EIGRP (Enhanced Interior Gateway Routing Protocol)
An advanced Distance Vector protocol (often called Hybrid). It is the only IGP that backs up routes (Feasible Successors), allowing for instant sub-second convergence.
1. The K-Values (Metric Calculation)
EIGRP uses a composite metric based on Bandwidth and Delay by default. Other factors like Load and Reliability can be used but are disabled by default (to prevent route flapping).
- K1: Bandwidth (Enabled)
- K2: Load (Disabled)
- K3: Delay (Enabled)
- K4: Reliability (Disabled)
- K5: MTU (Not used in calculation, but carried in packet)
Metric = (10^7 / Slowest_BW) * 256 + (Sum_Delay / 10) * 256
2. DUAL (Diffusing Update Algorithm)
DUAL is the brain of EIGRP. It guarantees a loop-free topology using two key concepts:
- Feasible Distance (FD): My total metric to reach the destination.
- Reported Distance (RD): My neighbor's metric to reach the destination. (Also called Advertised Distance).
The Feasibility Condition
To be a valid backup route (Feasible Successor), a neighbor must satisfy this rule:
RD < FD
My neighbor's distance to the destination must be STRICTLY LESS than my current distance. This proves the neighbor isn't routing back through me (Loop Prevention).
3. Route States: Passive vs Active
In EIGRP, you want your routes to be Passive.
- Passive: The network is stable. I have a valid route.
- Active: My primary route failed, and I have no Feasible Successor. I am actively querying my neighbors ("Do you have a path?").
Stuck In Active (SIA)
If a router sends a Query and doesn't get a Reply within 3 minutes, the route goes "Stuck In Active". The router kills the neighbor relationship to reset the state. This usually happens in large, flat networks or over slow links.
4. Packet Types
EIGRP uses RTP (Reliable Transport Protocol), not TCP or UDP.
| Type | Description |
|---|---|
| Hello | Discovery. Sent every 5s. Unreliable (No ACK). |
| Update | Routing info. Sent only when changes occur. Reliable (Requires ACK). |
| Query | "I lost my route. Do you have one?" Reliable. |
| Reply | "Yes I do / No I don't." Reliable. |
| ACK | "I received your Update/Query/Reply." Unreliable (Hello packet with no data). |
References
- RFC 7868: Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP) - The informational RFC that opened the standard.
- Introduction to EIGRP - Cisco's official guide on K-values, DUAL, and convergence.