Sriram, Kotikalapudi (Fed) wrote on 21/04/2024 16:53:
Q1: Consider an AS peering relationship that is complex (or hybrid)
meaning, for example, provider-to-customer (P2C) for one set of
prefixes and lateral peers (i.e., transit-free peer-to-peer (P2P))
for another set of prefixes. Are these diverse relationships usually
segregated, i.e., P2C on one BGP session and P2P on another? How
often they might co-exist within one single BGP session?
this sort of relationship is common but unintentional. You see it at
IXPs where P provides transit to C over a normal transit link, but they
unintentionally also peer via a route server. Oops.
Incidentally, this is one of the reasons why it's critically important
for route servers to provide prefix filtering knobs and why mandatory
interconnection (which thankfully only happens in a tiny handful of
IXPs) is such a bad idea.
P2P+P2C interconnection can also happen intentionally, but this is a
corner case - most transit providers don't like doing this, not least
for the simple reason that it moves traffic from the paid link to a
lower- or zero-revenue link.
I've not come across any situation where the two relationships happen on
the same bgp session.
Q2: Consider an AS peering relationship that is mutual transit (i.e.,
P2C relationship in each direction for all prefixes). Is this
supported within one single BGP session? How often the ASes might
setup two separate BGP sessions between them -- one for P2C in one
direction (AS A to AS B) and the other for P2C in the opposite
direction (AS B to AS A)?
This would probably be done with a single BGP session. Mutual transit is
not widely used in developed interconnection markets.
Nick
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