S7C8 Hot Standby Router Protocol RFC 2281 Provides fault tolerance Backup and load sharing

Traditional Routing Default gateways Proxy ARP RIP When router with default gateway fails, entire campus loses connectivity Proxy ARP Again, does not work when proxy router fails RIP Limitation 15 hops; slow convergence ICMP Router Discovery Protocol (IRDP) Finds new router (not a routing protocol) – RFC 1256

HSRP Explained Set of routers work together to form single virtual standby router (single MAC & IP address) Packets still routed when original router fails Standby router begins to function Routers exchange information to determine who forwards traffic Routers recognize own MAC & group MAC address Routers who only recognize single MAC address use HSRP MAC address when the active router and BIA address when in standby mode

HSRP Members Active router Standby router Virtual router Other routers Forwarding packets at present time Standby router Takes over when active router fails Monitors operational status of active HSRP router Limit of 255 standby groups on a given LAN Virtual router Name given to group of routers One router may be member of several groups Other routers

HSRP and ISL HSRP over ISL not possible <IOS 11.3 Eliminates situations when a single point of failure causes interruptions Define encapsulation type Configure IP address Enable HSRP

Multiple HSRP Groups Only active router forwards packets Each VLAN requires separate HSRP group Multiple standby groups may coexist – each group emulates a virtual router Routers may participate in multiple groups One IP and MAC address for each group Up to 255 standby groups on one LAN –load splitting can be achieved by distributing hosts among different standby groups

Electing an Active Router Hello messages sent and received Contain a priority level – default 100 Router with highest priority becomes active If priorities are same, highest IP address wins Only active router forwards packets

HSRP Messages Use port 1985 and multicast address Fields include: Version, Op Code, Hello/Group/Reserved, state, hellotime, holdtime, priority, group id, authentication, virtual IP address

HSRP States Initial – configuration change Learn – waiting to hear from active router Listen – knows virtual IP address; listens for hello messages Speak – sends periodic hello messages Standby – candidate to become next active router Active – forwarding packets; sends hellos

HSRP Configuration (config-if)# standby 10 ip n.n.n.n IP address is for virtual router (config-if)#standby 10 priority 110 (config-if)#standby 10 preempt Enables active router to reassume forward role (config-if)#standby 10 timers 10 20 Defaults are 3 and 10; range is 0-255 (config-if)#standby 10 track S01 9 (config-if)#no standby 10 track S01 9 (config)#show standby S01 group brief (config)#debug standby