network protocols handbook.pdf

272Protocols GuideCisco ProtocolsProtocol NameHSRP: Hot Standby RouterProtocolProtocol DescriptionHot Standby Router Protocol (HSRP) is designed to supportnon-disruptive failover of IP traffic in certain circumstances andto allow hosts to appear to use a single router and to maintainconnectivity even if the actual first hop router they are usingfails. In other words, the protocol protects against the failure ofthe first hop router when the source host cannot learn the IP addressof the first hop router dynamically. Multiple routers participatein this protocol and in concert create the illusion of a singlevirtual router. The protocol insures that one and only one of therouters is forwarding packets on behalf of the virtual router. Endhosts forward their packets to the virtual router.The router forwarding packets is known as the active router. Astandby router is selected to replace the active router should itfail. The protocol provides a mechanism for determining activeand standby routers, using the IP addresses on the participatingrouters. If an active router fails a standby router can take overwithout a major interruption in the host’s connectivity.HSRP runs on top of UDP, and uses port number 1985. Routersuse their actual IP address as the source address for protocolpackets, not the virtual IP address. This is necessary so that theHSRP routers can identify each other.Protocol Structure8 16 24 32bitVersion Op code State HellotimeHoldtime Priority Group ReservedAuthentication dataAuthentication dataVirtual IP address• Version -- HSRP version number. The current versionis 0.• Op code -- Type of message contained in the packet.Possible values are:0 Hello, sent to indicate that a router is runningand is capable of becoming the active orstandby router.1 Coup, sent when a router wishes to becomethe active router.2 Resign, sent when a router no longer wishesto be the active router.• State -- Internally, each router in the standby groupimplements a state machine. The State field describesthe current state of the router sending themessage. Possible values are: 0 Initial; 1 Learn;2 Listen; 4 Speak; 8 Standby; 16 Active.• Hellotime -- Approximate period between the Hellomessages that the router sends (for Hello messagesonly). If the Hellotime is not configured on a router,then it may be learned from the Hello message fromthe active router.• Holdtime -- The amount of time, in seconds, that thecurrent Hello message should be considered valid.(For Hello messages only.)• Priority -- Used to elect the active and standby routers.When comparing priorities of two different routers,the router with the numerically higher prioritywins. In the case of routers with equal priority therouter with the higher IP address wins.• Group -- Identifies the standby group. For TokenRing, values between 0 and 2 inclusive are valid. Forother media, values between 0 and 255 inclusive arevalid.• Authentication data -- Clear-text 8 character reusedpassword. If no authentication data is configured, therecommended default value is 0x63 0x69 0x73 0x630x6F 0x00 0x00 0x00.• Virtual IP address -- Virtual IP address used by thisgroup. If the virtual IP address is not configured ona router, then it may be learned from the Hello messagefrom the active router. An address should onlybe learned if no address was configured and the Hellomessage is authenticated.Related protocolsIIP, UDPSponsor SourceHSRP is a Cisco protocol and circulated by IETF RFC 2281.Referencehttp://www.javvin.com/protocol/rfc2281.pdfHot Standby Router Protocol