Replying out of order:
Question 2) Packet is damaged and has a bad CRC at the datalink layer, so
is dropped by next switch or router or end PC that verifies the CRC. (A hub
wouldn't notice). Retransmission is sent from the original source (IP host)
after not receiving an acknowledgement for that packet from the destination.
Question 3) Collison occurs, and is detected by Router B. Router B then
retransmits the packet. Source MAC address will be Router B, as the router
changes the source MAC to itself and the destination to the next hop. (In
this case, computer 2)
Question 4) Layer 3 sources and destinations will not change. (Ignoring
issues such as NAT here, for simplicity). Layer 2 MAC addresses will change
whenever the packet crosses a layer-3 device, as the router removes the old
datalink information, examines the layer 3 information, and repackages the
packet for the next hop device, changing the destination MAC address to the
next hop and the source MAC address to itself.
Back to Question 1) My first answer is that the packet is simply lost. The
protocol is simply IP, which provides best-effort delivery, and wouldn't be
retransmitted by the host (assuming it's not a TCP packet). I'm not
completely familiar with all types of serial line encapsulations, but I
believe it would be retransmitted by the router if it was over an x25 link.
Possibly PPP as well. Not really sure: What's the answer?
My two bits,
Joel Studtmann
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Derek Chung
Sent: Sunday, September 17, 2000 2:19 PM
To: [EMAIL PROTECTED]
Subject: CCIE Questions...
Question 1:
Router A and Router B are configured to route IP to each other over a serial
line. Host A is connected to Router A and Host B is connected to Router B. A
packet is sent from Host A to host B. A hit on the serial line causes an
error in the packet. Retransmission is sent by:
Question 2:
During the middle of a TCP conversion across a routed backbone, the network
receives a voltage spike and several of the packets are damaged. Where are
the packets retransmitted from?
Question 3:
Computer1 --------[Segment
A]-------RouterA--------------RouterB----------[SegmentB]--------------Compu
ter2
A packet is sent to Computer 2 from Computer 1. A collision occurs on
Segment B. Which device will retransmit the frame and what will the source
MAC address be (when the packet actually reaches Segment B)?
Question 4:
When computer A sends a frame to computer B across many routers, how will
the source and destination layer 3 addresses change? How will the source and
destination layer 2 addresses change?
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