Redundancy Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.


*** Theory ****

  • ICMP Router Discovery Protocol (IRDP) – Generates Router Advertisement heard by host on that segment. If a host hears more than one IRDP router, it will choose 1 as it primary and failover to second if need be. Host uses real IP and MAC. Hosts can send RS, requesting for RA.
  • Hot Standby Routing Protocol (HSRP) – Cisco. 1 router is primary. Host uses virtual MAC/IP. (pseudorouter).  Hello timers etc can be changed. Highest priority determines primary router (pre-empt is disabled by default , so changing priority will not take immediate effect)
    • 00-00-0c-070ac0xx = HSRP well known MAC. Xx is group number, e.g. 05 or 17 = 11 (16 + 1)
    • States
      • Disabled
      • Initial (init) – interface is up but HSRP not running
      • Learn – Learn about the active router etc
      • Listen – knows the VMAC and listening for Hellos
      • Speak – Sending hellos
      • Standby  – Sends hello and is candidate for active
      • Active – Router is forwarding to VIP
  • HSRP Interface tracking – Monitor additional interface e.g. (WAN). If WAN goes down, the router priority goes down allowing the other router to be high priority and takeover (pre-empt enabled) – config-if#standby 1 track serial 0 decrement
  • VRRP – Same as HSRP, but Active router is known as the master router, standby is backup. Multicast is MAC is 00-00-52-00-01-xx (xx is group number in hex). VRRP has pre-empt by default.
  • GLBP – Cisco only. Does load balancing.  Lets routers work on the load via round-robin. Host sees on gateway, but really there is multiple gateway. Host gets real MAC of router but  IP is VIP.
    • AVG – Active Virtual Gateway is the router with the HIGHEST GLBP priority (highest IP if tied).  It sends virtual MAC as ARP response, which is the same layer 3 address and is how load balance is achieved. There a stand by AVG and AVF
    • Algorithms – Round robin, host-dependent load balancing (Same MAC everytime), weighted – Percentage of traffic per Router


*** Commands ****

Config-if# ip irdp // enables IRDP

Config-if# (SHOW) standby 5 ip x.x.x.x// enables HSRP