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I am trying to better understand how routing works in practice. Which sources of information (can) go into the forwarding decision that a router makes? I have learned that BGP Updates from neighboring routers are just one source.

(My question relates to this answer in a previous thread: https://networkengineering.stackexchange.com/a/35052/30613)

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  • I think you need to clarify your question. Are you simply speaking of the routing table, or are you looking for how each routing protocol decides what to submit to the routing table?
    – Ron Maupin
    Sep 26, 2016 at 14:34
  • I was trying to understand your comment in the thread linked to above: > the information used to forward packets comes from many sources -- one of which could be the BGP application running on the router. (So I probably mean the routing table.)
    – zeitweise
    Sep 26, 2016 at 14:43

1 Answer 1

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A routing table gets routes in three ways:

  1. Directly connected networks
  2. Statically configured routes
  3. Routing protocols (RIP, EIGRP, IS-IS, OSPF, BGP, etc.)

It is possible that the routing process could receive multiple routes to the same destination from separate sources. The routing process must decide which of the possible routes to a destination to use. The first thing it looks at is the length of the mask, and the route to a destination with the longest mask wins. If there are still multiple routes of the same length, it needs a tie-breaker. Cisco call this AD (Administrative Distance), which is really how trusted the source of the route is. For instance, a directly connected network is the most trusted, and it will be used. There are tables, e.g. What Is Administrative Distance?

Select the Best Path

Administrative distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination. Administrative distance is a measure of the trustworthiness of the source of the routing information. Administrative distance has only local significance, and is not advertised in routing updates.

Note: The smaller the administrative distance value, the more reliable the protocol. For example, if a router receives a route to a certain network from both Open Shortest Path First (OSPF) (default administrative distance - 110) and Interior Gateway Routing Protocol (IGRP) (default administrative distance - 100), the router chooses IGRP because IGRP is more reliable. This means the router adds the IGRP version of the route to the routing table.

If you lose the source of the IGRP-derived information (for example, due to a power shutdown), the software uses the OSPF-derived information until the IGRP-derived information reappears.

Default Distance Value Table

This table lists the administrative distance default values of the protocols that Cisco supports:

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  • If the administrative distance is 255, the router does not believe the source of that route and does not install the route in the routing table.
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  • I thought that point needed to be stressed :)
    – Ron Trunk
    Sep 26, 2016 at 15:07

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