NAT and Switching questions

geezergeezer Member Posts: 136

Just working my way through the Sybex CCNA 801 'Lammle' book but have a couple of questions relating to

1). NAT - inside local, inside global etc

2). LAN switching regarding MAC thrashing

Point 1 : could someone clear up what the different local / global definitions are

Point 2 : On pages 352/3 he talks about Loop avoidance and that multiple copies of a frame can arrive from a different segment at the same time and also states that the MAC forward/filter table will be confused.

Switch A forwards the Unicast frame as it is directly connected and sends to Switch B but surely it wouldn't go to switch B?

In the diagram surely the switches will 'learn' Router C's MAC address and forward directly back out the interface where Router C can be contacted on the same local segment or did I miss something.

Don't see also why Switch B would broadcast the frame as it too would learn of the Router's port too?

I used to be undecided but now I'm not so sure.

There are only 10 types of people in the world: Those who understand binary, and those who don't!


  • NetstudentNetstudent Member Posts: 1,693 ■■■□□□□□□□
    Well I have the 5th edition delux so the page numbers are different, but Iwent to the STP chapter and I think I have the same diagram that is demonstrating multiple copies of a frame arriving at RouterC.

    Basically this is showing what would happen if STP was not on by default. So the server in the diagram has sent a unicast frame to routerC.

    To clear up your questions, this diagram is assuming that the MAC tables are empty so when the frame arrives on SwitchA, it will be fwd'ed out al ports except the port it came in on. IN this diagram, there are two ports connecting switch A and B and they are both fwd'ing because STP is not working and this is causing a LOOP. So when the frame gets broadcasted it goes out both ports of SWitchA that are connected to switchB, so switchB recieves 2 copies of the same frame.

    Also the server's MAC is the source of that broadcast frame, which means switchB will record that source MAC on 2 interfaces. Now the MAC table is corrupt. It will not know what is the right port to use when sending a frame to the server. IN all this mahem RouterC has recieved both of those broadcast packets which are identicle. Now switchB will be so caught up in trying to resume integrity of its MAC table, it will thrash to the point of not forwarding anything.

    I think what has confused you here is that terms he used for Broadcast and unicast. I think Todd used the term unicast a little too much for this demonstration. See in the diagram he has the word unicast writtin out, but in reality this frame went from a unicast to a broadcast. When switch A recived the unicast from the server and was unaware of RouterC's location, it changed the destination MAC to FFFF.FFFF.FFFF.

    As for NAT, these definitions can be kinda tricky. I had to read them 4 or 5 times before i really understood the difference between them when I was studying NAT.The cisco press book is excellent for NAT. YOu are going to need something better than Sybex for NAT on the CCNA. Unless you have dowloaded the extra PDF's on NAT, you really need another source. You can DL the PDF's on sybex webpage. The book + PDF's are a fine source.

    Anyways INSIDE LOCAL is the Private IP of a host inside your enterprise network. It's called inside because it's inside your enterprise network, it;s called local because it's a private address.

    INSIDE GLOBAL is the public IP that your inside host will be translated to as it leaves the enterprise network. It's called inside because it represents a host inside your enterprise network, it;s called global because that IP can take you anywhere on the GLOBE. Heh, just kidding, it;s called global because it is a public IP. Inside public is another name.

    Now OUTSIDE LOCAL is the IP of a host outside your enterprise network. This host would be trying to get in to your network from the outside. The outside local is a private IP that is given to a host coming into the enterprise network from the outside, but needs a private IP to route through the private network so that reply data can be sent back to the outside host. If the outside host were never translated, then any data that was replying to the outside host, would be trying to reply to a public IP. Your inside routers arn't going to have routes to some unknown public IP. Not in this case anyways.

    OUTSIDE GLOBAL refers to a host outside the enterprise network. The ourside global is the actual IP address of the outside host as it resides in the outside network or internet. it represents the outside host with a public IP so that it can be routed through the internet.

    The cisco press ICND book has an awesome pictorial of packets and their translations as they tranverse from the inside to the outside and vice versa. Anyways hope this helps.
    There is no place like BUT is my away from!
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