On Fri, 22 Nov 2002, David Kraut wrote:
> Here's the basic config. T1 from ISP goes to Cisco router external
> interface, Cisco internal Ethernet goes to mini hub, Gnat box pro external
> NIC is also connected to same mini hub. The Gnat box default gateway is the
> Cisco internal Ethernet so I understand how packets find their way from the
> Gnat to the Cisco but how does an external request find it's way from the
> Cisco internal NIC to the Gnat external NIC? Do I have to setup a gateway
> on the internal Cisco Ethernet or will the Cisco simply forward all packets
> to the subnet (mini hub) which includes the Gnat box external NIC?
To put it simply, your cisco has an IP address on the same subnet as the
external interface of the GNATBox.
When the cisco sees a packet whos destination is an address on a subnet it
is directly connected to, it checks its internal ARP table (I'll refer
only to tcp/ip over ethernet here) to see if it knows a hardware (MAC)
address of the device to direct the packet to.
If it doesnt find an ARP entry, it sends out an ARP broadcast to the
subnet in question. This query will be seen by the device who has the IP
we're looking for. This device will send back an ARP reply which contains
his hardware address.
Now the cisco router has the hardware address that relates to the IP
address of the device it is trying to reach. It then sends the packet
directly to that address via ethernet.
If the cisco sees a packet that is destined for a network that is NOT
directly connected, it consults its routing table. It looks for the most
specific route that is applicable, and forwards the packet down that path.
Without a specific route, it falls back to the default route (or default
gateway), which is essentially a wildcard route. ("anything that
I don't already know how to send, I send here.")
Does that help?
..david
---
David Raistrick
Systems Administrator - Global Technology Associates, Inc
[EMAIL PROTECTED]
Disclaimer: All opinions expressed are the opinions of
David Raistrick, not necessarily those of GTA, Inc.
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