Serial Link Encapsulation

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Jul 20th, 2007
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Textbooks like to go through the steps of what happens to a packet as it goes from client to server through a single router. This always makes sense...The client application payload gets handed down the TCP/IP stack to layer 3, layer 2, etc., then back up at the other end.

In the scenario, the client sends the packet to the default gateway which terminates on the router's Ethernet interface. The router sees the MAC address as its own, so it inspects the destination IP address and looks it up in the routing table to find out the outgoing router interface to send the packet. Here the scenario always shows what happens when the outgoing is an Ethernet ARPs the MAC address of the destination server because Ethernet NEEDS to know the destination MAC in order to build the frame. This makes perfect sense.

What happens though, if it is to go to an outbound serial interface? Whereas Ethernet requires a HARDWARE MAC address to send the frame... what does a SERIAL output buffer look for as a "hardware destination address" in order to package the layer 2 frame and send it on its way? I'm thinking we could be sending data via HDLC, PPP, Frame Relay(point-to-point and multipoint). For HDLC, PPP will it always just be a broadcast address? For Frame, will it use local DLCI?

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mohammedmahmoud Fri, 07/20/2007 - 08:10
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The adjacency table contains the layer 2 information you are talking about. The adjacency table is populated with data from the ARP table, the Frame Relay map table, and other tables of this type. The show adjacency command displays the adjacency table. (Its more advanced than the ARP table as it contains the entire layer2 header instead of having the destination layer 2 address and the outgoing interface only like the ARP table)

In simple words, Adjacency table contains a prebuilt Frame header for the next hop toward the destination, according to the routing decision the appropriate adjacency entry is used.

Pleas try to issue show adjacency on a router having different layer 2 technologies to see this.

I hope that i've been informative.


Mohammed Mahmoud.


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