Original subtitles

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So when sending traffic from 1 subnet to another subnet

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the layer 3 headers contain the source host IP address

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and the destination host IP address.

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But at layer 2 the source MAC address is the local host

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and the destination MAC address is the local router on the local segment.

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When the frame gets to the router the router will strip

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the layer 2 headers and then read the layer 3 headers

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to determine what to do with the traffic.

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So the destination IP address is 10.1.2.1 the router will firstly check

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to see if that’s a local IP address on the router

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and in this case it’s not, the router has these IP addresses.

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So its check it’s routing table to determine

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if it knows where the destination IP address is

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This IP address 10.1.2.1 is on subnet 10.1.2.0 which is out of F0/1.

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The router therefore knows it needs to send the traffic to host

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10.1.2.1 out of F0/1 it then checks its ARP cache

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to see if it has an entry for 10.1.2.1

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In this case let’s assume the router doesn’t have an ARP entry

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mapping IP address 10.1.2.1 to MAC address B, so it doesn’t know that.

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So to find that out it needs to send a broadcast unto the local segment

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requesting the MAC address of IP address 10.1.2.1

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so it will send out an ARP request message, the hub will flooded out

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of its ports and both B and D will receive the frame.

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D will receive the frame at layer 2 because its broadcast

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but at higher layers it will drop the message

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because it's an ARP request for another devices IP address.

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So host D drops the frame, but host B will receive at it

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layer 2 send it to high layer protocols, high layer protocols will see

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that this is an ARP request for the local IP address of this host.

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So PC B will process the ARP request

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and send back an ARP reply.

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The ARP reply will be sent to the hub with source MAC address of B

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destination MAC address of the router, the routers is the device

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that requested the IP address of the PC and the MAC address

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is on each interface of a router are different.

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In this case the MAC address used was H

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so the PC will reply back to that MAC address, so source MAC address is B

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destination MAC address is H, source IP address is 10.1.2.1

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destination IP address is 10.1.2.100, the IP address

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and MAC address of the routers F0/1 is used in the reply from PC B

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when the hub receives the traffic it will flood it out of all of its ports

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D will drop the frame the router however will process the traffic

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because the MAC address is its local MAC address.

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So the routers network interface card

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will receive the traffic at layer 2, it will then process the layer 3

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and layer 4 information and it will update its local ARP cache

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stating that IP address 10.1.2.1 as MAC address B.

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Now that the ARP cache is updated the router can send

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the original ping traffic to host B.

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So when the frame arrived at the router from host A

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it had a source MAC address of A, destination MAC address of G

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source IP address of 10.1.1.1 destination IP address of 10.1.2.1

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when it now sends that traffic out of F0/1, it rewrites the MAC address entries

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So the source MAC address is H, the local router's interface

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the destination MAC address is B

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the host that the router wants to communicate with.

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the source IP address is still the IP address of host A

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and the destination IP address is still the IP address of host B.

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It’s really important to remember that when traversing a router

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or a layer 3 switch, so for example when moving from 1 VLAN to another

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the layer 2 information is rewritten, the layer 3 information is left the same

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but every time traffic hops across a router

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or is sent from 1 VLAN to another VLAN

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the layer 2 information is rewritten in the frame

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When that traffic is received by the hub, it will flood it out of all ports

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D will drop the frame because the destination MAC address is B and not D.

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B will receive the frame at layer 2 because it's destined to itself

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and then it will process the layer 3 and layer 4 information.

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In this case it’s an ICMP echo message sent from A to B.

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so B is going to one to reply with an echo reply message.

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So B will reply with an echo reply but please notice that the echo reply

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is going to a destination IP address of 10.1.1.1 which is host A

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the source Mac address is B, the local PC

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but the destination MAC address is the router, device B is sending

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the traffic to its default gateway because it would have also done

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a logical end on the IP address and subnet and it would have worked out

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that IP address 10.1.1.1 is on a different subnet to itself.

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So it’s gonna send the traffic to its default gateway

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and in this case we would have configured the PC

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with the default gateway of 10.1.2.100

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The hub will flood the traffic out of all ports

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D will drop the frame once again because it’s not destined to itself.

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The router will process the frame at layer 2

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because the destination MAC address is its local MAC address.

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It will then strip the layer 2 information and read the layer 3 information

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to determine if it knows where the destination address is.

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In this case 10.1.1.1 is in subnet 10.1.1.0/24

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and that subnet is directly connected to F0/0 on the router.

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So the destination IP address is in a subnet that’s known by the router

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and it now knows out of which interface to send a traffic.

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So the router knows that it needs to forward this packet out of interface F0/0.

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The router will then rewrite the layer 2 headers.

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So the destination MAC address is A.

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The source MAC addresses is G,

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which is the MAC address of F0/0 on the router.

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The layer 3 information is left the same but the layer 2 headers are rewritten.

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The router forwards the frame to the hub.

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When a hub receives the traffic it will flood it out of all ports.

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C will drop the frame because it’s not destined to it.

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A will receive the frame because the destination MAC address is itself.

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It will then process the layer 2 information

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strip the layer 2 headers, forward it to higher layer protocols.

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The higher layer protocols will process the layer 3

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and layer 4 and upper layers and the ping will succeed in this example.

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Now in some cases you'll notice that when you ping a device

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the first ping fails and that’s typically because of the ARP request

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and replies that need to take place to populate the ARP caches

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of devices between the source and destination device.

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So don’t be concerned if you lose the first ping when pinging a remote device.

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It’s probably because the ARP cache is have been populated

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by the devices involved in the communication.

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The important thing to remember is that when you ping across a router

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or a layer 3 switch, the layer 2 information is updated at each hop

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but the layer 3 information remains the same

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unless Network Address Translation or NAT is used.

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When you move from 1 VLAN to another VLAN on a layer 3 switch

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or move from 1 interface to another on a router

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the layer 3 information is not changed, but the layer 2 headers are rewritten.

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So in summary a router is a layer 3 device, it makes routing decisions

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based on IP addresses and it rewrites MAC addresses

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layer 3 switches also operate at this layer.

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The layer 3 switch has layer 2 capability as well as layer 3 capability.

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When you send traffic from VLAN 10 to VLAN 20 for example

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the layer 2 frames are rewritten.

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The traffic is logically going through a router as the layer 3 switch

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implements routing capability and therefore the layer 2 MAC addresses

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are rewritten but the layer 3 information remains the same.