Router to Router communication

p1xelsp1xels Posts: 114Member
Hi

Can anyone please tell me how does one router handles the data as it travels from one Router to another router? I mean, say, I have the below topology-

pc1<->Switch <->Router1<->Router2<->Switch<->pc2

In the above the pc1 wants to send data or ping to pc2,so it inserts a source IP address as its own address,destination ip as that of pc2,source mac as that of pc1 and dest mac is unknown so it arp to FF.FF.FF.FF (correct me if I m wrong). Router 1 replies with its own mac address as proxy arp ,then when it receives this packet from pc1 it de-encapsulates or strips off the IP header and trailer (I'm not sure if its called IP or TCP/UDP header,please tell me) and encapsulates in hdlc or ppp ,right?

But here I am confused as I have read in many books and in the internet that the Router changes the MAC address of the packet ,i.e. it changes the source MAC to that of Router1(source MAC) and destination MAC address as that of the Router2 and without any change in source or destination IP address of the packet.

Then what is the purpose of HDLC encapsulation in this case if changing the MAC address suffices? HDLC doesn't change the MAC address! So which one of the above views are correct - is it HDLC encapsulation (as mentioned in Sir Wendell Odom's book) or is it change in MAC address or both when a packet traverses from one router to another Router as in the above case?

Any help will be greatly appreciated .

Thanks in advance

With best regards

Comments

  • bbarrickbbarrick Posts: 241Member
    Just my guess but I would think that the router would see that the destination ip is on interface xyz, strip the ethernet frame and apply the hdlc frame. Correct me if I'm wrong but I think I remember the book saying that HDLC leave's out the destination ip or something like that because it only has one place it can go?

    I don't remember reading anything about the router changing the MAC address...
  • FrankGuthrieFrankGuthrie Posts: 245Member
    p1xels wrote: »
    Hi

    Can anyone please tell me how does one router handles the data as it travels from one Router to another router? I mean, say, I have the below topology-

    pc1<->Switch <->Router1<->Router2<->Switch<->pc2

    In the above the pc1 wants to send data or ping to pc2,so it inserts a source IP address as its own address,destination ip as that of pc2,source mac as that of pc1 and dest mac is unknown so it arp to FF.FF.FF.FF (correct me if I m wrong). Router 1 replies with its own mac address as proxy arp ,then when it receives this packet from pc1 it de-encapsulates or strips off the IP header and trailer (I'm not sure if its called IP or TCP/UDP header,please tell me) and encapsulates in hdlc or ppp ,right?

    But here I am confused as I have read in many books and in the internet that the Router changes the MAC address of the packet ,i.e. it changes the source MAC to that of Router1(source MAC) and destination MAC address as that of the Router2 and without any change in source or destination IP address of the packet.

    Then what is the purpose of HDLC encapsulation in this case if changing the MAC address suffices? HDLC doesn't change the MAC address! So which one of the above views are correct - is it HDLC encapsulation (as mentioned in Sir Wendell Odom's book) or is it change in MAC address or both when a packet traverses from one router to another Router as in the above case?

    Any help will be greatly appreciated .

    Thanks in advance

    With best regards

    Router 1 will never remove the IP information, because the packet needs to be routed to the correct destination. A router however will change will change the Source and Destination MAC address of an IP packet. It will make itself the Source and the Destination MAC address IP the address of the next-hop.

    HDLC and PPP are encapsulation technics, so IP info will not be changed. The are utilized on point to point links.
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