportion to forwarder weighting

Host-related load sharing algorithm: As long as a virtual MAC address is also involved in traffic forwarding in the GLBP group, it is ensured that a host always uses this virtual MAC address for communication.

Cyclic load sharing algorithm: When the client sends an ARP request to resolve the MAC address of the default gateway, the MAC address contained in the ARP response received by each client is the MAC address of the next available router in the round robin algorithm.The MAC addresses of all routers are put into the address resolution response in order, and the MAC address corresponding to the default gateway IP address is returned to the client.

Weighted load sharing algorithm: The amount of load directed to a router depends on the weighting value advertised by the router.

The weighting of the GLBP determines whether a router acts as a virtual forwarder. The administrator can set the initial weight value and can specify a threshold.The administrator can configure the GLBP to track the interface status and set a decrement value that is used to reduce the weighting value when the tracked interface is closed.When the weight of the GLBP router drops below a certain value, the router will no longer become an active virtual forwarder.When the weighting rises above a certain value, the router can resume its active virtual forwarder role.

The weighting mechanism of GLBP is different from HSRP or VRRP. Only one threshold is defined in HSRP and VRRP. If the router priority drops below this threshold, the router becomes active.Two thresholds are defined in the GLBP: a lower threshold is used when the router weight is reduced, and a higher threshold is used when the router weight is increased.This double threshold mechanism is more flexible than a single threshold syste

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