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Thread: Routing

  1. #1
    Join Date
    Feb 2009
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    100

    Default Routing

    If PC A on Subnet A pings PC B on Subnet B and has to go through 5 routers, how many times does the Source Mac address change.
    I am under the understanding that once the TCP header leaves the PC A it (TCP Header/packet ) gets updated as it is leaving the exit interface of the first 1 router. But what happes, when it reaches to the 5th router.
    Does the header ( Source Mac Address) change when it goes from Router to router?

  2. #2
    Join Date
    Aug 2009
    Location
    California, US
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    Default

    The mac address is layer 2 and TCP/IP is layers 3 and 4.

    The layer 2 uses the mac address for sending packets on the local link only therefore, both the source mac and the destination mac will change on each link the packet travels. The source mac will be the mac address of the exit interface and the destination mac will be the mac address of the next destination device either a client or inbound interface of the next hop router

  3. #3
    Join Date
    Feb 2009
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    Default

    So if a packet passes through 5 routers, then does the source change 5 times?

  4. #4
    Join Date
    Nov 2008
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    Default

    PC A - Router A = first write of src/dst MAC
    Router A - Router B = first rewrite of src/dst
    Router B - Router C = second rewrite of src/dst
    Router C - Router D = third rewrite of src/dst
    Router D - Router E = fourth rewrite of src/dst
    Router E - PC B = fifth rewrite of src/dst

    So you see the MAC address gets written 6 times, but changes 5 times.
    CCNP R&S, CCNA DC
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  5. #5
    Join Date
    Aug 2009
    Location
    California, US
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    Default

    Ethernet uses the mac address to send frames between device on a network LINK.

    PC A ---> Router A Source mac is your pc Destination mac is Router A
    Router A --> Router B Source mac is Router A Desintation mac is Router B
    Router B --> Router C Source mac is Router B Desintation mac is Router C
    Router C --> Router D Source mac is Router C Desintation mac is Router D
    Router D --> Router E Source mac is Router D Desintation mac is Router E
    Router E --> PC B Source mac is Router E Desintation mac is PC B

  6. #6
    Join Date
    May 2010
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    Default

    Quote Originally Posted by DonB View Post
    Ethernet uses the mac address to send frames between device on a network LINK.

    PC A ---> Router A Source mac is your pc Destination mac is Router A
    Router A --> Router B Source mac is Router A Desintation mac is Router B
    Router B --> Router C Source mac is Router B Desintation mac is Router C
    Router C --> Router D Source mac is Router C Desintation mac is Router D
    Router D --> Router E Source mac is Router D Desintation mac is Router E
    Router E --> PC B Source mac is Router E Desintation mac is PC B
    Just to add some extra details.

    When the frame leaves the router, the source address in the frame is that of the mac address of the exit interface of the router. The destination mac address is always of that of the next immediate device. (Usually the receiving interface of the next router).

    The Source IP and Destination IP never changes.

  7. #7
    Join Date
    Feb 2009
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    Default

    GOT IT !!!!!

    thanks guys for your help with this issue.

  8. #8
    Join Date
    Jul 2009
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    Default

    Quote Originally Posted by Agnel View Post

    The Source IP and Destination IP never changes.
    Wat abt NAT !!!
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    Anurag Gambhir , Delhi
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  9. #9
    Join Date
    May 2010
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    Default

    Quote Originally Posted by anurag007 View Post
    Wat abt NAT !!!
    Good catch Anurag!
    I thought about it while I mentioned it earlier, but then it is applicable only if one of the routers have NAT enabled. You're right.

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