EIGRP Fundamentals
EIGRP uses diffusing update algorithm(DUAL) to identify network paths. EIGRP uses hop count, link speed and delay to factor into metrics for picking routes
Autonomous Systems
Each EIGRP process is assigned an autonomous systems(AS) number this represents a common routing domain. A router can participate in more than one AS but by default does not distribute routes learned from one AS to another AS(e.g.AS 1 will not know about routes from AS 2)
EIGRP Terms
• Successor Route; route with the lowest path metric
• Successor; first next hop router for the successor route
• Feasible Distance; the metric value for the lowest metric path to reach a destination.
• Reported Distance; the distance reported by a router to reach a prefix.
• Feasibility Condition; a condition under which a route can be considered a backup path.
• Feasible Successor; a route that satisfies the feasibility condition.
Topology Table
The topology contains all network prefixes with in a AS. Each entry has a network prefix, EIGRP Neighbors, Metrics, Values for obtaining the metric
In the topology table output P means the topology is stable it would switch to A when computing a new path.
HomeGarage#sh ip eigrp topology
EIGRP-IPv4 Topology Table for AS(1)/ID(172.16.10.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.10.0.0/16, 1 successors, FD is 3072
via 172.16.10.2 (3072/2816), Vlan172
EIGRP Neighbors
Neighbors advertise only incremental changes in the network. Then are 5 packet types. Hello, Request, Update, Query, Reply.
Path Metric
In general the higher the bandwidth of a connection the lower the metric of the link. The original EIGRP spec did not scale for multi-gigabit links. Wide Metrics allow support of multi-gigabit links
Load Balancing
EIGRP allows multiple Successor Routes(with the same metric) to be added to the routing table. This will allow for equal cost load balancing by default.
Failure Detection and Timers
EIGRP has a normal timer and slow speed link timer. 5 seconds for normal links and 60 for slow links for hello packets to be sent. The hold timer is 15 seconds for normal links and 180 seconds for slow links. If the hold timer reaches 0 a topology changes occurs. When a failure is detected a feasible successor will become the successor instantly.
Route Summarization
Scalability of an EIGRP AS depends on summarization as the size of an AS grows convergence may take longer. 192.168.0.1/24 and 192.168.1.1/24 exist on R1 and advertise to R2, R2 could tell R3 of the networks by advertising 192.168.0.0/16