At 10:21 AM 9/19/00, Ed wrote:
>Silly question... I was always under the impression that routers wouldn't
>retransmit.
The router has to obey CSMA/CD. This functionality is built into the
Ethernet chip set, which is just like the Ethernet chip set you would use
on a PC, server, etc.
>What about switches? Will they retransmit in the case of collisions, or do
>they leave
>it up to the hosts? (I thought that was the latter as well... ) :)
The switch retransmits when it senses a collision on a frame that it sent.
It has to obey CSMA/CD. Just as with a router, an Ethernet port on a switch
bounds the collision domain. If the originating host had to retransmit that
would mean the collision domain spanned the switch. The switch would have
to act like a hub and pass the jam signal back to the transmitter. This
would wreak havoc on various Ethernet timing parameters, etc.
You can't just memorize a rule that says the router doesn't retransmit,
even though it is true for almost all cases. Perhaps a follow-up study
question would be, in which other cases would the router retransmit?
Priscilla
>"Priscilla Oppenheimer" <[EMAIL PROTECTED]> wrote in message
>[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > Where did these questions come from? Please specify the source (book,
>etc.)
> > so that we know we aren't disobeying the NDA and giving you answers from
>an
> > actual test. These sure sound like questions from the CCIE written. Also,
>I
> > think it would be best to limit each e-mail message to one topic. It makes
> > discussions easier to follow.
> >
> > Despite all that, I can't resist giving you my answers! &;-) See below.
> >
> > At 05:18 AM 9/17/00, Derek Chung wrote:
> > >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:
> >
> > Host A retransmits the packet. The data-link layer protocols in use today
> > on serial lines, including PPP, Frame Relay, and Cisco HDLC, detect errors
> > but are not responsible for error correction or retransmission.
> >
> >
> > >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?
> >
> > The TCP sender. This is almost a repeat of the last question.
> >
> >
> > >Question 3:
> > >Computer1 --------[Segment
> >
> >A]-------RouterA--------------RouterB----------[SegmentB]--------------Comp
>u
> > >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)?
> >
> > Router B senses the collision and retransmits at the MAC layer. The MAC
> > address is Router B's Ethernet address.
> >
> >
> > >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?
> >
> > Layer 3 addresses won't change (unless you have NAT or tunnelling or
> > something else bizarre.) The Layer 2 addresses change each time a router
> > re-encapsulates the packet in a data-link-layer header than includes
> > layer-2 addresses.
> >
> > Priscilla
> >
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
> >
> > **NOTE: New CCNA/CCDA List has been formed. For more information go to
> > http://www.groupstudy.com/list/Associates.html
> > _________________________________
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> >
>
>
>**NOTE: New CCNA/CCDA List has been formed. For more information go to
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>_________________________________
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________________________
Priscilla Oppenheimer
http://www.priscilla.com
**NOTE: New CCNA/CCDA List has been formed. For more information go to
http://www.groupstudy.com/list/Associates.html
_________________________________
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Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
