routing a packet

Unanswered Question
Feb 26th, 2008

Hi all, when routing a packet, is the layer 2 info always needed ? ie the next hop mac address ?

I have this problem too.
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Hi Carl

In my opinion it depends on what you mean when you say "When routing a packet".

If you mean for making the routing decision of where to send the packet, then it is my understanding that the L2 MAC address is not required. Only the L3 IP address and subnetmask is required for this.

If you mean actually transporting the packet from A to B, then yes, A would require B's MAC address to be able to send the packet to B.

Best Regards,


Joseph W. Doherty Tue, 02/26/2008 - 05:13

Possibly not, if you're routing across a media that can not be multipoint.

If your reference is to Ethernet, the next physical MAC address will be necessary. It doesn't have to a "next hop", could be the final destination.

carl_townshend Tue, 02/26/2008 - 09:23

do you mean for example if we have an x21 serial connectiion using hdlc, it will not use the layer 2 address, only layer 3, so this only applies to ethernet ?

wouldnt the router maintain an arp table of the directly connected device though, for example a router ?

Joseph W. Doherty Tue, 02/26/2008 - 09:50

HDLC framing contains an address field, and I believe it can support multi-point. Using HDLC encapsulation on a dedicated point-to-point, don't know whether HDLC address field is actually used. In theory, no need for it.

Not positive, but suspect you'll not see anything in the usual ARP table but LAN interface (e.g Ethernet) MAC addresses, altough the protocol isn't limited to just LAN.


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