{"id":4911,"date":"2026-02-21T15:02:48","date_gmt":"2026-02-21T15:02:48","guid":{"rendered":"https:\/\/geekmungus.co.uk\/?p=4911"},"modified":"2026-02-21T15:02:48","modified_gmt":"2026-02-21T15:02:48","slug":"bgp-test-lab-part-6-bgp-attributes-path-selection-med-and-local-preference","status":"publish","type":"post","link":"https:\/\/geekmungus.co.uk\/?p=4911","title":{"rendered":"BGP Test Lab – Part 6 – BGP Attributes, Path Selection: MED and Local Preference"},"content":{"rendered":"\n

Border Gateway Protocol (BGP) path selection works out the best path when BGP has routes that it could choose from. There are a number of BGP Attributes which are evaluated (top to bottom) so that the path with the best attribute is selected for the path.<\/p>\n\n\n\n

BGP Attributes<\/h1>\n\n\n\n

There are 11 BGP Attributes, these are used by BGP to determine the “best path”. We’re not going to go into detail here about them, that will come in a later article; however we’re going to quickly review these so you can see what they are, then we’ll explore in this article how two of them, MED and Local Preference can be used.<\/p>\n\n\n\n

The key thing to remember is that each attribute has a priority from lowest to highest. When BGP attempts to determine the best path, it goes through the list from top to bottom. If there is a tie, for example you have two possible paths to the same network each with the same Weight<\/strong>, BGP just moves to the next attribute to see if it can determine the best path from the two it has learned.<\/p>\n\n\n\n

Priority<\/strong><\/td>Attribute<\/strong><\/td><\/tr>
1<\/td>Weight<\/td><\/tr>
2<\/td>Local Preference (LPref)<\/td><\/tr>
3<\/td>Originate<\/td><\/tr>
4<\/td>AS path length<\/td><\/tr>
5<\/td>Origin code<\/td><\/tr>
6<\/td>MED (Multi Exit Discrimator)<\/td><\/tr>
7<\/td>eBGP path over iBGP path<\/td><\/tr>
8<\/td>Shortest IGP path to BGP next hop<\/td><\/tr>
9<\/td>Oldest path<\/td><\/tr>
10<\/td>Router ID<\/td><\/tr>
11<\/td>Neighbour IP address<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Current Network Topology<\/h1>\n\n\n\n

We have the topology from the previous article. Nothing has really changed, but what we have done is stated that we want the link between Router A and Router C to be the “backup” path. And we want the link between Router B and Router D to be the “primary” path.<\/p>\n\n\n\n

Currently, however if you were to ping from Router G to the Router H (or vice versa) you’d find that it would favour the “BACKUP PATH”, in this article we’re going to explore techniques to influence and control this routing path to how we want it to work.<\/p>\n\n\n\n

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Why does it favour the Router A side? (with no specific mechanism to influence)<\/h1>\n\n\n\n

Although we’ll deep dive into the BGP Attributes in a later article, what we have found is that the network appears to favour the “BACKUP PATH” (Router A \u2194 Router C) for some reason. We’ve not yet implemented any kind of techniques to influence or direct traffic in any particular way, so clearly it is using an attribute to determine the “best path” as between Router A and Router C rather than between Router B and Router D.<\/p>\n\n\n\n

Let’s have a look on Router G.<\/p>\n\n\n\n

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OK, so we’re interested in the route to 10.2.2.0\/24 which is hosted on Router G. Let’s step down all the attributes and see if we can figure out how it’s making its decision. So stepping through the list above one by one from the top (first one used to determine):<\/p>\n\n\n\n