What action will a router take when it receives a frame with a broadcast MAC address

Hi @dolanduck.  ,

Lets say that PC1 is connected to SW1 and lets say that PC1 sends an ARP request with a broadcast destination MAC address FFFF.FFFF.FFFF . When the SW receives the frame, it checks the destination MAC address. For a broadcast address FFFF.FFFF.FFFF it will forward the frame to every port except the port in which the switch received the frame. That would be the action taken by the switch.

If the destination is in the same LAN, it will reply the broadcast frame with a unicast frame with PC1 Mac and IP address as the destination.

If the destination is in a different LAN, a router will construct an ARP request with a destination MAC address FFFF.FFFF.FFFF. Then, it will forward the request out of its directly connected ports. That would be the action taken by the router.

If the destination is reachable by the router (directly connected or through another switch), it will reply the broadcast frame with a unicast frame with R1 Mac address and PC1 IP address as the destination.

Answering your question, the switch and the router will forward the ARP requests and replies.

Hope this helps!

# 2020/05/05 ###### tags: `Daily Report` ==Routing Basics (Routers)== **WHY do I do this job?** Beacuse I need to understand the detailed information of Layer 3 (with router). **WHAT do I want to do?** Having eLearning with several videos. **Sum of job:** I know something about what is routers. ==Expected outcome== 1.Watch third-2 part of the third video. (Deadline:2400) 2.Make some notes what I’ve understood from the video. (Deadline:0200) [TOC] --- ## **Routing in Action - Part 1** - How data flows from a PC on one network to a PC on different network focussing on Layer 2 and Layer 3 addressing. (Major different between Ethernet switches and routers in action) - PC 1.2 wants to send data to PC 3.2. - First, the Application Layer sends the data down to the Transport Layer where Layer 4 header is added.  - Next, the Transport Layer sends the data down to the Network Layer which adds the Layer 3 header. These Layer 3 headers include the source and destination Layer 3 addresses. We use a simplify format, network.host rather than a full IP address.  - Moving on to Layer 2, before the source PC can add the Layer 2 header, it must determine if the destination PC is on the same network by examining the network portion of the destination Layer 3 address.  - In the previous example, when PC 1.2 wanted to send data to PC 1.8. - The sending PC's network portion was the same as the destination PC. Therefore, PC 1.2 simply sent out an address resolution request to determine PC 1.8 MAC address. - An address resolution request is sent to the broadcast MAC address, so all devices in the broadcast domain receive this request including the router.  - Since the Layer 3 address in the address resolution request does not match the router's Layer 3 address or in the case of IP networks the IP address, the router simply drops the packet. It does not forward broadcast flames out the other ports, hence reducing network traffic. - PC 1.8 is the only device that responds to this address request. After receiving the response, PC 1.2 will fill in the source and destination MAC addresses and transmit the frame.  - However, by examining the network portion of the Layer 3 address, the sending PC can determine that the destination PC is not on the same network and that the two PCs cannot communicate directly. - In this case the sending PC sends the data to the nearest router, also known as its default router or default gateway.  - If the source PC already knows the default gateway's MAC address, it skips the address resolution process and sends the packet to the router for further processing.  - If it doesn't know the router's MAC address, it sends out an address resolution request to determine, the router will respond with its MAC address. The PC 1.2 will be able to address its data to the router, so that the router can figure out how to forward it on to Network 3.  - Default gateway and Default router  - A default gateways different from the default route described earlier, a gateway is a device such as a router, which serves as a door or an access point to another network. - End-user devices such as PCs, send packets to their configured default gateway when the destination network number is not the same as their network number. - A Router, on the other hand, does not have default gateway configured since is the door to other networks, instead, a router uses a default route to direct traffic if it encounters network that is not in its routing table. - How does the PC know the Layer 3 address of its default gateway? This address is either configured by the network administrator or learned dynamically.  - However, as when sending to any device, the sending PC needs to determine the default gateway's Layer 2 or MAC address using the address resolution process before it can send the packet. - Once the PC determines the default gateway's MAC address the PC adds the Layer 2 Header using the router's MAC address for the destination address, and ts own MAC address for the source address. - The Layer 3 address identifies the final destination. The Layer 2 or MAC address identifies the stops made along the way to get to the final destination. Layer 2 addresses change with each stop along the route to the final destination - Conclusion  --- ## **Routing in Action - Part 2** - After Layer 2 encapsulation is complete, PC 1.2 converts the data to ones and zeros and sends it over the wire.  - The Ethernet switch connecting the source PC to the router receives the frame, examines the Layer 2 destination MAC address, and looks for a match in its MAC address table, which, on a switch, is commonly called a forwarding table.  - If the switch does not find the Layer 2 address in its table, it will send the frame out all ports except the one it came in on.  - In this case the switch finds a matching entry for the Layer 2 address, so it only forwards the frame out the port specified for that address. - Notice that this Layer 2 device did not look at the Layer 3 header or change the Layer 2 source and destination MAC addresses. - Next the router receives the frame and examines the destination MAC address to see if it is the intended recipient. If so, the router strips off the Layer 2 frame and examines the destination network portion of the Layer 3 address, which in this case is network number 3.  - The router looks in its routing table for this destination network number. The router finds network number 3 along with the Layer 3 address of the next router in the path to the destination network.  - Next, the router adds a new Layer 2 frame, including the source MAC address and the next-hop router's MAC address. - If the router does not know the MAC address for the next hop, it performs the address resolution process to determine the MAC address. - In this case, the MAC address for 2.2 is "bb". Notice that the Source Layer 2 address is now "aa" since the data is being set on a different port.  - Router and Switch  - In on Network 2, he switch examines the Layer 2 destination MAC address and looks for a match in its MAC address table.\ - Once it finds a match, it forwards the frame out the correct port.  - The next router repeats the strip and lookup process. - First, it checks the data's destination MAC address.  - Then, it strips the Layer 2 frame and examines the destination Layer 3 address.  - But this time, the destination network is directly connected to the router and the routing process is complete.  - At this point the router determines PC 3.2's MAC address, and adds the Layer 2 header including the appropriate destination MAC address and the source MAC address for the port on which it is sending the data. Then the router transmits the data.   - The switch connecting the router to the destination PC uses the destination MAC address to forward the frame to the correct PC.  - When the PC receives the frame, it first examines the destination MAC address. - If it is the PC's MAC address, the PC strips off the Layer 2 frame and examines the destination Layer 3 address. - If it matches, it strips the Layer 3 frame and examines the Layer 4 header. - This process is repeated, Layer by Layer until the application on the PC receives the data. - Our data's journey from PC 1.2 to 3.2 is complete.  --- ## **Reference** - Getting Started with Networking (eLearning) - https://learningportal.juniper.net/juniper/user_activity_info.aspx?id=769# - Module 3: Routing Basics - https://learningportal.juniper.net/core/user_scorm_launch.aspx

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