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]

OK, fair enough.

I'm asking you this because you were making the point/argument earlier that a data link layer/L2 address is not necessary for a point-to-point link, as if the insertion of an L2 address in an L2 header field depends on, from a topological perspective, whether the link is a point-to-point connection or not.

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

In truth, though, it's the technological requirements of the encapsulation protocol that necessitates the placement of that L2/data link layer address, not the manner in which the neighboring interfaces are connected.

Here is the output of a router whose ethernet interface is connected to another router's ethernet interface. It is, by your definition, a point-to-point connection. Nonetheless, it is an ETHERNET connection, and therefore the MAC address of the next hop's ethernet interface will be placed in the address field of the ethernet frame as it exits one router and heads to the other.

fhvrs001#sh cdp nei gi1/2 det

-------------------------

Device ID: fhvrs002

Entry address(es):

IP address: 19.210.146.234

Platform: cisco WS-C6503-E, Capabilities: Router Switch IGMP

Interface: GigabitEthernet1/2, Port ID (outgoing port): GigabitEthernet1/2

Holdtime : 130 sec

Version :

Cisco Internetwork Operating System Software

IOS (tm) s72033_rp Software (s72033_rp-ADVIPSERVICESK9_WAN-M), Version 12.2(18)SXF5, RELEASE SOFTWARE (fc3)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by cisco Systems, Inc.

Compiled Sat 08-Jul-06 02:32 by kellythw

advertisement version: 2

VTP Management Domain: 'fhvrs02'

Duplex: full

fhvrs001#sh ip arp | in 19.210.146.234

Internet 19.210.146.234 122 0021.d8cc.f440 ARPA GigabitEthernet1/2

fhvrs001#

As the ARP table shows, an L2 to L3 address mapping exists for the next hop router and that MAC address will be placed in the address field of the L2 header, EVEN THOUGH it is a point-to-point link.

[Richard Burts]

Joe

To the point of your original question, I do not know of a layer 2 protocol that does not have some concept of address as part of its header. For some protocols like HDLC the address field is not very useful. But it is still there. For other layer 2 protocols the address is there and is useful.

I would like to comment in particular about some of the discussion of Frame Relay. In terms of the encapsulation of Frame Relay it does not matter whether it is a point to point subinterface or a multipoint interface, every Frame Relay frame going out has the DLCI field, which is its layer 2 addressing. And the DLCI is important to the Frame Relay switch to make its forwarding decision, whether it is part of a point to point connection or is part of a multipoint connection.

[edit] In looking back through this thread I notice this statement in one of your posts:

"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"

I would say that for every entry in the routing table (and in the CEF tables which are used to make forwarding of packets more efficient) for every layer 3 next hop there is a corresponding layer 2 next hop. And that layer 2 next hop has some type of layer 2 address. In some, especially thinking of Ethernet, the layer 2 address is very obvious about what its useage is. For others, such as HDLC, it may be less obvious. But each layer 2 protocol has some concept of an address field related to the next hop.

HTH

Rick

[visitor68]

Hi, Rick:

Thanks for your information.

I agree with everything you have stated.

"To the point of your original question, I do not know of a layer 2 protocol that does not have some concept of address as part of its header. For some protocols like HDLC the address field is not very useful. But it is still there. For other layer 2 protocols the address is there and is useful."

Agree and that was exactly the point I was making.

"I would like to comment in particular about some of the discussion of Frame Relay. In terms of the encapsulation of Frame Relay it does not matter whether it is a point to point subinterface or a multipoint interface, every Frame Relay frame going out has the DLCI field, which is its layer 2 addressing. And the DLCI is important to the Frame Relay switch to make its forwarding decision, whether it is part of a point to point connection or is part of a multipoint connection."

Well stated and I agree. I focused on point-to-point FR because I wanted to address Edison's comments regarding such connections.

[aa321123]

Hi ex-engineer,

Few days ago, the router found its WAN2 . pub on an unknown.Mac ,  none of router mine.

Does this say :

A point to point  device has captured my Router ? Since nothing done NAT or p-p configured at my end.

The router-net speed down to 1/3 ISP plan with router whenever but , near 94% on modern most time though.

How to take over this ? Firmware updated twice covered the beta version .

Li