May 26th, 2004, 09:24 AM
Schools out for summer...
and I decided to use the time to brush up on what I have learned and prepare
for Module 4 of the CCNA. Total immersion...
I thought it a good idea to develop a good mental picture of how a packet (frame)
travels from a host on a LAN to a remote host and then back, using TCP/IP off course.
Somehow I got stuck on inbound return traffic on the interior interface of the default
gateway router. I had simply forgotten that a router maintains a ARP table that maps local
IP addresses to local MAC addresses, besides the routing and ACL tables, simplified.
In the process, I looked at the subject of MAC addresses a little closer.
I have a Catalyst 2900 / 12 port switch in my lab. As a networking device, you can assign an
IP address to the switch, which enables you to telnet into the device for management/configuration
purposes. But you need an IP/MAC address pair to resolve the node, hence the switch
has a hardware address as well. The "show interfaces" command at the prompt of the switch reveals
VLAN1 is up, line protocol is up
Hardware is CPU Interface, address is 0050.8046.6200 (bia 0050.8046.6200) ......
I can imagine this MAC address showing in my local ARP cache during a telnet session.
However, the MAC address serves another purpose. The above MAC address is the "base address"
from which all switch ports derive a sub-MAC address. These sub-MAC addresses are part of the
"bridge ID" that in conjunction with a priority value (default is 32768) form the basis of
electing/selecting the root bridge with Spanning Tree Protocol, a protocol used to prevent loops
(broadcast storms and redundant frames).
I've removed several lines that normally appear with this command to focus on the CPU interface and
FastEthernet0/1 is down, line protocol is down
Hardware is Fast Ethernet, address is 0050.8046.6201 (bia 0050.8046.6201)
FastEthernet0/2 is down, line protocol is down
Hardware is Fast Ethernet, address is 0050.8046.6202 (bia 0050.8046.6202)
FastEthernet0/3 is down, line protocol is down
Hardware is Fast Ethernet, address is 0050.8046.6203 (bia 0050.8046.6203)
FastEthernet0/11 is down, line protocol is down
Hardware is Fast Ethernet, address is 0050.8046.620c (bia 0050.8046.620c)
FastEthernet0/12 is down, line protocol is down
Hardware is Fast Ethernet, address is 0050.8046.620d (bia 0050.8046.620d)
As you can see, the switch ports have sub-MAC addresses incremented by 1 in hex,
derived from the base MAC. The format is slightly different, a dotted notation,
rather that the use of colons, but MAC addresses nevertheless.
Now, these MAC addresses have nothing in common with the switches' MAC-address tables,
that map the MACs of connected hosts to switch ports.
I think that this is an important distinction, that is not entirely clear when learning
about Cisco switches in general.
Hope this provides some insight...