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A question about Inter-vlan routing
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| What do you think happens if the route processor has no knowledge of the network segment for which the packet is destined?
A The communication breaks down.
B The route processor forwards the packet onto the next-hop router.
C The packet is discard and an ICMP replay is sent to the end-user device.
The answer is B.But I think it is C. | |
| Silkysmoothe1 2001-09-20, 9:19 pm |
| wouldn't the packet continue until it's max hop count is reached, and then it would drop the packet and send the ICMP.
I choose b
C would then be a natural order of sequence.
unless configured differently.
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| But the route processor has no knowledge of the network segment for which the packet is destined,In other words,it has no a entry in its routing table about the the destination network address.And I think the router discard the packet.Ask for more help,thanks. | |
| Mat P 2001-09-21, 12:19 am |
| It would send it out on the default route, I agree with Silkysmoothe1.
If there isn't a default route or TTL has expired it would drop it and send the ICMP. | |
| strikeattack 2001-09-21, 1:34 pm |
| The router looks at the IP address and picks the CLOSEST MATCH within its table. Now, should there be a match other than the default route, the router will take it. If there is not match other than the default route, all packets will eventually be matched to 0.0.0.0 if not route exists.
It sounds as if the packet is no involved in inter-vlan routing or MLS, but simply being routed to another subnet/destination... Possibly outside of the organization.
Just my thoughts. I would choose 'B' if asked this question. | |
| depamo 2001-09-23, 8:36 pm |
| To make it simple, I think that I know where they are going with this.
If the router doesn't know the destination network it will do one of two things, either send it to the default network or default route of last resort (these are a combination) or drop it.
I think that for this one we can assume from the wording of the answers that it will return an ICMP destination unreachable which is very realistic. Why waste your bandwidth if a destination is down?? Use your dynamic routing protocols to stop traffic before it gets on high use inter-block links. If you default-network the packets, they will have to travel to another router, it will have to go through the same scenario until you get somewhere that doesn't have a default route and actually knows that the network is probably just down. | |
| Retired-Mod 2001-09-28, 5:07 pm |
| I hate questions like this because to me it says there is no default route and therefore the packet will be dropped, but due to their answer they must mean "no direct knowledge of that route", thus ignoring the default route being present, yet that is the only way it will be passed to the next hop. I think the clue is that no ICMP message gets returned and therefore that answer is wrong leaving you with only B as an option.
Yankee | |
| doctorcisco 2001-09-28, 5:20 pm |
| quote:
Originally posted by Retired-Mod
I hate questions like this because to me it says there is no default route and therefore the packet will be dropped, but due to their answer they must mean "no direct knowledge of that route", thus ignoring the default route being present, yet that is the only way it will be passed to the next hop. I think the clue is that no ICMP message gets returned and therefore that answer is wrong leaving you with only B as an option.
Yankee
Won't the route processor send back ICMP Destination Net Unreachable?
doc | |
| depamo 2001-09-29, 1:09 pm |
| The only time a router will send a Destination-Unreachable is when the router does not have a route to follow and cannot forward the packet. This includes no routes in the routing table, no default route or default network defined so the packet cannot go further then this node.
Thought about this one for a while and this is another problem with test questions not clearly identifying with the real world. If you have a closed private network with no outside connections to the internet, chances are that you will get a destination unreachable at the leaf router because you will not use default routes/networks, no reason to.
If you have a real network that does connect to the internet and such, all your leaf routers will have default routes which will keep you from having to add tons of static routes to forward internet traffic or propogate all routes through the network. Of course you can redistribute static outbound routes which is the prefered method but it is merely a default route with another name.
That is just to let you know why the question is so weird for everyone to understand. Real networks having this problem would propogate the packet up to the edge router that would deny the packet since it is the only router with all the paths and no default route. From there it would send back an ICMP Destination Network Un-reachable to the host that sent the packet.
Hope that helps in unnderstanding the perplexing nature of this question. |
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