BGP Path Selection - Understanding BGP Path Selection
10 important questions on BGP Path Selection - Understanding BGP Path Selection
What are the BGP Path Attributes and in which order are they processed?
- Highest Weight
- Highest Local Preference
- Locally Originated
- Shorter Accumulated Interior Gateway Protocol (AIGP) metric
- Shortest AS_Path
- Best Origin Code
- Lowest MED
- External over Internal
- Closest IGP Neighbor
- Oldest eBGP Path
- Neighbor with Lowest BGP RID
- Neighbor with Lowest IP address
What are the 4 BGP Path Attribute Classifications, which are always supported and which are advertised between ASs?
- Well-known Mandatory
Always supported / Advertised between ASs - Well-known Discretionary
Always supported / Not advertised between ASs - Optional Transitive
Not always supported / Advertised between ASs - Optional Nontransitive
Not always supported / Not advertised between ASs
What are the characteristics of the BGP Weight attribute?
- First attribute processed
- 16-bit number (0-65535)
- Highest wins
- Not advertised, only locally assigned
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What are the characteristics of the BGP Local Preference value?
- Second attribute processed
- 32-bit number (0-4294967295)
- Highest wins
- Well-known discretionary, advertised within AS
- Default is 100
What are the characteristics of the Locally Originated value?
- Third decision point
- Preference in following order
- Routes that were advertised locally
- Networks that have been aggregated locally
- Routes received by BGP peers
What are the characteristics of the Accumulated Interior Gateway Protocol (AIGP) value?
- Optional non-transitive path attribute
- Included in advertisements throughout AS
- 32-bit value
- Following guidelines
- A path with AIGP is preferred over one without
- If next-hop needs recursive lookup, additional AIGP metric needs to be calculated.
- Next-hop without AIGP preferred over one with
- AIGP metrics recursively added with multiple lookups
- AIGP paths are compared using their metrics, either derived or actual.
What are the characteristics of the Shortest AS_Path value?
- Shorter AS_Path is preferred over longer AS_Path
- AS Numbers are prepended when send to the next AS
What are the characteristics/preference of the Origin Type value?
- IGP Origin
- EGP Origin
- Incomplete Origin
What is the BGP Attribute Multi-Exit Discriminator
- 32-bit value
- Set automatically to IGP metric
- Used for multiple routes coming from the same AS
- Path without MED is preferred
What are the characteristics of the Lowest IGP Metric BGP path attribute?
The question on the page originate from the summary of the following study material:
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