In a BGP setup, what indicates that two routers are IBGP peers?

In a BGP setup, the indication that two routers are IBGP (Internal Border Gateway Protocol) peers is that they belong to the same Autonomous System (AS). IBGP is used to facilitate routing information exchange between routers within the same AS, allowing them to share routing information while maintaining distinct routes to different networks. This means that for two routers to be considered IBGP peers, they must be configured under the same AS number.

The essence of IBGP relationships lies in the requirement that routers share a common AS, as opposed to EBGP (External Border Gateway Protocol), which is used for communication between routers in different autonomous systems. By requiring that the routers be within the same AS, BGP can make sure that routing decisions are consistently made within that local administrative domain.

Considering why the other options are not indicators of IBGP peers, being directly connected does not guarantee that routers belong to the same AS; routers can be directly connected but belong to different ASs as well. Different ASs signify an EBGP relationship rather than an IBGP one. Session encryption, while important for securing the communication between routers, does not have any relevance to their AS membership or their status as IBGP peers.