At 10:50 PM 1/14/02, Cisco Nuts wrote: >Sorry Priscilla but I am trying to understand, IP does not need a mac >address to get to the next hop( on a point-to-point netw - PPP or HDLC)!! >Why? Because it is a point-to-point as you say? But I thought ip was layer >three.
It is Layer 3, but in most cases to send an IP packet requires a Layer 2 address also. But not if there's only one possible recipient, the other end of the point-to-point link. I'm not sure I understand your confusion, but maybe this will help. Compare IP on a LAN to IP on a WAN. For example, assume you have configured IP addresses on some Ethernet stations and also on some router serial interfaces and you're doing some pinging. A LAN device sends an ARP packet to find the MAC address that maps to the destination IP address. On a point-to-point WAN, a device doesn't do this. It assumes there's only one place the packet can go -- to the other end. Also compare this to Frame Relay and ATM. In this case, a device learns in advance through Inverse ARP which data-link identifier to use when sending to an IP address. Maybe the point is just too obvious or you haven't thought about the fact that a point-to-point link is a special case.... Please send questions to the group. I like to answer to the group so that everyone benefits from the answer and any discussion that follows. Priscilla >I understand in terms of ATM or FR but ip on a point-to-point? >Am I missing something here? >Sorry, can you help? > > >>From: "Priscilla Oppenheimer" >>Reply-To: "Priscilla Oppenheimer" >>To: [EMAIL PROTECTED] >>Subject: Re: Mac address for Serial Ports?? [7:31898] >>Date: Mon, 14 Jan 2002 15:37:15 -0500 >> >>Serial interfaces don't have MAC addresses. IPX is a special case because >>the network-layer node address IS the MAC address. >> >>Think about frame formats and identifications for WAN protocols. What >>identifies the sender in Frame Relay? A DLCI. How about ATM? A VPI/VCI >>pair. How about PPP? No need because it's point-to-point. How about HDLC. >>No need because it's point-to-point (in Cisco HDLC anyway). >> >>How does a network layer address get mapped to a data-link identifier? >>Static mapping or Inverse ARP for some protocols. PPP has the NCP which >>sits between the two layers. >> >>Priscilla >> >>At 02:51 PM 1/14/02, Cisco Nuts wrote: >> >Hello, >> >Is there a way of verifying that a serial port in borrowing the mac address >> >of the Ethernet/Tr port when it is connecting to another router in an IP >> >network? >> >I see this in an ipx network but not in an ip network. sh ipx int s0 >> > >> >I tried the debug ip packet, detail, debug arp, debug broadcast etc. but I >> >am not seeing that the serial port is using a mac address. >> > >> >What test can I do on my router to check that the serial port does borrow >> >the first available Mac address of a Ethernet port on a router? >> > >> >Thank you. >> > >> >_________________________________________________________________ >> >Join the worlds largest e-mail service with MSN Hotmail. >> >http://www.hotmail.com >>________________________ >> >>Priscilla Oppenheimer >>http://www.priscilla.com >_________________________________________________________________ >Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp. > ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=31919&t=31898 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
