Why two routers connected via PPP don't use ARP but Ethernet requires ARP?

Question

I have read from @Ron Maupin's answer of this question that your two routers are connected via PPP (this is not uncommon), ARP will not be used , but if the two routers are connected via ethernet, then ARP will be used. My question are that -

(A) What does the mean two router connected by PPP(point to protocol)? As per as my understanding two routers are directly connected.

(B) Why if the two routers connected by PPP then ARP doesn't required?

(C) What does the mean that the two routers are connected via ethernet? As per as my understanding in between two routers there exists any layer2 device like switch or bridge.

(D) Why if the two routers connected via ethernet then ARP is required?

(E) If the two routers are directly connected then they can be communicated via IP address or Mac address or both?

Answer 1

(A) What does the mean two router connected by PPP(point to protocol)? As per as my understanding two routers are directly connected.

Routers (or any nodes) may be connected using PPP. PPP uses an underlying simple serial interface like RS-232 and provides data link layer functionality required by IP (data framing).

(B) Why if the two routers connected by PPP then ARP doesn't required?

These interfaces are point-to-point and use no addressing, so there's no use for ARP.

For completeness: The PPP framing specification (RFC 1662) does include an 8-bit address field, but since L2 addressing is not used (the framing is borrowed from HDLC), that address field is specified as a constant 0xff:

3.1. Frame Format

...

Address Field

The Address field is a single octet, which contains the binary sequence 11111111 (hexadecimal 0xff), the All-Stations address. Individual station addresses are not assigned. The All-Stations address MUST always be recognized and received.

Accordingly, a PPP frame with any other address is invalid.

(C) What does the mean that the two routers are connected via ethernet?

Routers may be connected via Ethernet with a point-to-point link (simple cable) or using switches. In any case, Ethernet is a point-to-multipoint network, requiring proper MAC addressing.

(D) Why if the two routers connected via ethernet then ARP is required?

ARP is only used for IPv4 over MAC-based networks, most prominently Ethernet/IEEE 802.3 or IEEE 802.11.

MAC-based networks require the protocol on top to specify the desired destination MAC address for their payload. IPv4 uses ARP with a broadcast address to learn that destination MAC. Then it uses the underlying network as L2 to deliver its packet.

You cannot run PPP on top of such a point-to-multipoint network, but there may be special variants like PPPoE.

PPPoE creates another L2 on top of Ethernet's L2, but there's no addressing between PPPoE and IP still (PPPoE uses MAC addressing downwards with Ethernet, ie. Ethernet's MAC addressing is controlled by PPPoE, not IP).

(E) If the two routers are directly connected then they can be communicated via IP address or Mac address or both?

Any nodes using IP use underlying data link layer (L2) protocols for local communication. The L2 protocol can vary along an IP path. Ethernet includes the required L2 protocol, a serial link requires PPP, (obsolete) SLIP or something similar.

Basically, routers communicate at the network layer (L3), using IP addresses. As pointed out above, they also need to use proper L2 addressing on certain networks, hence the need for ARP for IPv4 or IPv6's NDP as 'glue'.

Answer 2

This question is kinda impossible to answer. Different protocols use different means of transmitting its data.

OSPF uses IP, RIP uses IP/UDP, IS-IS uses layer2 directly. Also, addresses on multicast-capable links are different.

(A) and (B) are already answered.

(C) What does the mean that two routers are connected via ethernet? As per as my understanding in between two routers there exists any layer2 device like switch or bridge.

I think it means that the interfaces are Ethernet. Switches may or may not be there. The significance of Ethernet over PPP is that there may be multiple devices connected (e.g., with switches) and they can be reached using broadcast/multicast capabilities of Ethernet.

(D) Why if two routers connected via ethernet then ARP is required?

If the packet is sent over IP and is sent to a unicast IP (i.e., IP of another interface) as a destination, "normal" procedure for sending packet over Ethernet with ARP should happen.

Note, that for routing protocol packets this is a very big "IF". Most packets will be sent to broadcast or multicast addresses.

For normal forwarded packets (with payload) this holds.

(E) If two routers are directly connected then they can communicate via IP address or Mac address or both?

Two (directly connected) routers in the end always communicate via layer2 protocol of their link. That is, layer2 processing should be present on each interface. Each routing protocol decides what other protocols to stack on top of layer2. Note however, that when talking about routing protocol packets, in most cases* IP is sort of "dummy". It has addresses, but routers cannot yet route based on these addresses. It can only forward packet to the next direct hop.

^{This is not entirely true, some protocols allow to use IP addresses for non-direct neighbors (e.g. virtual links in OSPF), but they describe a procedure how this is accomplished using already computed routes.}