Cisco – Centralized MAC address database for Cisco switches

Take a look at Layer2 traceroute (for cisco).. Cdp should be running btw...

Router# traceroute mac 0000.0201.0601 0000.0201.0201
Source 0000.0201.0601 found on con6[WS-C2950G-24-EI] (2.2.6.6)
con6 (2.2.6.6) :Fa0/1 => Fa0/3
con5                 (2.2.5.5        )  :    Fa0/3 => Gi0/1
con1                 (2.2.1.1        )  :    Gi0/1 => Gi0/2
con2                 (2.2.2.2        )  :    Gi0/2 => Fa0/1
Destination 0000.0201.0201 found on con2[WS-C3550-24] (2.2.2.2)
Layer 2 trace completed
Router# 
Router# traceroute mac 0001.0000.0204 0001.0000.0304 detail 
Source 0001.0000.0204 found on VAYU[WS-C6509] (2.1.1.10)
1 VAYU / WS-C6509 / 2.1.1.10 :
                Gi6/1 [full, 1000M] => Po100 [auto, auto]
2 PANI / WS-C6509 / 2.1.1.12 :
                Po100 [auto, auto] => Po110 [auto, auto]
3 BUMI / WS-C6509 / 2.1.1.13 :
                Po110 [auto, auto] => Po120 [auto, auto]
4 AGNI / WS-C6509 / 2.1.1.11 :
                Po120 [auto, auto] => Gi8/12 [full, 1000M] Destination 0001.0000.0304 
found on AGNI[WS-C6509] (2.1.1.11) Layer 2 trace completed.
Router# 

First off - your English appears to me to be excellent. Secondly your summation of bridging (question 1) and consequently your understanding of the subject are very good. I would probably word some of that slightly different but your summation is fair and it appears that your understanding is good.

Let's make one assumption at this point to keep things simple - this is all Ethernet and IP.

Having said that there are two steps that I would add to your bridging summation that might clear up your understanding of routing (question 2) a little:

1. when your network stack is handed data to transmit there is a route table look up done (every device on an IP network contains a route table - to see it from a command prompt - on linux type "route", on windows type "route print".) to determine two things:

a) which interface to transmit the data out

b) if the destination is directly reachable or is remote (beyond a router).

2. Once the route is determined the network stack will ARP for a MAC address.

So when Alice is sending data to Charlie she does a route lookup - it is determined that his machine is directly accessible on the LAN. She then ARPs for his MAC address. Once the route has been determined and the MAC address has been acquired the network stack can then assemble the packet for transmission. This particular conversation as you mentioned only traverses switch A as it inspects the destination MAC.

Now that we have fleshed out question one a little better let's apply some of these concepts to question 2.

When Alice desires to send data to Bob the first step is a route lookup. It is determined that Bob is not on the LAN. Thus the route to Bob is the default route (assuming Router A is configured as such). (As your book points out Router A - and by extension Alice - don't know where Bob is. They only know the general direction - or "route".) So Alice ARPs for Router A. Then assembles the packet with destination IP address(network layer) of Bob and MAC address(datalink layer) of Router A. Upon receiving this packet Router A 1)strips off the MAC address(datalink layer) of Alice 2)does a route lookup 3)ARPs for router B 4)then reassembles the packet with MAC address of Router B. Router B repeats these steps ARPing for Bob and forwards the packet.

So the network layer is assembled at the start - source Alice and destination Bob - and never changes. The datalink layer on the other hand is deleted and reassembled each and every leg of the trip.