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Router forward Packet or frame

Can anybody tell me router forward packet or frame. when we connect two routers using leased line and we used PPP or HDLC protocol.It means HDLC and PPP are DLL protocol. when DLL comes means there is frame.Am I right. waiting for experts reply...

2 ACCEPTED SOLUTIONS

Accepted Solutions
Super Bronze

Re: Router forward Packet or frame

At DLL, serial might be assumed point-to-point while Ethernet can be assumed multipoint. With p-2-p, there's no need to resolve physical destination, but with multipoint there is. (NB: ARP isn't limited to just Ethernet, see http://en.wikipedia.org/wiki/Address_Resolution_Protocol)

Super Bronze

Re: Router forward Packet or frame

Normally no. IP hosts maintain ARP caches. Router will ARP once for destination, and use cached results until ARP cache times out. On Cisco routers, "show ip arp" (I think that's the command) will show the ARP cache.

14 REPLIES
Hall of Fame Super Gold

Re: Router forward Packet or frame

What DLL means ?

Usually, frame is a L2 term, packet a L3 term. Often the two terms are used indifferently.

Hall of Fame Super Silver

Re: Router forward Packet or frame

Hello Irfan,

using the layered OSI model a router works at layer3 but when it sends out a L3 PDU like an ipv4 packet it has to encapsulate it inside the correct L2 frame suitable for the outgoing interface:

so if the outgoing interface is a serial interface using PPP the ipv4 packet is inserted inside a PPP frame that has a protocol type in the header that tells it is carrying an ipv4 packet inside.

the frame is prepared and it is sent out as a sequence of 0 and 1 on the wire (OSI layer 1 line code)

if the encapsulation is Cisco HLDC the ipv4 frame is inserted into an HDLC frame and so on.

Hope to help

Giuseppe

New Member

Re: Router forward Packet or frame

In short you means to say it sends frame...for frame it requires mac.. ok will it send ARP request for each packet or once for each communication.And if we are using point to point link to connect routers it is necessary to send ARP.

waiting for reply...

Hall of Fame Super Gold

Re: Router forward Packet or frame

Do not jump to conclusions.

Frame does not mean necessarily MAC.

Serial protocols like HDLC, PPP, have no MAC address as you know it, in fact have a 4 bits address always set to broadcast.

There is no ARP on serial links.

New Member

Re: Router forward Packet or frame

And if we are using interface other than serail then what it does?

Hall of Fame Super Gold

Re: Router forward Packet or frame

It depends by the specific of the interface.

Super Bronze

Re: Router forward Packet or frame

Router forwards L3 packets (OSI network layer). Router encapsulates the L3 packets into L2 frames (OSI data link layer).

New Member

Re: Router forward Packet or frame

Will it send ARP on fast ethernet?

Super Bronze

Re: Router forward Packet or frame

"Will it send ARP on fast ethernet"

If you mean the router and IP, yes.

New Member

Re: Router forward Packet or frame

it will not send arp on serail and send arp on fast ethernet WHhy???

Super Bronze

Re: Router forward Packet or frame

At DLL, serial might be assumed point-to-point while Ethernet can be assumed multipoint. With p-2-p, there's no need to resolve physical destination, but with multipoint there is. (NB: ARP isn't limited to just Ethernet, see http://en.wikipedia.org/wiki/Address_Resolution_Protocol)

New Member

Re: Router forward Packet or frame

Thanks josephdoherty

New Member

Re: Router forward Packet or frame

josephdoherty,

For fast ethernet...

suppose PC A is communicating(copying or transfering a file) with PC B vai Router A and router B... will router A send ARP for each packet??

Super Bronze

Re: Router forward Packet or frame

Normally no. IP hosts maintain ARP caches. Router will ARP once for destination, and use cached results until ARP cache times out. On Cisco routers, "show ip arp" (I think that's the command) will show the ARP cache.

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