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This topic contains 1 reply, has 2 voices, and was last updated by  Flyingdoc 4 years, 11 months ago.

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  • #17869

    BReyn407
    Participant

    I don’t believe the information presented in the course on Inter-VLAN routing (and other places) is correct. The instructor decribes how a PC communicates to another PC on a different subnet via a so-called “Cisco Proxy ARP” feature.

    This does not jibe with any network training that I have ever received.

    Here is how I believe the *proper* communication takes place:

    Assumptions:
    1. Router r1 has two sub-interfaces configured for dot1q trunking on two VLANs (10 & 20) connected to the same switch as PC A (10.10.10.1/24 on VLAN 10) and PC B (20.20.20.3/24 on VLAN 20).
    2. Router r1 sub-interfaces are: f0/1.10 (10.10.10.254 VLAN 10) & f0/1.20 (20.20.20.254 VLAN 20) and Inter-VLAN routing is functional.

    Process:
    1. PC A (10.10.10.1/24) creates a packet destined for PC B (20.20.20.3)
    2. Since the destination IP is not on PC A’s subnet, no ARP packet is requested (contrary to the information presented in the CCNA lesson)
    3. Instead, PC A sends this packet to it’s default gateway (10.10.10.254), which is router r1’s interface f0/1.10
    4. Upon receiving this packet, router r1 looks at it’s routing table and finds the network 20.20.20.0/24 of which PC A’s packet destination is a member. i.e. PC B’s IP address of 20.20.20.3 is on the subnet 20.20.20.0/24
    5. Router r1’s routing table indicates that the subnet 20.20.20.0/24 is on interface f0/1.20
    6. Therefore, router r1 makes an ARP request on this interface (f0/1.20) – (this assumes PC B’s MAC is *not* already in router r1’s f0/1.20 ARP cache)
    7. PC B gets router r1’s ARP request and responds with its MAC address
    8. Router r1 receives PC B’s MAC address, adds it to its ARP cache, then delivers the packet to PC B

    Please correct me if I’m wrong.

    The instructor seems *very* knowledgeable on the subject-matter. I am quite impressed, have learned a great deal and am thankful. However, this “proxy ARP” explanation is not right.

    • This topic was modified 4 years, 11 months ago by  BReyn407. Reason: typo
    #21103

    Flyingdoc
    Participant

    I have to concur – and running a quick lab on it to check I found the following

    I pinged pc1 (10.10.10.1 from pc4 20.20.20.4)

    If the router sent back a proxy mask I would expect that to be stored in the apr cache under the ip 10.10.10.1 surely – instead on doing and apr -a request all I saw was the default gateway ip and its mac address.

    that bears out what the above user is saying

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