Category: BGP

BGP Path Attributes III


Category: Well-known discretionary
Preference: Highest value
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Wrong/Unknown Remote AS Number

Lately I got asked how to find the remote AS number for a BPG peering, especially if you do not have access to that remote router. In real life I’d say you should know which AS’ you peer with but in the Lab it might be a question to find out the remote AS.

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BGP Path Attributes I

The BGP path attributes are the characteristics for all BGP advertised routes. Additionally to the necessary informations to perform basic routing , the path attributes allow BGP to set routing policies and advertise them. Each attribute is a member of one of the following categories:

  • Well-known mandatory – This attribute has to be known and implemented by all BGP implementations
  • Well-known discretionary – This attribute has to be known by all BGP implementations, but does not have to be sent in an update message.
  • Optional transitive – This attribute does not have to be known by all BGP implementations but the BGP prozess has to accept the path within the attribute is found and forward it to other BGP peers, even though the BGP process doesnt know the attribute.
  • Optional non-transitive – An update with an attribute from this category can be ignored. The router does not have to advertise this route nor the path to its peers.

All examples in this document will use the following diagram

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