How is the administrative distance of ebgp and ibgp calculated

[Networking Guy]

Hi,

I know the AD of ebgp is 20 and that of ibgp is 200. But does anyone know how those AD values are calculated ? Is there a specific reason for ebgp to have a lesser value than ibgp. Kindly explain. 

[Philip D'Ath]

Is is simply the standard.

[Joseph W. Doherty · Accepted solution]

A Phillip has already noted, it's the standard, but I believe it's just Cisco's default preference.  I recall other vendors can AD weight their routing protocols differently.  (Although I also recall other vendors tend to keep the same general AD preferences.)  Also, on Cisco equipment, you can change Cisco's default AD weights.

As to why eBGP has a lower AD then iBGP, I believe that's because iBGP normally depends on some other IGP, whereas eBGP does not.

BTW, keep in mind the purpose of AD, it's used to set which prefix, provided by different routing protocols (or static routes), is preferred.  I.e. it's subjective, which is also why Cisco generally prefers (lower AD) their routing protocols over "similar" RFC routing protocols.

[Networking Guy]

Thanks Joseph. ibgp depends on igp explanation seems to be logical. 

[Julio E. Moisa · Accepted solution]

Hi

If a BGP router learns a prefix from a BGP peer with different AS (eBGP), the prefix will be known with AD 20.

If a BGP router learns a prefix from a BGP peer with the same AS (iBGP), the prefix will be known with AD 200. 

A BGP router will prefer eBGP over iBGP. And of course there are attributes like weight, local preference, origin code, etc. for path selection. 

Imagine a tie with attributes so the AD info will be the tiebreaker.

As it was mentioned previously it is a standard. 

I hope it is useful :-)