extended system id...

a fixed switch shares it's mac address with it's ports...

Base ethernet MAC Address: 00:09:B7:52:D7:80
the mac address of port 1 on this switch is:


asw2#sh int f0/1
FastEthernet0/1 is up, line protocol is up (connected)
  Hardware is Fast Ethernet, address is 0009.b752.d781 (bia 0009.b752.d781)

if your comfortable with hex, you can easily determine the rest of the ports mac addresses by counting up...


asw2#sh int f0/10
FastEthernet0/10 is down, line protocol is down (notconnect)
  Hardware is Fast Ethernet, address is 0009.b752.d78a (bia 0009.b752.d78a) = 10 hex

you knew that...

the default switch priority is 32768...

we know that the combination of priority and mac address gives us the BID
because 1 or many ports may belong to a vlan, we need more distinction...

the extended system id makes that yet more granular...


asw2#sh spann vlan 20

VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    32788
             Address     0009.b73f.ce80 (base ethernet)
             Cost        38
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32788  (priority 32768 sys-id-ext 20) 32768 + 20

the extended system id allows the vlan for the switch to have a unique BID

graphic below from http://adelzalok.wordpress.com/2011/09/29/anatomy-of-a-mac-address-bpdu-bid-and-the-802-1q-ethernet-frame-tag/  great site by the way... you haven't clicked on it yet?

packet traversing, mac changing...

the source ip will be that of the pc sending... the destination ip will be that of the destination host receiving... the source mac will be that of the router... the last router in the chain will be the source mac, and the destination mac will be that of the receiving host... the important thing to remember is that the ip of the sender and receiver never changes...  the source mac will be the last router forwarding to the destination hosts mac...

now, what was the question originally asked above...  leave comments...

spanning-tree song...

spanning-tree very pretty and the bpdu's are sweet,
but the root of the poor spanning, is impossible to eat,

Michael the network cat likes that song... reow
 

STP elects a root bridge (switch) and puts all root bridge interfaces into forwarding state

Each non root bridge (switch) determines which of its ports has the least administrative cost (best) to the root bridge and STP makes that port that switch’s root port.

The switch with the lowest (best) cost  to the root is put in forwarding state.

The lowest cost switch on each segment is the designated bridge (switch) and the interface on that switch is called the designated port.

            The root bridge’s (switch) ports are always in forwarding state and the root switch (bridge) is always the designated bridge on all connected segments.

            The non root bridge root port is always forwarding. This port receives the lowest cost BPDU from the root.

            Each LAN’s designated port is always forwarding and the bridge forwarding the lowest cost BPDU is the segment’s designated bridge (switch)

             All other ports are blocking. No forwarding frames, no receiving frames.

         At first each switch claims to be root by sending BPDU’s that contain:

                 The root bridge ID- a combination switch priority and MAC address, lower number, higher priority

             The cost to reach the root- again the lower, the better

             And it’s own bridge ID

can't we just call a root bridge a root switch instead...

a poor man's routing...

routing rudiments... 

given: hosts are directly connected, host_A pings host_B

with a nod to lammle, odom, et al

1) host_A creates an echo request for host_B (ICMP)

2) ICMP hands the request to IP, which makes a packet that contains the IP source address, the IP destination address and a protocol field

3) IP determines whether the destination is a local or remote network

4) in this case it is remote, so the packet needs to go to the remote network determined by host_A’s OS (default gateway 172.16.10.1)

5) the MAC address of 172.16.10.1 (E0) is known by host_A. this supposes the arp cache has resolved E0’s IP address to a hardware address. (the output of arp -a will display E0’s MAC address) the packet will be framed by the datalink layer and shipped to the MAC address of E0. hosts only communicate with MAC addresses on the local network

6) if not already resolved, the host sends an arp broadcast to the local network. Upon receipt of the resolution, the host cache’s the MAC address of E0 and encapsulates the packet in a frame with control information.

In both the above cases (hardware address known, hardware address resolved) the framed destination MAC address (E0) and source MAC address (host_A), an ether-type field (network layer IP), the packet, and FCS (frame check sequence which holds the result of a cyclic redundancy check CRC) are shipped to the MAC address of the default gateway, not the MAC address of the remote host

7) the frame thusly prepared, is ready to traverse the physical layer bit by bit

8) at the router, the packet is taken from the frame and off loaded to IP. The remainder of the frame is discarded.

a good place to jump