At 02:22 PM 6/23/02, LongTrip wrote: >hmmm maybe there was a misunderstanding on my part of an earlier post that >mentioned "The only time you see the virtual MAC address is on the original >request from the host. Forwarded requests and replies don't use it. ".
Each request from the host uses the virtual MAC address in the destination. In my experiments, I was only doing a single ping. There was just one request. >I understood this to mean that after the initial set up of communications >that the virtual mac address was not used in subsequent data transmissions. You jumped to the wrong conclusion. Theoretically, the client doesn't even know any other address. How could it use it? Also, how could redundancy work if it used an actual address for an interface that might go down?????? In actuality, the client could know other addresses, by the way, since the ping (or whatever) replies that the router forwards come from the router's real MAC address. But the PC ignores this. Some operating systems could use it though. UNIX used to just reverse the MAC addresses on the next packet. (Long story, not relevant). Also, you might find it interesting (and confusing) to know that the ARP reply from the active HSRP router actually does come from the real address. But the ARP data in the reply supplies the virtual MAC address. Here is the ARP reply from the active HSRP router after the client ARPed for the virtual IP address of the gateway, which was 10.10.0.3. Notice that the source Ethernet address and the Sender's Hardware address in the ARP data don't match? Cool, eh? Ethernet Header Destination: 00:00:0E:D5:C7:E7 Source: 00:00:0C:05:3E:80 Protocol Type: 0x0806 IP ARP ARP - Address Resolution Protocol Hardware: 1 Ethernet (10Mb) Protocol: 0x0800 IP Hardware Address Length:6 Protocol Address Length:4 Operation: 2 ARP Response Sender Hardware Address:00:00:0C:07:AC:00 Sender Internet Address:10.10.0.3 Target Hardware Address:00:00:0E:D5:C7:E7 Target Internet Address:10.10.0.10 >This will be one for a lab experiment on my part. Until I see it the result >with my own eyes it will be a question. Why is it a question? I did a bunch of research for you. Why don't you read what I have written and what others wrote? (Although doing your own research is a good idea too.) Priscilla >Kim > > > > > > > From: "Thomas E. Lawrence" > > Date: 2002/06/23 Sun PM 01:08:17 EDT > > To: [EMAIL PROTECTED] > > Subject: Re: Re: HSRP [7:47177] > > > > Perhaps this will help explain > > > > >http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_c > > /ipcprt1/1cdip.htm#xtocid23 > > > > Yes, HSRP creates a single "virtual" IP and MAC pair. Yes, when one router > > fails, the standby router "assumes" control of this virtual IP and MAC >pair. > > > > From an end station standpoint, nothing has changed. The end station knows > > the virtual IP, as configured in it's own settings, or as received as part > > of its DHCP configuration. In either case, no end station knows all of the > > IP's of all of the members of the HSRP group. Unless things have changed > > recently, there is no way to configure multiple default gateways on a > > Windows machine, at least. This is the reason HSRP, and now VRRP, were > > developed. If the end station does not already know the MAC of the default > > gateway, it sends an ARP request, as is standard operating procedure for >any > > host seeking the MAC of an IP. The active router replies with the virtual > > MAC. > > > > You may also want to refer to the VRRP RFC. VRRP is the open standard > > intended to replace the several proprietary methods that now exist. The > > first couple of pages provide a good explanation and a good background of > > the problem to be solved. > > > > ftp://ftp.isi.edu/in-notes/rfc2338.txt > > > > Tom > > > > > > > > ""LongTrip"" wrote in message > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > So you are saying the client never sees the MAC address of RouterA? It > > only > > > sees the MAC address of the "Virtual Router"? > > > > > > Kim > > > > > > > > > > > From: "Michael L. Williams" > > > > Date: 2002/06/23 Sun AM 11:29:24 EDT > > > > To: [EMAIL PROTECTED] > > > > Subject: Re: HSRP [7:47177] > > > > > > > > This isn't quite right. See comments below. > > > > > > > > "Kim Graham" wrote in message > > > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > > > This brings up a question. I understand that after the initial "hi I > > > will > > > > > be handling your requests please use me as your destination mac > > address". > > > > > (Router talking to client). > > > > > > > > > > But what happens when the initial router fails and HSRP kicks in? > > After > > > an > > > > > unreachable, would ClientA send out an arp or would RouterB initiate > > the > > > > > arping to re-establish connections to any client that was using > > RouterA > > > > > after it noticed that RouterA was not responding? > > > > > > > > > > Scenario: > > > > > > > > > > > > > > > ClientA ----- RouterA/B(HSRP) ------ ClientB > > > > > > > > > > ClientA sends a packet to ClientB > > > > > ClientA talks to the Virtual RouterA/B -- RouterA/B sends to ClientB > > > > > RouterA/B tells ClientA -- RouterA will be handling your requests. > > > > > > > > Router A never tells Client A that "Router A will be handling your > > > > requests". As you mentioned, Client A talks to the Virtual Router via > > the > > > > Virtual IP address which it ARPs to find the Virtual MAC. Client A > > never > > > > knows which of the HSRP routers is "intercepting" and processing it's > > > > requests.... When Client A sends a frame to the Virtual MAC to go out > > of > > > > it's gateway, both Router A and Router B "hear" the packet, but only >the > > > > HSRP Active router will process it. So if, the janitor steps in and > > > unplugs > > > > Router A, then after Router B misses enough Hello packets from Router >A, > > it > > > > declares itself the Active HSRP router for that HSRP group, and at that > > > > point it starts to process the information sent to the Virtual > > IP/Virtual > > > > MAC. This is all transparent to the end clients, Client A in this > > example. > > > > So as far as Client A knows, it's still sending traffic to the Virtual > > IP > > > > via the Virtual MAC address it has in its ARP cache..... > > > > > > > > HTH, > > > > Mike W. ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=47262&t=47177 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
