strange ARP problem

ray at redshift.com ray at redshift.com
Sat Mar 18 05:48:50 UTC 2006


At 08:59 PM 3/17/2006 -0800, Glenn Dawson wrote:
| At 08:34 PM 3/17/2006, ray at redshift.com wrote:
| >I'm having a strange issue here and thought maybe someone on this list might
| >have some ideas.  I have tried to figure it out for a couple of days, but no
| >luck yet.  The problem seems to be around reporting of arp information.
| >
| >Here is my basic config.  I have my workstation (a windows XP box) with 2 IP's
| >on a private network segment (both with /24 subnet masks)
| >
| >192.168.10.250
| >192.168.20.250
| >
| >the 10.250 and 20.250 are connected out to a small switch. Also connected to
| >that small switch is a mail server as shown below.
| >
| >[ workstation  ]                         [ mail server  ]
| >[192.168.10.250]-------[ small  ]--------[ 192.168.10.15]
| >[192.168.20.250]-------[ switch ]--------[ 192.168.20.15]
| >                            |
| >                            |
| >                   [router 192.168.10.1]
| >                            |
| >                        public IP
| >
| >10.15 handles SMTP to the public, 20.15 is for admin and POP to/from the
| >workstation on 20.250
| >
| >Okay, so the problem is that when I fire up the Workstation (it's running
| >Windows XP), the arp data for 192.168.20.15 comes back with the incorrect Mac
| >address.  It ends up with the Mac address for 10.15, instead of 20.15 - which
| >keeps the machines from talking correctly.  If you delete the ARP table and
| >re-arp, then it's perfectly fine from then on.  Totally odd.
| >
| >Then the other night I noticed the following errors (see below) from the mail
| >server.  It seems to be related, but I can't pin point the source or 
| >what might
| >cause something like this.
| >
| >Does anyone have any ideas what could be causing this?
| >
| > > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
| > > arp: 192.168.20.250 is on bge0 but got reply from 00:e0:81:32:e0:a0 on fxp0
| > > arp: 00:30:48:51:ce:f0 is using my IP address 192.168.20.15!
| > > arp: 00:30:48:51:ce:f0 is using my IP address 192.168.20.15!
| > > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
| > > arp: 00:30:48:51:ce:f0 is using my IP address 192.168.20.15!
| > > arp: 192.168.10.15 is on lo0 but got reply from 00:30:48:51:ce:f0 on bge0
| > > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
| > > arp: 192.168.10.15 is on lo0 but got reply from 00:30:48:51:ce:f0 on bge0
| > > arp: 192.168.20.250 is on bge0 but got reply from 00:e0:81:32:e0:a0 on fxp0
| > > arp: 192.168.10.15 is on lo0 but got reply from 00:30:48:51:ce:f0 on bge0
| > > arp: 192.168.10.1 is on fxp0 but got reply from 00:30:48:52:08:03 on bge0
| >
| >here is the ifconfig from the mail server:
| >
| >[ray at mail ray]$ ifconfig
| >fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
| >         inet 192.168.10.15 netmask 0xffffff00 broadcast 192.168.10.255
| >         ether 00:30:48:51:ce:f0
| >         media: Ethernet autoselect (100baseTX <full-duplex>)
| >         status: active
| >bge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
| >         options=1b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING>
| >         inet 192.168.20.15 netmask 0xffffff00 broadcast 192.168.20.255
| >         ether 00:30:48:51:ce:f1
| >         media: Ethernet autoselect (100baseTX <full-duplex>)
| >         status: active
| >lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
| >         inet 127.0.0.1 netmask 0xff000000
| >
| >If anyone has any idea, please let me know.  Thanks!
| 
| This is exactly why it's ill-advised to have two network interfaces 
| on different networks connected to the same physical network.
| 
| If you actually need two different networks (although from your 
| description I don't see a reason why you would) then use a single 
| physical interface and assign it an IP from each network.  Or, get a 
| switch that has VLAN capabilities and keep the two networks separated.
| 
| -Glenn

The switch should act as a bridge - so from the standpoint of the computers,
shouldn't it effectively be two different networks?  In other words, if I ran
20.x across a different switch, would that matter?  I've always understood that
a switch was something you viewed as basically a bunch of individual little
bridges.

I can drag out another switch and test that idea and see if the problem goes
away.  This problem did start when I ran everything through 1 switch - I just
didn't think that would any specific negative impact.

Thanks for the note back Glenn :-)

Ray
 



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