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. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=47238&t=47177 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
