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Home > Archive > CCNA > November 2001 > Why ARP then?
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| If there is no entry for a network on the routing table the router will drop the packet
PERIOD! as Mr Lammle stated, so why ARP then.
As usual I might be missing something here.
Is it fine to say once we have a routing table in place then ARP is not used? either forward or discard?
TIA
ps
Todd ccna 507 study guide
page # 246 under IP routing in our network | |
| Silkysmoothe1 2001-11-10, 2:58 pm |
| If the router does not recognize/notice an address,,the router will shoot the packet out a (default/dedicated) port,,,there by not dropping the packet totally,,,
You are correct to believe that the packet will be forwarded out a designated port.
To say that a packet will be dropped totally without a network address in the header/because a routing table does not have that network ID,,doesn't sound right to me,,, hopefully someone can bring light to your question.
,,,,silky,,, | |
| rheingold 2001-11-10, 3:14 pm |
| You still need to resolve or map a known destination IP address to a MAC sublayer address. The MAC address will change with
every hop, but the source and destination IP address will not change.
Hope this helps. | |
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| My understanding was that, if there is an entry in the routing table then no problem, but if there is NO entry then the ARP process will start, but according to Mr. Lammle statement the packet will be dropped if there is no entry in the routing table.
So if that the case it is either there is entry then forward OR there is No entry then drop!!!! | |
| Silkysmoothe1 2001-11-10, 5:48 pm |
| There is only a few instances where a router will drop a packet,,,assuming there is nothing wrong with the network,,,(cross talk,,, access list,,power/router failure)I can only recall a packet being dropped when the Hop count expires. otherwise, the router will be configured to have a default port (where) all unknown packets will be sent/flushed out on..... | |
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| You have to remember that routing over serial connections is all Layer 3 logical addressing and routing over an ethernet connection is were Layer 2 addressing comes in and the need for a router to use ARP.
When a router looks a packet up in his routing table and it's destination network addressis out a serial connection it doesn't need arp because it's staying in layer 3, unless it's using frame-relay in which case it uses IARP. And if a router gets a packet w/ a destination network address it hasn't learned it will be dropped unless a default route has been configured.
A router only uses arp when it see's a packets destination network is out it's ethernet port. It then looks at the whole IP address and resolves it to a MAC by using ARP. | |
| Silkysmoothe1 2001-11-10, 10:00 pm |
| thanks for the clarification Ant,,, | |
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| Thanks to all. Ant, that was a good explanation, thanks. I will try to summarize what Ant said please correct me if I am wrong.
quote:
When a router looks a packet up in his routing table and it's destination network addressis out a serial connection it doesn't need arp because it's staying in layer 3, unless it's using frame-relay in which case it uses IARP
packet out serial port = Layer 3 addresing
quote:
A router only uses arp when it see's a packets destination network is out it's ethernet port. It then looks at the whole IP address and resolves it to a MAC by using ARP.
packet out ethernet port = ARP is used
quote:
And if a router gets a packet w/ a destination network address it hasn't learned it will be dropped unless a default route has been configured.
No default route = DROP the packet
So that means the initial statement is TRUE (If there is no entry for a network on the routing table the router will drop the packet )
, the only thing that Mr. Lammle missed is to add is UNLESS THERE IS A DEFAULT ROUTE DEFINED | |
| doctorcisco 2001-11-12, 10:56 am |
| quote:
Originally posted by Ant
You have to remember that routing over serial connections is all Layer 3 logical addressing and routing over an ethernet connection is were Layer 2 addressing comes in and the need for a router to use ARP.
Not really. In frame relay, there is a layer 2 address ... the DLCI. The DLCI must be associated with an IP address using frame-relay map commands or inverse ARP.
X.25 is more involved, because it has its own layer 3 addressing as well.
HTH,
doctorcisco | |
| strikeattack 2001-11-12, 2:19 pm |
| I still don't understand what ARP'ing has to do with routing. ARP'ing is only done once that routing logic has already taken place. | |
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| quote:
Originally posted by doctorcisco
Not really. In frame relay, there is a layer 2 address ... the DLCI. The DLCI must be associated with an IP address using frame-relay map commands or inverse ARP.
X.25 is more involved, because it has its own layer 3 addressing as well.
HTH,
doctorcisco
If you read the rest of my post I mentioned that Frame-Relay is an exception and that it uses a layer 2 resolved by IARP | |
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| ARP will resolve IP address to MAC address.
routers dont read mac addresses only IP addresses, its only when they move out of interface they need the MAC address to cross a bridge or switch.
i hope it makes sense | |
| strikeattack 2001-11-17, 7:51 pm |
| It does. Indeed, I posed the question several posts ago what routing logic has to do with ARP'ing for the same reasons you just gave. |
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