What will A router do if it cannot determine where to forward an incoming packet

How Routers Transfer Packets

Routers transfer packets in the following manner:

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• How Routers Transfer Packets
• What will the router do to forward the packet?
• Which portion of the network layer does a router use to forward packets?
• When a router receives a packet it assesses whether it should be forwarded to a remote destination host How will the packet be forwarded by the router?
• What role does a router play on a network?

• All nodes on an IP network maintain routing information in routing tables. These tables contain information of how to reach systems that are attached to both local and remote networks. The routing tables are generated from local configuration information and from routing protocol messages that is exchanged with neighboring systems.

• When a host system initially sends a packet, it looks up the packet's destination address in its routing table to determine if the destination is on the local network. If yes, the packet goes directly to the host with that IP address. If not, the packet goes to a router on the local network.

• When a router receives a packet, the router checks its routing table to determine if the destination address is for a system on one of it's attached networks or if the message must be forwarded through another router. It then sends the message to the next system in the path to the destination.

• This process is repeated on each router that receives the message until the message reaches the destination system.

Refer to Chapter 2, Configuring a System as a Router, in Configuring an Oracle Solaris 11.2 System as a Router or a Load Balancer .

The following figure shows a network topology with three networks that are connected by two routers.

Figure 1-4  A Network Topology With Three Interconnected Networks

Router R1 connects networks 192.9.200.0/24 and 192.9.201.0/24. Router R2 connects networks 192.9.201.0/24 and 192.9.202.0/24.

If Host A on network 192.9.200.0/24 sends a message to Host B on network 192.9.202, the following events occur:

1. Host A examines its routing tables for the path to 192.9.202.10. The local network address range does not cover this address, but there is a previously learned default route through router R1 that covers the address. Therefore, Host A sends the packet to Router R1.

2. Router R1 examines its routing tables. No local network's address range covers the destination address, but there is a known route to network 192.9.202.0/24 through Router R2 that covers the address, Router R1 sends the packet to Router R2.

3. Router R2 is connected directly to network 192.9.202.0/24. The routing table lookup reveals that 192.9.202.10 is on the attached network. Router R2 sends the packet directly to Host B.

If a router receives a packet whose IP destination address is not in its routing table, I believe it will send it to its default port.

Suppose that we erase that entry from the table. Now, I think that the router will send an ARP request in order to find where to forward it. I have two questions regarding this process:

• If there is no ARP response (i.e. there is no route from the router to the destination address), will it discard it and send an ICMP message to the origin stating that the destination was unreachable?
• Will in any case a router discard a packet just because the destination address is not in its routing table (assuming there is no 'default' entry)? Or will it always check if it is possible to reach the destination via ARP requests?

Ron Maupin♦

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asked Feb 18, 2018 at 22:14

ARP is a LAN protocol that resolves a layer-3 to a layer-2 address. A router, like any host will use ARP for a host that is on a LAN to which it is directly connected.

Routers will look at the destination address on a packet, and try to find a match in its routing table. If it cannot find a match it will drop the packet and send an ICMP message to the source to tell it that is has no route to the destination network.

The default IPv4 network, 0.0.0.0/0, or default IPv6 network, ::/0, encompasses every IP address for the protocol, and every address for the protocol will match a route to the default network. Having a default route in the routing table prevents the router from dropping packets with unknown destinations.

answered Feb 18, 2018 at 22:20

Ron Maupin♦Ron Maupin

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4

Onces the packet reaches router . Router will verify routing table . If in routing table route to destination is found ,router will forward packet to next hop as per routing table . If route show its directly connected networks .Then further checks for ARP table and forward traffic to L2 switch , in L2 switch mac -address table is verified and forward packet towards specific destination.

If router is not present in routing table of router . Router will just drops the packet .

answered Jan 1, 2021 at 3:16

Sagar UragondaSagar Uragonda

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What will the router do to forward the packet?

Which portion of the network layer does a router use to forward packets?

When a router receives a packet it assesses whether it should be forwarded to a remote destination host How will the packet be forwarded by the router?

What role does a router play on a network?