I found this question interesting.
Maybe, I missed it, but I'm surprised that there wasn't more discussion
on
this one.
> 192.168.1.16/29 ----- R1 ----- 192.168.1.32/29
> |
> |192.168.1.252/30
> |
> 192.168.1.48/29 ----- R1 ----- 192.168.1.64/29
>
>node at address 192.168.1.18 sends a packet to address
> 192.168.1.255.
>which node or nodes will receive the packet?
> a) All nodes on all subnetworks
> b) Only the node at address 192.168.1.255
> c) Only the nodes on subnetwork 192.168.1.16 that have broadcast
reception
> enabled
> d) All nodes on subnetwork 192.168.1.16
> e) All nodes on subnetworks 192.168.1.16, 192.168.1.32, and
192.168.1.48
My answer is D.
The question says nothing about *accepting* the packet,
so it's reduced to a routing question:
Onto which segments, if any, will the routers send this packet?
I believe that the question, as stated, doesn't have enough information
to
be answered absolutely (but, then which ones do ;>).
Certainly, all nodes on the local segment, 192.168.1.16, will receive
the
packet (whether or not they accept the packet is entirely another
question,
although we hope that the author understands the distinction |>)
So, D) could be a correct answer.
B is out, since it is at a minimum a subnet broadcast address, not a
specific node address.
E and C
(Let's leave out the question of,
"...what the meaning of "is", is."
)
are also right out.
So, we are left with whether A is correct.
Under classful rules, with subnet prefix length /29, the address
192.168.1.255
is host 7 on subnet 192.168.1.248/29.
This 7/-3 is a subnet broadcast address, -1.
In addition, 248/5 is the -1 subnet. So we have the address:
network=192.168.1 subnet=-1 host=-1
.
This is supposed to be *recognized* as an "all-subnets" broadcast by
hosts
and must be accepted (RFC1122) by all hosts in all subnets of
192.168.1.0/24
network.
But, again, but this is irrelevant to the question of who *receives* the
packet.
Under classless rules, the address
192.168.1.255
is simply host 7 on network prefix 192.168.1.248/29.
Thus, this would simply be a directed broadcast.
The question is what will R1 (and R2) do with the packet?
At least two questions remain for the diagram:
. are the routers configured to forward directed subnet broadcasts?
. do they ignore all-subnets broadcast address
Are we to assume
no ip directed-broadcasts
?
Under 11.3, the default is
ip directed-broadcast
.
OTOH, this only has effect when
ip forward-protocol ...
is set.
Presumably, in the absence of facts (I hate those things -- they always
get
me confused), we would assume the lowest possible configuration, so we
would
*not* assume that any broadcasts being forwarded.
Presumably, also the all-subnets broadcast would also be blocked.
So, I have to answer D.
***However***
Now, more interesting, is what if we ***did*** have
ip forward-protocol ...
and ip directed-broadcast
on all interfaces?
*Now*, what would R1 do with the packet?
Will he see it as a directed broadcast to 192.168.1.252/30 only?
Or will he see it as an all-subnets broadcast and put it on
192.168.1.32/29
as well?
I frankly don't know what the behavior of IOS is vis-a-vis the
all-subnets
broadcast issue.
But, the all-subnets broadcast is deprecated as early as RFC1812.
Seeing it as an all-subnets broadcast would require that the routers be
configured for some kind of deprecated classful behavior.
I would therefore think that a classless (for lack of a better term)
router
with no local 192.168.1.0/24 addresses would simply treat the address
192.168.1.255 as any other routable directed broadcast address.
If so, in our case, R1 & R2 know about 192.168.1.252/30, so the packet
would
be put on that segment (ignoring, for the moment, that the connection
between R1 & R2 looks like a point-to-point serial connection).
Now,
All of the above having been said, I tried this on my own network:
(which consists of 4 segments connected by a 3640 and a few other
segments connected by some 25xx. The segments have several Unix and
windows hosts on them.
)
pluto ## ping 10.255.255.255 # network's mask (nonVLSM)= 255.255.252.0
PING 10.255.255.255: 64 byte packets
64 bytes from 10.10.120.12: icmp_seq=0. time=4. ms
64 bytes from 10.10.120.1: icmp_seq=0. time=5. ms
64 bytes from 10.10.120.11: icmp_seq=0. time=6. ms
64 bytes from 10.10.120.10: icmp_seq=0. time=7. ms
64 bytes from 10.10.120.12: icmp_seq=1. time=0. ms
64 bytes from 10.10.120.10: icmp_seq=1. time=2. ms
64 bytes from 10.10.120.11: icmp_seq=1. time=3. ms
64 bytes from 10.10.120.1: icmp_seq=1. time=4. ms
64 bytes from 10.10.124.200: icmp_seq=1. time=58. ms
So all the local hosts responded, as expected (answer D).
But then there was this 10.10.124.200 response. I have no idea what
this
box is (I no longer have control over the network).
Either it's one of the 3640's interfaces, or I have a rogue box on my
10.10.120.0/22 subnet, or the 3640 passed the packet somewhere.
Oooh, my head's starting to hurt, now.
As Linda Richman would say,
"Discuss it amongst yourselves."
-------------------------------------------------
Tks | <mailto:[EMAIL PROTECTED]>
BV | <mailto:[EMAIL PROTECTED]>
Sr. Technical Consultant, SBM, A Gates/Arrow Co.
Vox 770-623-3430 11455 Lakefield Dr.
Fax 770-623-3429 Duluth, GA 30097-1511
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