MTU does not play a role in calculating the EIGRP metric!
K5 has a specific term, a modifier, and the delay is cumulative, not just the one on a single link.
I appreciate EIGRP for being very powerful precisely because of the granularity of its metric, supporting load balancing on links with different costs, surpassing OSPF in some situations as an IGP, with the drawback that you need to have only Cisco devices in the network.
Support for IP, AppleTalk, and IPX is an advantage in some heterogeneous networks. Also, manual summarization on any interface is a plus.
If we start with K5=0, the last part of the equation simplifies, that’s clear. Good convergence is achieved by modifying the Hold and Hello timers.
If the interface through which the neighbor relationship is established goes down and the IOS changes its state to anything other than Up/Up, then the router instantly knows that the neighbor has failed. In some cases, the interface remains Up/Up while the connection can no longer be used. In such cases, EIGRP convergence relies on the expiration of the Hold Timer, which is 15 seconds on FastEthernet links and 60 seconds on T1 links.
If a Hello packet is not received within 15 seconds, the neighbor relationship drops. For such situations, the Hello and Hold Timers are reduced, accepting higher bandwidth consumption in exchange.
A note: IOS allows the Hold Timer to be smaller than the Hello Timer, in which case the neighbor relationship drops and comes back up, causing route flapping.
With maximum-path
and variance
, the router can balance traffic proportionally to the metric, meaning that a lower metric sends more packets, while the other routes remain in the table for fast convergence in case the best route fails.
1. EIGRP Metric Calculation
- MTU (Maximum Transmission Unit): Unlike some routing protocols, EIGRP does not use MTU in its metric calculation. The metric is based on bandwidth, delay, reliability, and load.
- K5 Modifier: K5 is a coefficient in the EIGRP metric formula. If K5=0, the formula simplifies, ignoring reliability and load.
- Cumulative Delay: The delay in EIGRP is cumulative across all links in the path, not just the delay of a single link.
Example: If a path has three links with delays of 100, 200, and 300 microseconds, the total delay used in the metric calculation is 600 microseconds.
2. Granularity and Load Balancing
- EIGRP supports unequal-cost load balancing, allowing traffic to be distributed across multiple paths with different metrics. This is controlled by the
variance
command. - Example: If the best path has a metric of 100 and another path has a metric of 200, setting
variance 2
allows the second path to be used for load balancing.
3. Support for Multiple Protocols
- EIGRP supports IP, AppleTalk, and IPX, making it versatile in heterogeneous networks.
- Example: In a network with legacy AppleTalk devices, EIGRP can route traffic between AppleTalk and IP networks seamlessly.
4. Manual Summarization
- EIGRP allows manual summarization on any interface, reducing the size of routing tables and improving efficiency.
- Example: If a router has routes for 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24, it can summarize them as 192.168.0.0/16.
5. Hello and Hold Timers
- Hello Timer: The interval at which EIGRP sends Hello packets (default: 5 seconds on high-speed links, 60 seconds on low-speed links).
- Hold Timer: The time a router waits without receiving a Hello packet before declaring a neighbor down (default: 15 seconds on high-speed links, 180 seconds on low-speed links).
- Example: On a FastEthernet link, if a Hello packet is not received within 15 seconds, the neighbor is considered down.
6. Fast Convergence
- EIGRP achieves fast convergence by detecting neighbor failures through interface state changes or Hold Timer expiration.
- Example: If an interface goes down, EIGRP immediately removes routes through that interface and recalculates paths.
7. Route Flapping
- If the Hold Timer is set smaller than the Hello Timer, the neighbor relationship may flap (repeatedly go up and down), causing instability.
- Example: If Hello Timer is 10 seconds and Hold Timer is 5 seconds, the neighbor relationship will drop and re-establish frequently.
8. Traffic Balancing with maximum-path
and variance
maximum-path
: Controls the number of equal-cost paths EIGRP can use (default: 4).variance
: Allows unequal-cost load balancing by specifying a multiplier for the best metric.- Example: If the best path has a metric of 100 and
variance
is set to 2, paths with metrics up to 200 can be used for load balancing.
Conclusion
EIGRP is a powerful and flexible routing protocol, especially in Cisco-dominated networks. Its granular metric calculation, support for multiple protocols, and advanced features like unequal-cost load balancing and manual summarization make it a strong choice for many network environments. However, its reliance on Cisco hardware and potential for route flapping are important considerations.