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- Mark Scott
August 7, 2005, 3:37 am
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Please can anyone confirm my thoughts on encapsulation with regards to
1. User has data to send (Layer 7)
2. Layer 4 Information is added = Segment
3. Layer 3 information (IP address) is added = packet
4. Layer 2 information (MAC) is added = frame
a layer sees information from an upper layer only as DATA.
If a frame hits a router, it extracts the packet and reframes it ready for
transmission to the next router.
Where my thinking goes off track is regarding Routers MAC addresses, would
the router then ARP for the next hop mac address then wrap the packet up
What happens when the packet / frame is passed between routers and switches?
Re: Encapsulation questions
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Mark Scott wrote:
ARP is a function to find the MAC address for an IP that is unknown or
currently not in the table. It is not performed on a per packet or per
Routers and switches are quite different when it comes to encapsulation
as routers live one level higher so must perform additional steps.
A router when receieving a packet is acutally a frame destined to it's
MAC address. It will discard the layer 2 infomation (only if it is the
recipient) and then pass the packet onto route engine (for lack of a
better phrase). From there it will look up the destination in it's FIB
(forwarding information base also known as the route table) and find out
the next hop. It will find the attached interface for the next hop
encapsulate the packet with a layer 2 with the destination MAC address
of the next hop. If the next hop's MAC address table is not in the ARP
table it will perform the ARP at this time.
If the packet is destined for a connected interface instead of a next
hop router. The router will create the frame with the destination MAC
address of the final destination (the dest ip in the packet). Again if
this MAC isn't in the ARP table the router will send an ARP request and
figure it out.
Of course the above assumes everthing is ethernet. There are some
differences when it comes to other layer 2 protocols.
As far as switches, they will not strip layer 2 information as they
work on this layer (quite the same as a router doesn't strip the layer 3
information). So a swith will use it's CAM table to find where to send
the frame. If the destination is unknown the switch will flood the
frame out all ports looking for the reply to add to it's CAM table.
There is one more layer of encapsulation that both switches and router
perform and that is Layer 1... or turning the frame into bits. This is
hardly mentioned as it is just that easy. When the bits get to the
other side they are turned back into a frame.
There are ofcourse even some more advanced topics for both switches and
routers, but I hope this answers your question.
<bennetb at gmail dot com>
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