Routing and MAC addresses

[visitor68]

When a packet that is being routed moves from one router's serial interface to another router's serial interface, what MAC addresses are used for the source and destination?

Thanks

[visitor68]

Let me rephrase that, which source and destination L2 addresses are used if that serial link is a point to point connection (like HDLC encap)?

Of course MAC addresses would be used if the L2 technology of that router to router connection was ethernet, so my question was poorly worded.

I assume that if the router to router connection were a frame relay link, the L2 source and destination addresses of the IP datagram would be the DLCIs, correct?

[Istvan_Rabai]

Hi Joe,

As far as I remember there is some number in the HDLC frame as a layer2 address, but it has no significance on point-to-point links.

When you send a frame on a point-to-point link, only the device on the other end of the link can receive it.

The layer2 addresses, like mac-address and DLCI are significant only on multiaccess networks.

Cheers:

Istvan

[visitor68]

Hi, Istvan:

"The layer2 addresses, like mac-address and DLCI are significant only on multiaccess networks."

True, but nonetheless, the data-link layer includes its own information that it appends to an IP datagram. More specifically, the IP datagram is encapsulated by the data-link layer/L2 frame, which has its own header. That header includes source and destination L2 address information for that layer. Something has to be placed in those fields to satisfy the requirement.

Also, frame relay is indeed a multiaccess network, which makes the L2 address scheme all the more significant. I do believe that with frame relay, the source and destination address fields for the header of that Frame Relay frame are populated with the local and remote DLCIs for that connection.

But I am still at a loss when it comes to HDLC. What source and destination addresses populate the corresponding fields when HDLC encapsulation is used?

[Istvan_Rabai]

Hi Joe,

I have now made a packet capture specifically for you:

The HDLC header is 4 bytes in length.

The 1st byte is the layer2 destination address: 0x0F (for all packets and for both directions).

The 2nd byte is 0x00.

The 3rd and 4th bytes contain the protocol (next header information), in this case IP: 0x0800.

I hope this helps you.

Cheers:

Istvan

[visitor68]

Thanks, Istvan. That was very kind of you.

[Edison Ortiz]

In the context of Cisco HDLC, it uses SLARP (Serial Line ARP) between the end-points and no Ethernet Frame information (MAC Address) is send on the packet. It's pure IP src-dst. Attached you will find the differences between a ping done in a LAN segment vs a ping done in a cHDLC connection (download WireShark if you want to view its detail).

Additional information on cHDLC can be found on this condensed document:

http://en.wikipedia.org/wiki/Cisco_HDLC

HTH,

__

Edison.

[visitor68]

Hi, Edison:

Thanks for your input. I do believe that you may be a bit confused regarding my question. SLARP is a subset of cHDLC that allows a router to configure its own IP address for an interface configured with HDLC encapsulation. Within the lexicon of SLARP, the word "address" refers to a 32-bit L3/IP address.

My question involved the L2 address(es) that a router inserts in the address fields of a L2 frame's header - with a particular concern with an HDLC header. As I can see from Istvan's sniffer output, as well as the link you sent me, the L2 address field will be 0x0F for Unicast and 0x8F for Broadcast HDLC packets.

The reason I even began thinking of this was that I was refreshing my memory on how a packet traverses a routed IP network and the manner in which the packet is handled hop-by-hop by the routers. I knew that the L3 source and destination addresses will not change (barring NAT or some other special circumstance), but the L2 addresses will as they traverse the network hop-by-hop -- datalink hopping, if you will. It then dawned on me that serial interfaces do not have MAC addresses - that MAC addresses are associated with Ethernet.

I needed to know how the router resolves the IP address next-hop in its routing table with a L2 next hop for it to use in its L2 header as it forwards the packet to the next hop router. For Ethernet interfaces, the IP addresses resolves to a MAC address, but what about serial interfaces, I thought.

Hence my question...

Am I making sense?

Thanks

[Edison Ortiz]

Am I making sense?

Not really.

On LAN segments, the MAC is needed in order to know where to send the packets to. It provides a unique identifier for each device in a LAN segment.

On a Point-to-Point WAN segment, there is only ONE device at the remote send. Why the packet overhead?

__

Edison.

[visitor68]

Edison:

"Why the packet overhead?"

I don't create the protocols - I only try to understand how they work. :-)

HDLC is a point-to-point WAN technology that does utilize an L2 address field, as the very link you gave me points out. The address field is not the address of the sending or receiving node, but is used more as an identifier of the type of packet being sent -- unicast or broadcast. Nonetheless, the field exists and it must be populated. With what? That was my question.

Also, Frame Relay, which has its roots in LAPD, and can be configured as a point-to-point connection, also utilizes an L2 address field in the header. The Frame Relay header will include an address field in which the DLCI is placed, regardless of the fact that the router may be directly connected to another router. The reason, I imagine, is that the underlying assumption of FR is that it is an NBMA technology that can be implemented as a point-to-point.

In short, all data link layer technologies utilize an address scheme. And that address scheme is used to forward IP packets from hop to hop over a routed network.

[Edison Ortiz]

HDLC is a point-to-point WAN technology that does utilize an L2 address field, as the very link you gave me points out.

And Layer2 address field does not make use of MAC address hence my comment on the packet overhead. The MAC address information isn't needed. It's a point-to-point link.

Also, Frame Relay, which has its roots in LAPD, and can be configured as a point-to-point connection, also utilizes an L2 address field in the header. The Frame Relay header will include an address field in which the DLCI is placed, regardless of the fact that the router may be directly connected to another router. The reason, I imagine, is that the underlying assumption of FR is that it is an NBMA technology that can be implemented as a point-to-point.

But you will notice, when implementing point-to-point subinterfaces in Frame-Relay, you don't need to map an IP to a DLCI. You do need to map an IP to a DLCI on multi-point frame-relay either via reverse ARP or static.

HTH,

__

Edison.

[visitor68]

Edison:

And Layer2 address field does not make use of MAC address hence my comment on the packet overhead. The MAC address information isn't needed. It's a point-to-point link.

I never said it did. In fact, I specifically said that MAC addresses are associated with Ethernet, not point-to-point serial HDLC links. My question involved the address field of the HDLC header. HDLC uses an address field and I wanted to know what gets inserted in that field.

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&topicID=.ee71a06&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.2cc2e4ce/0#selected_message

Quick question: Can you define what you mean when you say "point-to-point"? Is your definition topology based or address based?

[Edison Ortiz]

HDLC uses an address field and I wanted to know what gets inserted in that field.

Did you have a moment to check my packet capture? It does show what gets inserted on that field..

Quick question: Can you define what you mean when you say "point-to-point"?

In the context of Frame-Relay? That's one kind of subinterface.

In the context of HDLC, it's based on Layer1/2 connectivity between to end-points.

__

Edison.

[visitor68]

Edison:

I did get the answer to my original question several posts ago -- between the website you sent and Istvans trace.

How about in the context of Ethernet? If I have 2 routers connected to each other through a routed Ethernet interface, would you, according to your definition of point-to-point, consider that link -- that connection -- a point-to-point link?

[Edison Ortiz]

How about in the context of Ethernet? If I have 2 routers connected to each other through a routed Ethernet interface, would you, according to your definition of point-to-point, consider that link -- that connection -- a point-to-point link?

Yes

No other device can connect on that segment, right?