Thanks for the excellent answer Rick. I now have it hammered down. PS : for
all those who spotted my Gaff, I was on different networks at either end.
Blush.
Regards,
Phil.
----- Original Message -----
From: "Rick Seiler"
To:
Sent: Tuesday, June 05, 2001 1:04 AM
Subject: RE: Transparent Bridging ? [7:7126]
> OK, more than you ever wanted to know...
>
> The setup of the PCs is really no different than if they are on the same
> hub, switch vlan, or directly connected via a twist cable. The
configuration
> of the PCs has to be done in one of two ways:
>
> 1. Set the PCs to the same ip subnet and mask (the default gateway doesn't
> matter, it won't be used).
>
> 2. Set the PCs to entirely different ip subnets (or the same, as long as
the
> ip address is not identical or a broadcast address) and set the default
> gateway the same as the ip address.
>
> First Option:
> -------------
>
> Set the PCs to the same ip subnet and mask (the default gateway doesn't
> matter, it won't be used).
>
> For example:
>
> PC#1:
> IP: 10.100.1.20
> MASK: 255.255.255.0
> GW: 0.0.0.0 (or blank, depending on OS)
>
> PC#2:
> IP: 10.100.1.21
> MASK: 255.255.255.0
> GW: 0.0.0.0
>
> The reason the default gateway doesn't matter is because both PC's are on
> the same IP subnet. The default gateway is only used if you try to
> communicate (ping) an ip subnet that is not local to the PC.
>
> On PC#1, ping your loopback (ping localhost on Windows),
> then ping your interface (ping 10.100.1.20) see above,
> then ping the other PC (ping 10.100.1.21).
>
> The reason for pinging your loopback and your own interface is to verify
> that your IP stack is functioning and configured properly before you blame
> the 'network'.
>
> Since the two PCs don't actually use the IP addresses to communicate, you
> can see what is actually going on by typing 'arp -a' in Windows to see the
> local ARP cache. You should see the IP address of PC#2 (10.100.1.21) and
> the MAC address. When you typed 'ping 10.100.1.21' on PC#1 (above), PC#1
> actually:
>
> a. Sent an ARP request on the wire (you will see this on your sniffer)
> looking for the MAC address that answers to IP address 10.100.1.21.
>
> b. Provided only one machine on this segment (hub, switch vlan, etc.) is
> configured with this IP address, PC#2 will be the only PC to answer that
ARP
> request.
>
> c. PC#1 will populate its local ARP cache with the IP address to MAC
address
> mapping
>
> d. All communication with PC#2 will be with the MAC Address of PC#2 (not
the
> IP Address). The reason for the IP address is to make it easier for
humans
> to manage device addressing, the computers use only layer 1 (the cabling,
> hub) and layer 2 (mac address, bridge) to send information to each other.
>
> This is why you cannot ping an IP address on a different subnet
> (192.168.255.1 for example). The PC will try to use a default gateway to
> get there, which isn't configured and doesn't exist.
>
>
> To illustrate this point a little better, let me explain the second option
> for configuring the PCs:
>
> Second Option:
> --------------
>
> Set the PCs to entirely different ip subnets (or the same, as long as the
ip
> address is not identical or a broadcast address) and set the default
gateway
> the same as the ip address.
>
> For example:
>
> PC#1:
> IP: 192.168.255.26
> MASK: 255.255.255.0
> GW: 192.168.255.26
>
> PC#2:
> IP: 10.1.50.201
> MASK: 255.0.0.0
> GW: 10.1.50.201
>
> Notice that the IP address and default gateway are identical on each
> individual PC.
>
> Now, why would you ever do this? To illustrate a point. If you would
ping
> PC#2 (10.1.50.201) from PC#1 (192.168.255.26), it will work!!! (Assuming
you
> started by verifying that you could ping localhost and your own
interface).
>
> Why does this work? Because, by setting the ip address and default
gateway
> the same, you tell the PC to ARP for everything. Even though the two PCs
> are configured on different IP subnets, the PCs don't care because they
> really use their MAC addresses to communicate. So, PC#1 sends and arp
> request for 10.1.50.201 on the wire and PC#2 responds, PC#1 adds the MAC
> address to its ARP cache and will send all further IP packets destined for
> 10.1.50.201 to the MAC address of PC#2.
>
>
> Does this clear anything up? Here are sample configs for your two
routers:
>
> R1
> --
>
> ! the following line is NOT necessary if you don't put ip addresses
> ! on the individual interfaces, like this config
> !
> no ip routing
> !
> interface ethernet0
> no ip address
> bridge-group 1
> no shut
> !
> interface serial0
> desc DCE
> no ip address
> clockrate 56000
> bridge-group 1
> no shut
> !
> bridge 1 protocol ieee
> !
> end
>
>
> R2
> --
>
> ! the following line is NOT necessary if you don't put ip addresses
> ! on the individual interfaces, like this config
> !
> no ip routing
> !
> interface ethernet0
> no ip address
> bridge-group 1
> no shut
> !
> interface serial0
> desc DTE
> no ip address
> bridge-group 1
> no shut
> !
> bridge 1 protocol ieee
> !
> end
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