Since AD is a vendor thing and not a standard, i guess you'll have to ask Cisco.

My idea would be that since you are more likely to filter prefixes on ingress eBGP rather on IGP and eBGP usually carries the majority of prefixes, eBGP maybe was considered more "trustworthy" for the majority of traffic.

In any case, eBGP AD can be changed (plus the network backdoor option) and some recommendations propose so.

PS: If i remember correctly, Juniper does it the "right" way.

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Tassos

Hank Nussbacher wrote on 10/4/2018 7:52 μμ:

While giving a routing lecture today someone asked me
"Why was eBGP assigned an administrative distance of 20 which is better
than OSPF's administrative distance of 110.  What was the logic behind
that decision?"
I was unable to think of an answer.
Ideas?

Thanks,
Hank