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IP Routing Process(Frame Creation)& CISCO Basic BACS FIREWALL/ Access lists


When a frame is created by the datalink layer; where is the source and destination IP address located? I know the source & mac address is visible.

2.Also I dont quite understand what happens at/with the ETHER-TYPE FIELD???

3.What role does the protocol field play here?

4.Why is blocking source port protocols wrong, when limiting access of the network to the internet to eg// http https only??

5.I dont undertand the INSPECT Process very much in Cisco (BASIC)CBACS Firewalls when processing traffic.


Re: IP Routing Process(Frame Creation)& CISCO Basic BACS FIREWAL

The frame (layer two) is created containing a packet (layer three). The IP Addresses are contained in the packet.

At layer two, frames/switches have no knowledge of IP, only MAC addresses.

In days past (long past, at this point), different manufacturers used Ethertype to identify their frames. The applications of the time would only look at frames containing their Ethertype.

The protocol field identifies the which stack (in a multi-protocol environment, like IP and IPX) the frame contents should be handed to.

Blocking the source port doesn't work, because the originator could choose any port number to send from. They send TO a specific port that the server is listening on (like 23 for Telnet). The source port could be anything, but the destingation port for Telnet is "well known" at port 23.

INSPECT / CBAC is "deep packet inspection" ... rather than just looking for a value at a certain offset (like a source MAC or source IP), it peels the frame and peels the packet to look at the contents ... then making an assessment of the contents to decide if it's traffic to block or pass based on the PERMITs and DENYs of the configuration.

Good Luck


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