EIGRP - internal vs external routes

External EIGRP routes have a different administrative distance (AD) from internal EIGRP routes because they are considered less trustworthy due to their source and the potential variability in their reliability. Here's why:

1. Source of Routes:

  • Internal EIGRP Routes (AD 90): These routes are learned directly from other EIGRP routers within the same autonomous system (AS). They are part of a cohesive and consistent routing domain managed under a single administrative authority.
  • External EIGRP Routes (AD 170): These routes are introduced into EIGRP through redistribution from other routing protocols (like OSPF, RIP, or BGP) or from different EIGRP autonomous systems. They originate outside the EIGRP routing domain.

2. Trustworthiness and Reliability:

  • Consistency: Internal routes are deemed more reliable because they are part of a consistent and controlled environment. All routers within the same AS share the same routing policies and metrics.
  • Variability: External routes may come from diverse sources with different routing policies, metrics, and potential misconfigurations. This variability can introduce inconsistencies or less optimal paths.

3. Administrative Distance Purpose:

  • The administrative distance is a value that routers use to select the best path when multiple routes to the same destination are available from different routing protocols.
  • Lower AD Preferred: A lower AD indicates higher trustworthiness. Routers prefer routes with the lowest AD when multiple routes to the same destination exist.

4. Preventing Routing Loops and Suboptimal Routing:

  • By assigning a higher AD to external routes, routers prefer internal EIGRP routes over external ones, which helps prevent potential routing loops and ensures that traffic stays within the optimized internal paths whenever possible.

5. Interaction with Other Routing Protocols:

  • Comparison with OSPF: OSPF has an AD of 110. By setting external EIGRP routes to an AD of 170, routers will prefer OSPF routes over external EIGRP routes but still prefer internal EIGRP routes over OSPF.
  • This hierarchy ensures that the most reliable and preferred paths are chosen first, based on their source and trust level.

External EIGRP routes have a higher administrative distance (AD 170) than internal EIGRP routes (AD 90) because they originate from outside the EIGRP autonomous system or are redistributed from other routing protocols. This higher AD reflects the reduced trust in external routes compared to internal ones, ensuring that routers prefer more reliable, internally learned routes over potentially less consistent external routes.

Links:

https://forum.networklessons.com/t/eigrp-external-route-path-selection/1336/12?u=lagapidis

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