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TOPIC: switching tables.

switching tables. 6 years 10 months ago #33515

  • freakhead
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say i have two 4 port switches, A and B interconnected to each other.
A connects hosts 1,2,3 through a,b,c ports
B conencts hosts 5,6,7 through a,b,c ports

and let A and B interconnect through d -d port.

incase 1 needs to send data to 7, it is understandable that A(d-port) is mapped to switch B physcial address in A tables, but is it also needed that 7 details are also registered?? in that case is it not redundancy?

Is it alright to have a common switching table for the entire LAN where each swicth can look up?
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Re: switching tables. 6 years 9 months ago #33673

  • krik
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Hi,

Actually each switch maintains a mac-address-table (or previously called CAM table) where it memorizes which source MAC addresses it has seen behind which ports (and if VLANs are supported by the switch in which VLAN). When a switch needs to forward a frame, it performs a lookup of the destination MAC in the mac-address-table and use the learned port to forward the frame. If the switch does not know where the MAC address is it floods the frame through all the ports excepted the ingress port.

So, assume switch A and B mac-address-table are empty, 1 wants to send a frame to 7.
1) 1 sends the frame to 7's MAC
2) A receives 1's frame on port a and memorizes 1's MAC is behind port a
3) A performs a lookup for 7's MAC in its mac-address-table and doesn't find it, the frame is then flooded through b, c and d
4) B receives 1's frame on port d and memorizes 1's MAC is behind port d
5) B performs a lookup for 7's MAC in its mac-address-table and doesn't find it, the frame is then flooded through a, b and c
6) B receives the frame
7) Now 7 sends back a frame to 1
8) B receives 7's frame on port c and memorizes 7's MAC is behind port c
9) B performs a lookup for 1's MAC in its mac-address-table and find it is behind port d, the frame is then sent to A through d (and only d)
10) A receives 7's frame on port d and memorizes 7's MAC is behind port d
11) A performs a lookup for 1's MAC in its mac-address-table and find it is behind port a, the frame is then sent to 1 through a (and only a)
12) from now, the mac-address-tables are correctly populated for 1 and 7 and they can communicate together without flooding.

I hope this helps...
Christophe Lemaire
www.exp-networks.be/blog/
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Re: switching tables. 6 years 9 months ago #33680

  • S0lo
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Neat explanation krik :)
incase 1 needs to send data to 7, it is understandable that A(d-port) is mapped to switch B physcial address in A tables, but is it also needed that 7 details are also registered?? in that case is it not redundancy?

The switche's own MAC is not necessary for this scenario to work. Yes, as kirk mentioned, Switch A CAM table will ultimately/finally contain the MAC of 7 as behind it's port d. And Switch B CAM table will contain the MAC of 1 as behind it's port d
Is it alright to have a common switching table for the entire LAN where each swicth can look up?

No it's not done this way as far as I know.
Studying CCNP...

Ammar Muqaddas
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www.firewall.cx
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