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I know that a routing metric can take into account several statistics, for example:

  • measuring link utilisation (using SNMP)
  • number of hops (hop count)
  • speed of the path
  • packet loss (router congestion/conditions)
  • latency (delay)

How are these metrics actually assembled? What metrics are actually widely used in routing nowadays?

closed as too broad by Mike Pennington, Teun Vink, generalnetworkerror, Ricky Beam, cpt_fink May 27 '15 at 2:55

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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Let's take some common routing protocols in use today:

EIGRP -- uses a combination of bandwidth and total delay (also by default a function of bandwidth). EIGRP can optionally use other criteria, such as load, reliability and MTU size, but in 99.9% of networks, these are not used.

OSPF -- uses a single metric. In the Cisco world, it is bandwidth.

BGP -- Everything else being equal, BGP uses hop count, but for BGP a "hop" is an entire autonomous system.

Since IS-IS seems to be making a comeback, I'll include it as well. IS-IS uses an static administrative metric. In other words, the metric is whatever you set it to.

Link utilization and other "transient" values cause more problems than they solve. And in modern networks, it just isn't important enough to bother with.

Note that these are the default values. You as the network administrator can change them to suit your needs. BGP in particular is designed to allow more administrative control over the path selection.

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wrote an article explaining routing protocols and selection

http://www.slideshare.net/shaun_hummel/routing-protocols-31653494

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    SE disencourages link-only answers due to their volatile nature. Please consider adding more details to this answer. – Teun Vink May 26 '15 at 18:26
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The metrics are only part of the routing process. The longest prefix match always wins among the routes actually installed in the routing table, while the routing protocol with the lowest administrative distance always wins when installing routes into the routing table.

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    Can someone share why this answer is no good? I am interested in learning this routing stuff. – Ron Royston May 25 '15 at 3:08
  • The only part of your answer that references metrics is a vague one-liner. "The metrics are only a part of the routing process." What does that mean? Answers should have supporting information, that is directly relevant to the OPs question. – Jordan Head May 25 '15 at 3:32
  • << What metrics are actually widely used in routing nowadays? >> Is it just me or is the person unaware of the actual process of route selection? In other words metrics are only part of part of the process. The majority of you obviously have no experience. – Ron Royston May 25 '15 at 21:49
  • You're probably right, if someone is asking about what routing metrics are they might not have any idea how the general route selection process works (longest match, etc). The problem with your answer is that it lacks detail, yes metrics are a part of the routing process - great. A good answer includes examples/references, you yourself said it, OP might not have any idea what that process is. Why not elaborate on how the process works along with the longest match concept. The more RELEVANT detail, the better. Here's an example of a great answer: tinyurl.com/l7ca8tn – Jordan Head May 25 '15 at 22:24
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    Oh and statements like "The majority of you have no experience" is flat out not constructive, which is the entire point of this site. Lots of people with no experience in particular areas, or none at all come here to ask questions. You didn't have experience at one point, and I could jump to conclusions based on your answer history - but that's rude. – Jordan Head May 25 '15 at 22:31

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