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[ Pobierz całość w formacie PDF ]
.Availability, Scalability and Load Balancing 537 addressed to one of these IP addresses for ICMP pings, TCP connections,etc.Primary IP AddressAn IP address selected from the set of real interface addresses.Onepossible selection algorithm is to always select the first address.VRRPadvertisements are always sent using the primary IP address as the source ofthe IP packet.Virtual Router MasterThe VRRP router that is assuming the responsibility of forwarding packetssent to the IP address(es) associated with the virtual router, and answeringARP requests for these IP addresses.Note that if the IP address owner isavailable, then it will always become the master.Virtual Router BackupThe set of VRRP routers available to assume forwarding responsibility for avirtual router should the current master fail.11.1.3 VRRP OverviewVRRP specifies an election protocol to provide the virtual router function describedearlier.All protocol messaging is performed using IP multicast datagrams (seemulticast in 9.1, Multicasting on page 467), thus the protocol can operate over avariety of multiaccess LAN technologies supporting IP multicast.Each VRRPvirtual router has a single well-known MAC address allocated to it.The virtualrouter MAC address is used as the source in all periodic VRRP messages sent bythe master router to enable bridge learning in an extended LAN.A virtual router is defined by its virtual router identifier (VRID) and a set of IPaddresses.A VRRP router can associate a virtual router with its real addresses onan interface, and can also be configured with additional virtual router mappings andpriority for virtual routers it is willing to back up.The mapping between VRID andaddresses must be coordinated among all VRRP routers on a LAN.However,there is no restriction against reusing a VRID with a different address mapping ondifferent LANs.The scope of each virtual router is restricted to a single LAN.To minimize networktraffic, only the master for each virtual router sends periodic VRRP advertisementmessages.A backup router will not attempt to pre-empt the master unless it hashigher priority.This eliminates service disruption unless a more preferred pathbecomes available.It's also possible to administratively prohibit all pre-emptionattempts.The only exception is that a VRRP router will always become master ofany virtual router associated with addresses it owns.If the master becomesunavailable then the highest priority backup will transition to master after a shortdelay, providing a controlled transition of the virtual router responsibility withminimal service interruption.The VRRP protocol design provides rapid transition from master to backup tominimize service interruption, and incorporates optimizations that reduce protocolcomplexity while guaranteeing controlled master transition for typical operationalscenarios.The optimizations result in an election protocol with minimal runtimestate requirements, minimal active protocol states, and a single message type andsender.The typical operational scenarios are defined to be two redundant routersand/or distinct path preferences among each router.A side effect when theseassumptions are violated (for example, more than two redundant paths all with538 TCP/IP Tutorial and Technical Overview equal preference) is that duplicate packets can be forwarded for a brief periodduring master election.However, the typical scenario assumptions are likely tocover the vast majority of deployments, loss of the master router is infrequent, andthe expected duration in master election convergence is quite small ( [ Pobierz całość w formacie PDF ]
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.Availability, Scalability and Load Balancing 537 addressed to one of these IP addresses for ICMP pings, TCP connections,etc.Primary IP AddressAn IP address selected from the set of real interface addresses.Onepossible selection algorithm is to always select the first address.VRRPadvertisements are always sent using the primary IP address as the source ofthe IP packet.Virtual Router MasterThe VRRP router that is assuming the responsibility of forwarding packetssent to the IP address(es) associated with the virtual router, and answeringARP requests for these IP addresses.Note that if the IP address owner isavailable, then it will always become the master.Virtual Router BackupThe set of VRRP routers available to assume forwarding responsibility for avirtual router should the current master fail.11.1.3 VRRP OverviewVRRP specifies an election protocol to provide the virtual router function describedearlier.All protocol messaging is performed using IP multicast datagrams (seemulticast in 9.1, Multicasting on page 467), thus the protocol can operate over avariety of multiaccess LAN technologies supporting IP multicast.Each VRRPvirtual router has a single well-known MAC address allocated to it.The virtualrouter MAC address is used as the source in all periodic VRRP messages sent bythe master router to enable bridge learning in an extended LAN.A virtual router is defined by its virtual router identifier (VRID) and a set of IPaddresses.A VRRP router can associate a virtual router with its real addresses onan interface, and can also be configured with additional virtual router mappings andpriority for virtual routers it is willing to back up.The mapping between VRID andaddresses must be coordinated among all VRRP routers on a LAN.However,there is no restriction against reusing a VRID with a different address mapping ondifferent LANs.The scope of each virtual router is restricted to a single LAN.To minimize networktraffic, only the master for each virtual router sends periodic VRRP advertisementmessages.A backup router will not attempt to pre-empt the master unless it hashigher priority.This eliminates service disruption unless a more preferred pathbecomes available.It's also possible to administratively prohibit all pre-emptionattempts.The only exception is that a VRRP router will always become master ofany virtual router associated with addresses it owns.If the master becomesunavailable then the highest priority backup will transition to master after a shortdelay, providing a controlled transition of the virtual router responsibility withminimal service interruption.The VRRP protocol design provides rapid transition from master to backup tominimize service interruption, and incorporates optimizations that reduce protocolcomplexity while guaranteeing controlled master transition for typical operationalscenarios.The optimizations result in an election protocol with minimal runtimestate requirements, minimal active protocol states, and a single message type andsender.The typical operational scenarios are defined to be two redundant routersand/or distinct path preferences among each router.A side effect when theseassumptions are violated (for example, more than two redundant paths all with538 TCP/IP Tutorial and Technical Overview equal preference) is that duplicate packets can be forwarded for a brief periodduring master election.However, the typical scenario assumptions are likely tocover the vast majority of deployments, loss of the master router is infrequent, andthe expected duration in master election convergence is quite small ( [ Pobierz całość w formacie PDF ]