GLBP
The Gateway Load Balancing Protocol (GLBP) is a Cisco-proprietary protocol designed to overcome the limitations of existing redundant router protocols. Some of the concepts are the same as with HSRP/VRRP, but the terminology is different and the behavior is much more dynamic and robust.
To provide a virtual router, multiple switches (routers) are assigned to a common GLBP group.Instead of having just one active router performing forwarding for the virtual router address,
all routers in the group can participate and offer load balancing by forwarding a portion of the overall traffic.
The advantage is that none of the clients has to be pointed toward a specific gateway address; they can all have the same default gateway set to the virtual router IP address. The load balancing is provided completely through the use of virtual router MAC addresses in ARP replies returned to the clients. As a client sends an ARP request looking for the virtual router address, GLBP sends back an ARP reply with the virtual MAC address of a selected router in the group. The result is that all clients use the same gateway address but have differing MAC addresses for it.
Active Virtual Gateway
The trick behind this load balancing lies in the GLBP group. One router is elected the
active virtual gateway (AVG). This router has the highest priority value, or the highest IP address in the group, if there is no highest priority. The AVG answers all ARP requests for the virtual router address. Which MAC address it returns depends on which load-balancing algorithm it is configured to use. In any event, the virtual MAC address supported by one of the routers in the group is returned.
The AVG also assigns the necessary virtual MAC addresses to each of the routers participating in the GLBP group. Up to four virtual MAC addresses can be used in any group. Each of these routers is referred to as an
active virtual forwarder (AVF), forwarding traffic received on its virtual MAC address. Other routers in the group serve as backup or secondary virtual forwarders, in case the AVF fails. The AVG also assigns secondary roles.
Assign the GLBP priority to a router with the following interface configuration command:
Switch(config-if)# glbp group priority level
GLBP group numbers range from 0 to 1023. The router priority can be 1 to 255 (255 is the highest priority), defaulting to 100.
As with HSRP, another router cannot take over an active role until the current active router fails. GLBP does allow a router to pre-empt and become the AVG if it has a higher priority than the current AVG. Use the following command to enable pre-empting and to set a time delay before pre-empting begins:
Switch(config-if)# glbp group preempt [delay minimum seconds]
Routers participating in GLBP must monitor each other’s presence so that another router can assume the role of a failed router. To do this, the AVG sends periodic hello messages to each of the other GLBP peers. In addition, it expects to receive hello messages from each of them. Hello messages are sent at
hellotime intervals, with a default of 3 seconds. If hellos aren’t received from a peer within a
holdtime, defaulting to 10 seconds, that peer is presumed to have failed. You can adjust the GLBP timers with the following interface-configuration command:
Switch(config-if)# glbp group timers [msec] hellotime [msec] holdtime
The timer values normally are given in seconds, unless they are preceded by the
msec keyword,to indicate milliseconds. The
hellotime can range from 1 to 60 seconds or from 50 to 60,000 milliseconds. The
holdtime must be greater than the
hellotime and can go up to 180 seconds or 180,000 milliseconds. You always should make the
holdtime at least three times greater than the
hellotime to give some tolerance to missed or delayed hellos from a functional peer.
Active Virtual Forwarder
By default, GLBP uses the periodic hello messages to detect AVF failures, too. Each router within a GLBP group must send hellos to every other GLBP peer. Hellos also are expected from every other peer. For example, if hellos from the AVF are not received by the AVG before its holdtime timer expires, the AVG assumes that the current AVF has failed. The AVG then assigns the AVF role to another router.
Naturally, the router that is given the new AVF role might already be an AVF for a different virtual MAC address. Although a router can masquerade as two different virtual MAC addresses to support the two AVF functions, it doesn’t make much sense to continue doing that for a long period of time. The AVG maintains two timers that help resolve this condition.
GLBP Load Balancing
The AVG establishes load balancing by handing out virtual router MAC addresses to clients in a deterministic fashion. Naturally, the AVG first must inform the AVFs in the group of the virtual MAC address that each should use. Up to four virtual MAC addresses, assigned in sequential order, can be used in a group.
You can use one of the following load-balancing methods in a GLBP group:
■
Round robin―Each new ARP request for the virtual router address receives the next available virtual MAC address in reply. Traffic load is distributed evenly across all routers participating as AVFs in the group, assuming that each of the clients sends and receives the same amount of traffic. This is the default method used by GLBP.
■
Weighted―The GLBP group interface’s weighting value determines the proportion of traffic that should be sent to that AVF. A higher weighting results in more frequent ARP replies containing the virtual MAC address of that router. If interface tracking is not configured, the maximum weighting value configured is used to set the relative proportions among AVFs.
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Host-dependent―Each client that generates an ARP request for the virtual router address always receives the same virtual MAC address in reply. This method is used if the clients have a need for a consistent gateway MAC address. (Otherwise, a client could receive replies with different MAC addresses for the router over time, depending on the load-balancing method in use.)
On the AVG router (or its successors), use the following interface configuration command to define the method:
Switch(config-if)# glbp group load-balancing [round-robin
|
weighted
|
host-dependent]