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The Virtual Router Redundancy Protocol (VRRP) is a computer networking protocol that provides for automatic assignment of available Internet Protocol (IP) routers to participating hosts. This increases the availability and reliability of routing paths via automatic default gateway selections on an IP subnetwork. The protocol achieves this by creation of virtual routers, which are an abstract representation of multiple routers, i.e. master and backup routers, acting as a group. The default gateway of a participating host is assigned to the virtual router instead of a physical router. If the physical router that is routing packets on behalf of the virtual router fails, another physical router is selected to automatically replace it. The physical router that is forwarding packets at any given time is called the master router. VRRP provides information on the state of a router, not the routes processed and exchanged by that router. Each VRRP instance is limited, in scope, to a single subnet. It does not advertise IP routes beyond that subnet or affect the routing table in any way. VRRP can be used in Ethernet, MPLS and token ring networks with Internet Protocol Version 4 (IPv4), as well as IPv6. The protocol is described in IETF publication RFC 5798, which is an open standard, but Cisco claims that a similar protocol with essentially the same facility is patented and licensed;〔(IETF source )〕 however in reply to a direct request Robert Barr of Cisco replied in 2001 that they will not assert any patent claims unless someone tried to assert a claim against Cisco. IBM also claims covering patents and their statement is readable on the IETF webpage. ==Implementation== A virtual router must use 00-00-5E-00-01-XX as its Media Access Control (MAC) address. The last byte of the address (XX) is the Virtual Router IDentifier (VRID), which is different for each virtual router in the network. This address is used by only one physical router at a time, and it will reply with this MAC address when an ARP request is sent for the virtual router's IP address. Physical routers within the virtual router must communicate within themselves using packets with multicast IP address 224.0.0.18 and IP protocol number 112.〔(Section 5.2.4. Protocol )〕 Routers have a priority of between 1-255 and the router with the highest priority will become the master. The default priority is 100, for address owner the priority is always 255. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Virtual Router Redundancy Protocol」の詳細全文を読む スポンサード リンク
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