Multiple access and full/half duplex point-to-point links

The usual suspects in the case of autonegotiations are:

• bad cables
• bad ports, SFPs or NICs
• port configs (are they both set to auto and do they both list the 1000BASE-T FD ability?)
• bugs, compatibility or legacy issues

I would not recommend a fixed speed and duplex setting in this (or almost any) situation. The days of frequent autoneg issues and incompatible implementations are gone, on recent equipment such issues are very rare. If autoneg is not working, switching to a fixed setting is a workaround, hiding the underlying issue that causes the autonegotiation to fail. Not fixing this issue might allow it to bite you later.

Also, the probability of speed or duplex mismatches and the like is actually much higher in an environment with fixed settings, caused by left over settings when moving equipment around.

As an example, if in your case the cable is to blame, setting both sides to 1000 fixed could disable the link with the faulty cable completely. Autonegotation gracefully degrades you to 100. It is of course up to you or your monitoring equipment to detect and fix this.

Interesting blog post, with links some vendor recommendations: "EtherealMind on Autonegotiation"

You seem to be confused about logical topology (bus, ring, star). Ethernet has a couple of different topologies, but token ring has only the logical ring topology.

Full-duplex ethernet doesn't use CSMA/CD because it has separate send and receive paths between two devices. When you share send and receive paths, then you must use CSMA/CD in order to detect collisions (two devices sending at the same time). With full-duplex, there is no possibility of collisions; the send path on one device is the receive path on the other end, and vice versa.

A hub means you must use half-duplex because the possibility of collisions exists, and you must detect the collisions, so you use CSMA/CD. The devices connected to the hub take turns sending (and resending when there are collisions). Each will have the opportunity to get frames on the wire, so, yes, they can ping each other at the same time.

For ethernet: Coaxial cable only has a single path, so it must be half-duplex. UTP cable as point-to-point between two devices can use full-duplex because each device has separate send and receive paths, but on a hub, you have multiple devices trying to send on on the same path, and receive on the same path, so you must use half-duplex with CSMA/CD. Fiber will have separate send and receive paths.

For token ring: there can be no collisions because only one device possesses the token at a time, and no device can send without possession of the token.

For Wi-Fi: all devices share the medium, so there are going to be collisions. Wi-Fi doesn't use CSMA/CD. CSMA/CA was developed for Wi-Fi to try to avoid collisions.