MAC address are similar result in similar ip address

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April 23rd, 2006, 04:30 PM
Tiger Shark

Because the switch logs the IP address to the _physical_ port the request for the net came in on and updates it's MAC table accordingly. Thus, once the response from the server comes back it is sent to the port that the request came in on. The switch will not simply keep updating it's tables for every single request because that would cause havoc on the network so once the first laptop has "reserved" the port on the switch then it will stay there in the switches table until something significant happens to alter it.
April 23rd, 2006, 04:45 PM
zillah

Thanks Tiger Shark

1- What will the explaination be if that AP is connected to cisco router directly (there is no switch, that means there is no CAM table) ?

2- What will the the explaination be if I want to do the same test at home with ADASL linksys router ?
April 23rd, 2006, 07:06 PM
ZT3000

Quote:

Originally posted here by zillah
Now since we have two laptop with same MACs and same IPs (we knew, why they have got the same IPs),,if we have been asked why when one laptop access the internet first the second can not,,,,what should be the answer ?

For my answer, I would first ask, What topology are we discussing?
There are great differences depending on the topology.

Which one? Star, Bus, Ring, Tree, Mesh??

(I'm supposing you are using STAR, but still it's smarter to ask which topology first).
April 23rd, 2006, 08:44 PM
zillah

Quote:

For my answer, I would first ask, What topology are we discussing?

Star topolgy

1- First Secnario AP is connected to a cisco router 2600 directly without switch , both laptops are connecting to the AP wirelessly.

2- Second scenario AP is connected to a cisco switch 2900 and a switch is connected to a cisco router , both laptops are connecting to the AP wirelessly.
April 23rd, 2006, 08:45 PM
Nokia

It is possible to have two identical MAC addresses on a network - they can be on different VLANS or different internetworks - I.E on different interfaces on a router and even on the same swich, depending on the make on model of it.

They will not have 100% functionality, but they will have some.

A switch will update its filter table as and when it needs to and will periodicaly flush entries that have not been active.

If two MAC's connected to the same switch they would be entered on to the table (on CISCO some 2900 series and below there is no duplicate MAC address checks) - data will be passed to either host depending on where they come in the table - the table is parsed until it comes to an entry that matches the destination MAC - once it has found the entry, the frame is passed out of the correct port - the table is not searched in its entirity only until if finds a matching entry.

Hence you can send data out but are not guarenteed to receive any. :p
April 23rd, 2006, 09:27 PM
zillah

Quote:

It is possible to have two identical MAC addresses on a network - they can be on different VLANS or different internetworks - I.E on different interfaces on a router and even on the same swich, depending on the make on model of it.

Since it is possible to have two identical MAC,,,what did you mean by "depending on the make on model of it"

Quote:

If two MAC's connected to the same switch they would be entered on to the table (on CISCO some 2900 series and below there is no duplicate MAC address checks)

You meant that the process of learning (building CAM table) can map same MAC address to two different ports,,,didn't you ?

IP address for laptop A is 10.70.72.93/24 , and MAC is 00-91-4C-2A-2B-2C.say this is connec
IP address for laptop B is 10.70.72.141/24, and MAC is 01-80-4C-29-22-29.

Quote:

data will be passed to either host depending on where they come in the table - the table is parsed until it comes to an entry that matches the destination MAC - once it has found the entry, the frame is passed out of the correct port

Suppose (my real sceanrio not like this ) I have got laptop A with IP address 10.70.72.93/24 , and MAC is 00-91-4C-2A-2B-2C connected wired to port 10

I have got laptop B with IP address 10.70.72.93/24 (has been changed after I modified its MAC), and MAC 00-91-4C-2A-2B-2C (same as A) connected wired to port 20

Laptop C with ip address 10.70.72.150/24 , and MAC CC-3C-4C-22-33-33 connected wired to port 30

Now CAM table will be:

Code:

MAC Port 00-91-4C-2A-2B-2C 10 00-91-4C-2A-2B-2C 20 CC-3C-4C-22-33-33 30

Now if laptop C (MAC CC-3C-4C-22-33-33 port 30) tries to send frame to Laptop with MAC address 00-91-4C-2A-2B-2C how does the switch know the traffic should exit from which port ?
April 23rd, 2006, 09:33 PM
zillah

If I have got 3 or 4 PCs connected to AP wirelessly, and that AP is connected to a cisco switch wired,,,Does't the cisco switch build CAM table for MAC address for AP only ?
April 23rd, 2006, 10:58 PM
Nokia

Hello mate,

Quote:

Since it is possible to have two identical MAC,,,what did you mean by depending on the make on model of it

I meant you can have two MAC's on the same switch depending on the make and model of the switch - some of the more intelligent switches will activly look for duplicate MAC's in its filter table and some won't.
Now I am not saying that you will be able to send or receive any frames, all I am saying is some switches will keep both entries in it's table.

Quote:

Suppose (my real sceanrio not like this ) I have got laptop A with IP address 10.70.72.93/24 , and MAC is 00-91-4C-2A-2B-2C connected wired to port 10

I have got laptop B with IP address 10.70.72.93/24 (has been changed after I modified its MAC), and MAC 00-91-4C-2A-2B-2C (same as A) connected wired to port 20

Laptop C with ip address 10.70.72.150/24 , and MAC CC-3C-4C-22-33-33 connected wired to port 30

Now CAM table will be:

Code:

MAC Port 00-91-4C-2A-2B-2C 10 00-91-4C-2A-2B-2C 20 CC-3C-4C-22-33-33 30

Now if laptop C (MAC CC-3C-4C-22-33-33 port 30) tries to send frame to Laptop with MAC address 00-91-4C-2A-2B-2C how does the switch know the traffic should exit from which port ? [/B]

A CAM table is updated by what is known as 'last packet updates' - as the name suggests if you duplicated a MAC on say port 1 and sent a frame out to another port the table will be updated with your spoofed MAC as being on port 1 - some switches will now delete the real MAC from its table and some will just leave it there.

If it gets deleted you will effectivley own that MAC and receive traffic destined for that host, untill the real host sends a frame through the switch - then as that is the 'last packet'[updates] to be received from that MAC the switch will now update itself accordingly.

If you happen to be on a switch that doesn't actively search for duplicate MAC's it will now have two entries for the MAC - when it parses its table now, as soon as it comes across the relevant MAC it will stop parsing and send the frame. So if your entry is higher up in the filter table than the other one you will get the data, if not the other host will get the data.

Quote:

Originally posted here by zillah
If I have got 3 or 4 PCs connected to AP wirelessly, and that AP is connected to a cisco switch wired,,,Does't the cisco switch build CAM table for MAC address for AP only ?

More than one MAC can be assosiated with a port.
A wireless AP is infact a hub, so say you had 4 hosts on the AP and the AP was wired in to port 1 on the switch, the switch would have 4 MACs assosiated with the port - it would just send the frame out of the port, the frame would reach the hub and obviously go out to all hosts.

Hope it helped!
April 23rd, 2006, 11:19 PM
Nokia

One thing I forgot to mention, some of the catalyst switchs use the Timeout method for MAC filtering. 300 secs (5 mins) is the default - so if it has not received or sent any traffic to the MAC for 300 seconds it is flushed from the table.

This is good for bigger networks as IIRC the max size for a CAM table is 8000 entries - but it is usually altered or even disabled on smaller networks to reduce the ammount of time taken in the switching process.
April 23rd, 2006, 11:25 PM
Nokia

Quote:

Because the switch logs the IP address to the _physical_ port the request for the net came in on and updates it's MAC table accordingly. Thus, once the response from the server comes back it is sent to the port that the request came in on. The switch will not simply keep updating it's tables for every single request because that would cause havoc on the network so once the first laptop has "reserved" the port on the switch then it will stay there in the switches table until something significant happens to alter it.

I take it the first part about a switch loging IP addresses to a port was a typo! You meant MAC address of course, since we are talking layer 2 switches!

Switches will and do constanlty update/check their tables - some use 'last packet updates' and others use default timeouts.

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