VRRP

虚拟路由冗余协议（VRRP:Virtual Router Redundancy Protocol就是一种很好的解决方案。在该协议中，对共享多存取访问介质（如以太网）上终端IP设备的默认网关（Default Gateway进行冗余备份，从而在其中一台路由设备宕机时，备份路由设备及时接管转发工作，向用户提供透明的切换，提高了网络服务质量。为了避免由这个默认网关造成的单点故障，可以在一个广播域中配置多个路由器接口，并在这些路由器上运行VRRP（虚拟路由器冗余协议）。简单来说，VRRP是一种容错协议，它为具有组播或广播能力的局域网，（如以太网）设计，它保证当局域网内主机的下一跳路由器出现故障时，可以及时的由另一台路由器来代替，从而保持通讯的连续性和可靠性。为了使VRRP工作，要在路由器上配置虚拟路由器号和虚拟IP地址，同时产生一个虚拟MAC地址，这样在这个网络中就加入了一个虚拟路由器。而网络上的主机与虚拟路由器通信，无需了解这个网络上物理路由器的任何信息。一个虚拟路由器由一个主路由器和若干个备份路由器组成，主路由器实现真正的转发功能。当主路由器出现故障时，一个备份路由器将成为新的主路由器接替它的工作。在VRRP协议中，有两组重要的概念：VRRP路由器和虚拟路由器，主控路由器和备份路由器。VRRP路由器是指运行VRRP的路由器，是物理实体，虚拟路由器是指VRRP协议创建的，是逻辑概念。一组VRRP路由器协同工作，共同构成一台虚拟路由器。该虚拟路由器对外表现为一个具有唯一固定IP地址和MAC地址的逻辑路由器。处于同一个VRRP组中的路由器具有两种互斥的角色：主控路由器和备份路由器，一个VRRP组中有且只有一台处于主控角色的路由器，可以有一个或者多个处于备份角色的路由器。VRRP协议使用选择策略从路由器组中选出一台作为主控，负责ARP相应和转发IP数据包，组中的其它路由器作为备份的角色处于待命状态。当由于某种原因主控路由器发生故障时，备份路由器能在几秒钟的时延后升级为主路由器。由于此切换非常迅速而且不用改变IP地址和MAC地址，故对终端使用者系统是透明的。只有当这个VRRP组中的所有的路由器都不能正常工作时，该域中的主机才不能与域外界通信。VRRP中只定义了一种报文——VRRP报文，这是一种组播报文，封装在IP报文上，由主路由器定时发出来通告它的存在，使用这些报文可以检测虚拟路由器各种参数，还可以用于主路由器的选举。

VRRP (virtual route redundancy protocol) is a good solution. In this protocol, the default gateway of the terminal IP device on the shared multi access media (such as Ethernet) is redundantly backed up, so that when one of the routing devices goes down, the backup routing device takes over the forwarding work in time, provides transparent switching to users, and improves the quality of network service. To avoid a single point of failure caused by this default gateway, you can configure multiple router interfaces in a broadcast domain and run VRRP (Virtual Router Redundancy Protocol) on these routers. In short, VRRP is a fault-tolerant protocol, which is designed for local area network (such as Ethernet) with multicast or broadcast capability. It ensures that when the next hop router of the host in the local area network fails, it can be replaced by another router in time, so as to maintain the continuity and reliability of communication. In order to make VRRP work, the virtual router number and virtual IP address should be configured on the router, and a virtual MAC address should be generated at the same time, so a virtual router is added to the network. The host on the network communicates with the virtual router without knowing any information about the physical router on the network. A virtual router consists of a main router and several backup routers. The main router realizes the real forwarding function. When the main router fails, a backup router will become the new main router to take over its work. In VRRP, there are two important concepts: VRRP router and virtual router, master router and backup router. VRRP router refers to the router running VRRP, which is a physical entity. Virtual router refers to the creation of VRRP protocol, which is a logical concept. A group of VRRP routers work together to form a virtual router. The virtual router is a logical router with unique fixed IP address and MAC address. Routers in the same VRRP group have two mutually exclusive roles: the master router and the backup router. There is only one router in the master role in a VRRP group, and there can be one or more routers in the backup role. VRRP uses the selection strategy to select one router from the router group as the master, responsible for ARP corresponding and forwarding IP packets, and other routers in the group are standby as backup roles. When the master router fails for some reason, the backup router can be upgraded to the master router after a delay of several seconds. Because this switch is very fast and does not change the IP address and MAC address, it is transparent to the end-user system. Only when all routers in this VRRP group are not working properly, the hosts in this domain cannot communicate with the outside of the domain. VRRP only defines one kind of message, which is a kind of multicast message. It is encapsulated in IP message and sent out by the main router regularly to announce its existence. These messages can be used to detect various parameters of the virtual router, and can also be used for election of the main router.

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