Virtual Router Redundancy Protocol (VRRP) is an IETF proposed standard that is very similar to HSRP. This is an open standard to allow vendor interoperability, but (according to Cisco) is not as reliable or quick to converge as HSRP. Cisco advises to always run HSRP rather than VRRP, unless on a multi-vender infrastructure.

In HSRP, there is one active router, one standby router, and perhaps additional listening routers. VRRP has one master router and one or more backup routers. When these groups are initialized, messages are exchanged to elect the master or active router. These messages include group identifier, priority, virtual IP, and hello interval. In HSRP, active and standby routers then send periodic hello messages. In VRRP, only the master sends messages which are known as advertisements.

If the hello or advertisements have not been received within a certain interval by the standby or backup routers, they automatically assume the active/master role. When the original router comes back up again, it will send out a hello/advertisement and may regain the primary role again.

VRRP customization is option. As soon as a VRRP group is enabled, it begins to operate. Be sure to customize VRRP prior to initialization, otherwise a master router may have been elected before you finish customization.

Gateway Load Balancing Protocol (GLBP) is similar to HSRP and VRRP in that it provides near-instant fail-over capability using virtual IP and MAC addresses. Additionally, GLBP allows load-balancing between multiple routers in a group. The following platforms support GLBP:

• Cisco 2600 series routers
• Cisco 3620 router
• Cisco 3640 router
• Cisco 7200 series routers
• Cisco 7500 series routers
• Cisco Catalyst 6000 MSFC

Following is a list of GLBP definitions:

Term	Definition
Active Virtual Gateway (AVG)	One AVG is elected per GLBP group. This router is responsible for assigning virtual MAC addresses to the rest of the member routers. Other group members provide backup to this router in case it goes down.
Virtual Forwarder (VF)	This is an abstract entity in a GLBP group. It may assume responsibility for a virtual mac address.
Active Virtual Forwarder (AVF)	One virtual forwarder within a GLBP group is elected the active virtual forwarder. The AVF is responsible for forwarding packets sent to a certain virtual mac address
Primary Virtual Forwarder (PVF)	A primary virtual forwarder is a virtual forwarder that has been assigned the virtual MAC address by the active virtual gateway.
Secondary Virtual Forwarder (SVF)	A secondary virtual forwarder is a virtual forwarder that has learned the virtual MAC address from a Hello message.

Single Router Mode (SRM) allows multiple MSFCs to work redundantly in a router. Only one MSFC is visible to the network at a time. All interfaces on the secondary MSFC are kept in a 'line down' state, though the rest of the configurations are identical.

If the designated router fails, the backup changes all its interface states to 'link up'. Existing switch processor entries forward data while the MSFC builds its routing tables. Once those are done being rebuilt, the entires in the switch processor are updated.

Server Load Balancing (SLB) allows a virtual server to represent an entire server farm. With SLB set up, client requests to a virtual ip address are load-shared across multiple servers. This virtual ip address is set up one each of these servers as a local loopback address. With this configured, the addition or failure of servers goes unnoticed by clients.

The following platforms support SLB:

• Catalyst 6000 Series
• Cisco 7200 Seroes

The following commands can be used for troubleshooting SLB:

Command	Definition
show ip slb conns [vserver virtserver-name] [client ip-address] [detail]	Displays all connections handled by SLB, or optionally only the connections associated with a specific virtual server or client.
show ip slb dfp [agent ip_addr port] [detail] [weights]	Display information about DFP and DFP agents as well as about weights assigned to real servers.
show ip slb reals [vserver virtserver-name] [detail]	Displays information about the real servers defined to IOS SLB/
show ip slb serverfarms [name serverfarm-name] [detail]	Displays information about the server farms defined to IOS SLB.
show ip slb stats	Displays IOS SLB statistics.
show ip slb sticky [client ip-address]	Displays information about the sticky connections defined to IOS SLB
show ip slb vservers [name virtserver-name] [detail]	Displays information about the virtual servers defined to IOS SLB.

Debug Command	Description
all	Displays all IOS SLB debug messages.
conns	Displays debug messages for all connections being handled by IOS SLB, including WSP events and states.
dfp	Displays debug messages for DFP. To display debug messages for the DFP agent subsystem, use debug ip dfp agent.
firewallfarm	Displays debug messages related to firewall load balancing
icmp	Displays all Internet Control Message Protocol debug messages for IOS SLB
natpool	Displays debug messages related to the IOS SLB client NAT pool.
probe	Displays debug messages related to probes.
reals	Displays debug messages for all real servers defined to IOS SLB.
replication	Displays debug messages related to IOS SLB stateful backup virtual server.
sessions	Displays debug messages for all sessions being handled by IOS SLB.
vservers	Displays debug messages for all virtual server defined to IOS SLB.