What happens when a router receives a packet for a network that isn t listed in the routing table

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.

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Ch. 9. chapter review
What happens when a router receives a packet that does not match any routes?
What happens if a router receives a packet but does not have an entry in its routing table for the destination network and does not have a default route configured?
What happens when a packet arrives on a router interface?
What happens to a packet if the router does not find the network containing its IP address?

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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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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Ch. 9. chapter review

Question	Question	Answer	Answer
3. which is true regarding dynamic routing? a. static routes are best in large networks and thus better to use than dynamic routing protocols. b. static routes are automatically added to the routing table but dynamic routes must be added by hand	3. you must use a DNS and WINS server when configuring dynamic routing d. dynamic routes are automatically added to the routing table.	3. D. Dynamic routing scales well in large networks and routes are automatically added into the routing table. static routing is done by hand, one route at a time into each router.	3
4. which of the following is true for MAC addresses? A. MAC addresses are never local on the LAN and always pass through a router B. MAC addresses are always local on the LAN and never go through or past a router	4. C. MAC addresses will always be the IP address of Fa0/0 interface. D. None of the above.	4. B. Media Access Control (MAC) addresses are always local on the LAN and never go through and past a router.	4
5. what is it called when protocols update their forwarding tables after changes have occurred? a. name resolution b. routing c. convergence d. ARP resolution	5.	5. C. Routing convergence is the time required by the routing protocols to update the routing tables (forwarding tables) on all routers in the network.	5
6. what command would be used to view the ARP cache on your host? A. C:\ > show ip route B. C:\ >show ip arp C. C:\ >show protocols D. C:\ >arp -a	6	6. D. The arp -a command will show the ARP cache on your host	6
7. what happens when a router receives a packet for a network that isn't listed in the routing table? a. it forwards the packet to the next available router b. it holds the packet until the address is updated in the routing cable	7. c. the router will use RIP to inform the host that it can't send the packet d. none of the above	7. D. Hope you answered D! A router will not send a broadcast looking for the remote network- the router will discard the packet.	7
8. which of the following is not a distance vector protocol? A. RIPv1 B. RIPv2 C. OSPF D. IGRP	8	8. C. RIPv1 and 2 and IGRP are all distance vector (DV) protocols. routers using a DV protocol send all or parts or their routing table in a routing-update message at a regular interval to each of their neighbor routers.	8
9. Which two of the following are link state protocols? A. RIPv1 B. RIPv2 C. OSPF D. IS-IS E. IGRP	9	9. C,D. Open Shortest Path First (OSPF) and Intermediate System-to-Inermediate System (IS-IS) are link state (LS) routing protocols.	9
10. Which of the following is a hybrid routing protocol? A. RIPv2 B. EIGRP C. IS-IS D. IGRP	10	10. B. The only protocol you could select is Enhanced Interior Gateway Routing Protocol.	10
11. What does the acronym IGRP stand for? A. Interior Gateway Routing Protocol B. Inside Gateway Redundancy Protocol C. Interior Group Reliability Protocol D. Interior Gateway Redundancy Protocol	11	11. A. Interior Gateway Routing Protocol is a DV interior gateway protocol.	11
12. what EGP protocol is used on the internet? A. GGP B. EGP C. BGP D. IGP	12	12. C. Border Gateway Protocol (BGP) is the most popular choice for ISPs or really large corporations.	12
13. What are the two categories of IGP protocols? a. link state b. static c. distance vector d. EGP	13	13. A,C. Distance vector (DV) and link state (LS) are the two routing protocols to remember.	13
14. What two pieces of information does a router require to make a routing decision? a. destination network (address) b. destination MAC address c. application layer protocol d. neighbor router	14	14. A,D. A frame uses MAC addresses to send a packet on the LAN. The frame will take the packet to either a host on the LAN or a router's interface if the packet is destined for a remote network.	14
15. where does a frame have to carry a packet if it is destined for a remote network? a. default gateway b. neighbor host c. switch d. hub	15	15. A. I hope you said A! Packets specifically have to be carried to a router in order to be routed through a network.	15
16. where along the IP routing process does a packet get changed? a. router b. host A c. destination device d. host B	16	16. C. Remember that the frame changes at each hop but that the packet is never changed in any way until it reaches the destination device.	16
17. when all routers in a network agree about the path from one point to another, the network is said to be what? a. dynamic b. static c. secure d. converged	17	17. D. When the routing tables are complete because they include info about all networks in the internetwork, they are considered converged.	17
18. what type of request must a client send if it does not know the destination MAC address? a. ARP broadcast b. multicast c. ICMP redirect d. reverse ARP	18	18. A. This is step 6 in the IP routing process. if the hardware address isn't in the ARP cache of the host, an ARP broadcast is sent out onto the local network to search for the hardware address.	18
19. you need to perform maintenance on a router in your corporate office. it is important that the network does not go down. what can you do to accomplish your goal? a. configure BGP on the router b. implement NAT on the router c. configure on the ...	19...router a static route that temporarily reroutes traffic through another office. d. implement convergence on the router	19. C. The best answer would be to reroute traffic using a temporary static route until the maintenance is complete on the router.	19
20. when are you most likely to see a Request Timed Out message? a. when an unknown error has occurred b. when you have used the arp -a command incorrectly c. when a known error has occurred d. when you are using a hybrid routing protocol	20	20. A. You are most likely to see a Request Timed Out message when (if) a packet is lost on the way back to the originating host for an unknown error. remember, if the error occurs because of a known issue, you are likely to see..	20. a Destination Unreachable message
1. which is not a routing protocol? A. RIP B. RIPv2 C. RIPv3 D. EIGRP	1	1. C. Yup, you got it. RIP, RIPv2, and EIGRP are all examples of routing protocols.	1
2. Which of these best describes dynamic routing? A. All network addresses must be hand-typed into the routing table. b. only a portion of the network address must be hand-typed into the routing table. c. routing tables are updated automatically when	2. changes occur in the network D. A and B	2. C. In dynamic routing, routers update each other about all the networks they know about and place this info into the routing table. this is possible because a protocol on one router communicates with the same protocol running on neighbor routers. if	2. changes occur in the network, a dynamic routing protocol automatically informs all routers about the event.

What happens when a router receives a packet that does not match any routes?

The router forwards any packet whose destination address fails to match any entry in the routing table to the next hop of the default static route.

What happens if a router receives a packet but does not have an entry in its routing table for the destination network and does not have a default route configured?

What does a router do when it receives a packet whose destination address is not listed in the routing table and the router does not have a default route configured? It discards the packet.

What happens when a packet arrives on a router interface?

When a multicast packet is received on an interface, the router interprets the source address in the multicast IP packet as the destination address for a unicast IP packet. The source multicast address is found in the unicast routing table, and the outgoing interface is determined.

What happens to a packet if the router does not find the network containing its IP address?

If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone.