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One of my favorite routing subjects to discuss. Usually, I’ll be asked what is a better IGP OSPF or EIGRP and my answer is always… it depends. There a couple compelling reasons why EIGRP is still a great IGP after so many years. Here are a few in my opinion; ease of deployment, convergence speed (without tweaks), and offset capability. Of course, there are some disadvantages; scale, ease of deployment (your not required to take a deep look at your topology like OSPF), and it’s proprietary.

The EIGRP Fundamentals:

Uses the DUAL algorithm to prevent routing loops and propagate topology information.
Split horizon/DUAL are responsible for maintaining a loop free topology. Split Horizon is disabled on frame-relay physical interfaces but, enabled on all others.
EIGRP is classless
MD5 only authentication (use “debug eigrp packet” to verify)
Constraining bandwidth and cumulative delay are the default metrics used (ToS:0, K values:10100)
EIGRP (using default K’s) Metric = 256*(Bw + Delay), EIGRP Metric = 256*((10^7 / min. Bw) + Delay)
Maximum paths is 4, the range is 1-6.

Metric Weights: Default TOS0, K1=1, K2=0, K3=1, K4=0, K5=0 (must match to form neighbor)

usec (microsecond) is the delay value on IOS routers. The EIGRP formula is 10 of usec. Delay of 100000usec is actually, 100000/10=10,000

Plug those numbers in now. 256*(1544 (t1)+12,000)
Decimals are rounded DOWN to the nearest WHOLE number.

RD: Is there neighbors metric to that interface/route.
FD:  The metric for the lowest metric path to reach subnet.
Feasibility condition: RD must be lower than FD.
Successor route: Lowest metric route.
FS: Not the successor, but can be used when the successor fails without introducing a loop.

P, 1 successors, FD is 2693120
        via (FD:2693120/RD:2181120), Serial0/0/0.1

Variance: If other FS have a better metric (lower) than the product of variance multiplier * FD they are added to the RIB.

Hellos sent to via IP protocol 88. 5 seconds on PTP/LAN and 60 seconds on multipoint links with less than 1Mbps. Dead time is 3 x the hello.

Network statement is what interface is running the EIGRP process.

“sh ip eigrp nei” “Q Count” value of 0 means no updates to be sent and the network has converged. 

Disabling split horizon on the hub can cause route replication in the topology table.

“ip default-network” can advertise default (no really but a candidate).

“ip summary-address eigrp xxx” will suppress local all other routes unless you use “leak-map”

Poison the summary (AD 255) to allow longer match without dropping via null0

 Unequal cost load balancing using Variance: 

SIA Timer: config-router> “timer active-time X” This is disabled by default, so it will continue until the end of time… Disables the timers and permits the routing wait time to remain active indefinitely.

IMPORTANCE OF EIGRP ROUTER ID: This is a new one for me. I found out that any external routes injected into EIGRP are tagged with the redistributing router’s RID. This is used for route loop prevention. You can also use this as a filter mechanism by changing a router’s ID to the originating router and preventing that prefix from entering the topology/route table. Tricky stuff!!!