CCNA Routing & Switching : EIGRP metric

To distribute routing information throughout a network, EIGRP uses non-periodic incremental routing updates. That is, EIGRP only sends routing updates about paths that have changed when those paths change. The basic problem with sending only routing updates is that you may not know when a path through a neighboring router is no longer available. You can not time out routes, expecting to receive a new routing table from your neighbors. EIGRP relies on neighbor relationships to reliably propagate routing table changes throughout the network; two routers become neighbors when they see each other's hello packets on a common network. EIGRP sends hello packets every 5 seconds on high bandwidth links and every 60 seconds on low bandwidth multipoint links. 5-second hello: broadcast media, such as Ethernet, Token Ring, and FDDI point-to-point serial links, such as PPP or HDLC leased circuits, Frame Relay point-to-point subinterfaces, and ATM point-to-point subinterface high bandwidth (greater than T1) multipoint circuits, such as ISDN PRI and Frame Relay 60-second hello: multipoint circuits T1 bandwidth or slower, such as Frame Relay multipoint interfaces, ATM multipoint interfaces, ATM switched virtual circuits, and ISDN BRIs The rate at which EIGRP sends hello packets is called the hello interval, and you can adjust it per interface with the ip hello-interval eigrp command. The hold time is the amount of time that a router will consider a neighbor alive without receiving a hello packet. The hold time is typically three times the hello interval, by default, 15 seconds and 180 seconds. You can adjust the hold time with the ip hold-time eigrp command. Note that if you change the hello interval, the hold time is not automatically adjusted to account for this change - you must manually adjust the hold time to reflect the configured hello interval. It is possible for two routers to become EIGRP neighbors even though the hello and hold timers do not match. The hold time is included in the hello packets so each neighbor should stay alive even though the hello interval and hold timers do not match. While there is no direct way of determining what the hello interval is on a router, you can infer it from the output of show ip eigrp neighbors on the neighboring router. If you have the output of a show ip eigrp neighbors command from your Cisco device, you can use Output Interpreter (registered customers only) to display potential issues and fixes. To use Output Interpreter, you must have JavaScript enabled. router# show ip eigrp neighbors IP-EIGRP neighbors for process 1 H Address Interface Hold Uptime SRTT RTO Q Seq Type (sec) (ms) Cnt Num 1 10.1.1.2 Et1 13 12:00:53 12 300 0 620 0 10.1.2.2 S0 174 12:00:56 17 200 0 645 rp-2514aa# show ip eigrp neighbor IP-EIGRP neighbors for process 1 H Address Interface Hold Uptime SRTT RTO Q Seq Type (sec) (ms) Cnt Num 1 10.1.1.2 Et1 12 12:00:55 12 300 0 620 0 10.1.2.2 S0 173 12:00:57 17 200 0 645 rp-2514aa# show ip eigrp neighbor IP-EIGRP neighbors for process 1 H Address Interface Hold Uptime SRTT RTO Q Seq Type (sec) (ms) Cnt Num 1 10.1.1.2 Et1 11 12:00:56 12 300 0 620 0 10.1.2.2 S0 172 12:00:58 17 200 0 645