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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?

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Ron Maupin
    Sep 24 at 15:53
  • Names refer to specific things. When you see PPP, don't assume that because you know what "point-to-point" means you know everything about PPP — look it up!
    – hobbs
    Sep 24 at 16:32
  • @Ron ARP is used for ipv4 to know the Mac address in layer2. But NDP is used for ipv6 in layer3 to know about Mac address in layer 3?
    – Alok Maity
    Sep 25 at 5:56
  • No. NDP has a feature that works like ARP, but it uses multicast instead of broadcast (interrupts everything) that IPv6 does not have. NDP sends the request to a special multicast group that is based on the last 24-bits of the layer-3 address to which the target is subscribed. Multicasts only interrupt those listening on the group address, and that is probably only the target host, so multicast is less disruptive to the network.
    – Ron Maupin
    Sep 25 at 14:17
  • @Ron Maupin then l3 multicast isn't happening in l3 but it converts into l2 multicasting and multicast happening in l2 by Mac address?
    – Alok Maity
    Sep 25 at 14:30
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(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 (or ipV6's NDP) as 'glue'.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Teun Vink
    Sep 25 at 9:29
  • @Zac If "there's no addressing between PPPoE and IP still "- how layer2 and layer3 will be communicated?
    – Alok Maity
    Oct 2 at 11:56
  • @AlokMaity We've moved to chat.
    – Zac67
    Oct 3 at 13:32
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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 layer 2 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 layer 2 protocol of their link. That is, layer 2 processing should be present on each interface. Each routing protocol decides what other protocols to stack on top of layer 2. 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.

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  • But my question is that any two router connected in network layer in one side suppose sender side, it has not come yet data link layer. Then how they are communicated?
    – Alok Maity
    Sep 24 at 15:06
  • sorry, i do not understand that sentence because of the language. could you rephrase it?
    – Effie
    Sep 24 at 15:08
  • Suppose Host A wants to communicate with B. A has already crossed the it's application , transport layer. Now A comes in network layer. Now network layer has two router which are connected. My question how the two router communicated? Now presently A is in sender side in network layer, never enter it's data link layer.
    – Alok Maity
    Sep 24 at 15:16
  • Is the question - how A knows which router to send data to?
    – Effie
    Sep 24 at 15:19
  • The communication always goes down the whole stack. Once the communication reaches layer 3, layer 3 determines outgoing interface/next hop for the packet. Then, the communication goes into that interface's layer 2 for the interface/next hop with appropriate layer 2 processing. Then layer 2 passes its frame to layer 1 and data is physically sent over wire/air/whatever it is.
    – Effie
    Sep 24 at 15:22

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