Actually, I never specified that the packet in questioin number 1 had to be
UDP. Question 1 only specifies that the routers are routing IP. It could
be an ICMP, IGMP, RIP, EIGRP, etc... packet. Not all IP packets use a
transport protocol. Granted, 99% of user traffic does, but the question
doesn't specify. I'm just not completely familiar with all of the possible
data-link protocols available over a serial link, and which ones (if any)
would detect the bad packet and have the last station retransmit. I think
x25 would, as its overengineered for providing error-checking, and think PPP
might.
Joel Studtmann
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Ed
Sent: Sunday, September 17, 2000 7:16 PM
To: [EMAIL PROTECTED]
Subject: Re: CCIE Questions...
I agree with your answers excpet for #1. Just because a transport layer
protocol hasn't been specified, I don't think we can assume UDP. I would
have to guess that the original host retransmits. I don't think the router
would,
since it doesn't care about anything above the 3rd layer unless otherwise
configured with QOS type stuff...
Ed
""joel.studtmann"" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> 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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