s vermill wrote: > > Anyone smart on interworkings of Cisco routers care to clarify > something for me? I was in a discussion with someone in > another forum. It was being discussed how pings from a local > ethernet interface to a local serial interface on the same > router actually cross the WAN to which that serial interface is > attached and are returned by the distand-end router.
This doesn't sound believable to me. It sounds like the discussion mangled some slightly related topics and applied them to this situation. ;-) Or that someone misinterpreted the output from "debug ip icmp." Or perhaps someone jumped to this conclusion due to the fact that you have to map your serial interface IP address to a DLCI to ping it in a Frame Relay environment. To forward a frame a router looks into its routing information base, (whether that be the routing table, fast cache, or whatever), and determines how to forward the frame. In this case, it would see that the destination network is directly connected and from there it would presmumably check its running config and see that the destination node is directly connected also. So there's no need to send the frame out an interface. Anyway, I tested it. I tried various serial encaps, including HDLC, Frame Relay, and PPP. I was consoled into two routers at once, Router A and Router B, to make it easy to see the output from "debug ip icmp" on both routers. The router serial interfaces are connected back-to-back. When I pinged from Router A's e0 to Router A's s0, Router B did not see the packets. I don't believe they crossed the serial link. To be absolutely sure I would want to use a serial protocol analyzer, but alas, those are too expensive for the self-employed. But I'm 99% convinced by the testing that I did. Priscilla > I know > this has been discussed in the past but I didn't find anything > in the archives that exactly answers my question. > > It kinda makes sense that a local serial interface will > encapsulate an echo packet that it receives and put it on the > wire (it only knows how to encapsulate in one direction and > de-encapsulate in the other). It makes sense that the > distant-end router will return it, based on the destination IP > in the packet. What I'm a little fuzzy on is why CCO says that > the echo reply must also be sent accross the WAN and be > returned by the distant end router. If it were the interface > itself that had to generate the echo reply, I guess the same > logic as before would apply. But does the router or the > interface actually generate the reply? If it isn't the > interface itself, it seems the router would simply generate a > reply back towards the source IP (the ethernet interface - > which is essentially itself). Is it required that an ICMP > packet actually be given life on a hardware interface and thus > the one-directional encapsulation issue comes back into play > again? > > Or am I just really confused about it all? > > Thanks all, > > Scott > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=53153&t=53148 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
