IP routing

'IP routing uses frames to transport packets between routers and to the destination host.'

Is this statement correct? I thought IP routing between routers used only packets.

Does this mean that a Frame would have the source MAC address of the exit router interface and a destination MAC address of the receiving routers interface?

Are MAC addresses used between routers?

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Comments

Heero

Assuming the connection between the routers is ethernet, yes, that is correct. A frame is generally just the layer 2 unit of data that is actually put on the wire. The packet is encapsulated in a different frame for transport on each "hop."

Bolton07

Do Frames get sent over Serial Links as well?

Heero

yep. they would just would not use ethernet for the layer 2 frame, and instead a layer 2 protocol for serial links, like PPP or HDLC. It is still a frame though.

Bolton07

How does a Frame change at every hop on its way to its destination?

Bolton07

Does that mean MAC addersses aren't used between routers over serial links?

Heero

MAC addresses are not used by routers over serial links. When using HDLC or PPP, it is a point to point link. Only 2 devices on the link ever. It is not multi-access like ethernet. Because of this, no physical addressing is even needed in this situation, and the router just encapsulates the packet into a PPP or HDLC frame and puts it on the link.

Serial interfaces can be used for things like frame-relay though, where it can be multi-access. With frame relay, you use DLCIs for physical addressing. They function a bit differently than MAC addresses though.

Every layer 2 protocol has its own frame format. MAC addresses are only part of the ethernet layer 2 protocol.

chmorin

Bolton07 wrote: »

Does that mean MAC addersses aren't used between routers over serial links?

It depends on the encapsulation. HDLC uses a destination address, but it is not a MAC address. I never learned, or don't remember, the nitty-gritty part of it. I know PPP's address field in the frame is disregarded since it is a point to point link.

Bolton07

Hope you don't mind all the questions.

What do these two statements simply mean (This is for CCENT)?

1 'IP routing uses frames to transport packets between routers and to the destination host.'

2 'The frame changes at each hop but that the packet is never changed or manipulated in any way until it reaches the destination device'

chmorin

Bolton07 wrote: »

Hope you don't mind all the questions.

What do these two statements simply mean (This is for CCENT)?

1 'IP routing uses frames to transport packets between routers and to the destination host.'

2 'The frame changes at each hop but that the packet is never changed or manipulated in any way until it reaches the destination device'

Look at it this way: A packet is a sheet of paper that has someone's name on it. A Frame is an envelope with an address. John needs to send joe a message. So say the paper has the name 'joe' on it and it needs to get to him. Well first John needs to see if joe is in the house. When he is not, then it will have to be mailed. It gets put into the envelope (the frame) and then hands it to Rick to get sent to Joe's house. When it gets to the house someone named steve opens the envelope (the frame) and sees that it is for joe. Steve then hands the paper to joe.

So it goes:

John -> Rick -> Mail -> Steve -> Joe
Host -> Router -> Media -> router -> Host

Packet -> Packet -> Frame and Packet -> packet -> packet
Paper -> Paper -> Envelope -> Paper -> Paper

So you start with a packet, and when it gets transmitted via layer 3 protocols it uses a layer 3 frame, which can be seen as an envelope with a Senders address and a Destination address.

So...

1) Each router puts packets into frames to send them over the link.
2) Each router can take apart the frame (envelope) and change its destination to the next hop, all without changing the packet (paper).

Bolton07

I thought the packet had to be put in a Frame by the original host for it to get to the local Router interface to be routed to the remote network.

chmorin

Bolton07 wrote: »

I thought the packet had to be put in a Frame by the original host for it to get to the local Router interface to be routed to the remote network.

Son-of-a... (DOE!)

You're right, but the example I gave does not exactly put that into perspective. That was trying to explain the path a packet takes between a serial link. Anytime a packet goes over a layer 2 or above link it will be encapsulated into a frame with the MAC address information (in one way shape or form).

It is a little hard to explain everything since there are many different means for communication to take place. For the most part it is safe to assume that the host creates the packet that has the destination IP and the source IP in it. That is your layer 3 information that a router will read to figure out which hops to send it through. The packet gets put into a frame with the destination MAC and source MAC, which is for those single hop transfers.

Different encapsulations like serial links change the rules again, but that is pretty much the truth for Ethernet.

PS:

You're trying to learn, don't apologize for asking questions