PC4-MAC is used only in frames to the right side of the router.PC1-MAC is used only in frames to the left side of the router.While analyzing all the details, keep the following in mind from the earlier discussions: Which Addresses are Learned?Īrmed with all these concepts, you can then analyze the question one half at a time. First, from PC4 to R1, it has source MAC PC4-MAC and destination MAC R1-G0/1-MAC, and after R1 routes the packet to the left, it has source MAC R1-G0/0-MAC and destination MAC PC1-MAC. That return IP packet has a similar encapsulation, but with the opposite MAC addresses. Once the packet sent by PC1 arrives at PC4, then PC4 replies. That new frame uses these Ethernet addresses: Once R1 receives the frame, it de-encapsulates the IP packet, and re-encapsulates it in a new Ethernet frame, forwarding that frame out its G0/1 interface. So, what addresses are used? PC1 sends its IP packets to PC4’s destination IP address (10.1.2.4), encapsulated in an Ethernet frame. And those packets use different Ethernet headers and trailers, with different source and destination MAC addresses, which then impacts switch learning. That means the IP packet going from PC1 to PC4 first flows to R1, then from R1 to PC4. The IP packets must flow through router R1. Based on the topology, and the IPv4 addressing, PC1 and PC4 are in different subnets. The next step is to think about what happens with encapsulation, and the combination of Layers 2 and 3. Explanation: Source and Destination MACs for the Various Frames In short, the question is about dynamically-learned MAC addresses, and those addresses only. The question also told us that the switches did not use any statically configured MAC table entries. Port security causes the switch to out static MAC addresses into the MAC address table, and the question told us that port security was not used.
Don’t read here until you try the question!įigure 102 is just a repeat of the figure from the question, for handy reference.įigure 102: Network Used for Question 102 Explanation: Only Dynamic MAC Table EntriesĪll the different conditions in the question exist for one reason: to make sure you know that the switches will learn their MAC table entries dynamically, with no static entries. The question focuses on switch learning, with a little layer 3 weaved in. Today’ post gives the letter answer, and explains why, for this latest #CCENT question.
200-301 V2 Part 4: Network Architectures.200-301 V2 Ch12: Miscellaneous IP Services.200-301 V2 Ch05: Securing Network Devices.200-301 V2 Ch04: Security Architectures.200-301 V1 Ch25: Configure IPv6 Routing.200-301 V1 Ch24: Configure IPv6 Addressing.200-301 V1 Ch18: Troubleshooting Routing.200-301 V1 Ch16: Router Addresses and Routes.200-301 V1 Ch14: Analyzing IPv4 Subnets.200-301 V1 Ch12: Classful IPv4 Networks.