Changing the Hello timer does not automatically change the Hold timer. Additionally, Hello and Hold timers do not need to match between routers for an EIGRP neighbor relationship to form. Successor Path and Feasible Successor path: EIGRP enabled routers will send HELLO Packets to each other and become neighbors. EIGRP neighbors will exchange routing information which will be saved in the topology table. The best path from the topology table will be copied in the routing table. Such path is called Successor. Metric of the successor path is called feasible distance. After finding the successor path, EIGRP will find another path as a backup path to successor to give redundancy is called feasible successor. The metric used to find feasible successor is called Advertised distance (AD) or Reported distance (RD).This Advertised distance (AD) is the distance between neighbor of the source and destination. In short, Advertised distance: How far the destination is away for your neighbor. Feasible distance: The total distance to the destination. Successor: The best path to the destination Condition for choosing feasible successor: Advertised distance (AD) must be less than the metric of successor path Advertised distance of feasible successor < Feasible distance of successor.
SHARAFAT ALI Mob No. -8305012450 4 Note: While calculation best paths, one thing you must keep in mind is Split horizon rule which states “whatever you learn on an interface you don’t advertise back out of the same interface.” Feasible Successors provide EIGRP with redundancy, without forcing routers to reconverge when a topology change occurs. If no Feasible Successor exists and a link fails, a route will enter an Active (converging) state until an alternate route is found. EIGRP Route States An EIGRP route can exist in one of two states, in the topology table: • Active state • Passive State A Passive state indicates that a route is reachable, and that EIGRP is fully converged. A stable EIGRP network will have all routes in a Passive state. A route is placed in an Active state when the Successor and any Feasible Successors fail, forcing the EIGRP to send out Query packets and re-converge. Multiple routes in an Active state indicate an unstable EIGRP network. If a Feasible Successor exists, a route should never enter an Active state. You can check the status of states by using: Router# show ip eigrp topology To view only active routes in the topology table: Router# show ip eigrp topology active EIGRP Metrics EIGRP can utilize 5 separate metrics to determine the best route to a destination: 1. Bandwidth (K1) 2. Load (K2) 3. Delay of the Line (K3) 4. Reliability (K4) 5. MTU (K5) By default, only Bandwidth and Delay of the Line are used. K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0
SHARAFAT ALI Mob No. -8305012450 5 EIGRP Passive Interface In EIGRP (and OSPF) the passive interface command stops sending outgoing hello packets, hence the router cannot form any neighbor relationship via the passive interface. This behavior stops both outgoing and incoming routing updates.
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- Summer '20
- Cisco Routers, Sharafat Ali