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Community Member

Can someone help me to understand ?

Hello,

I have two physical network (10.100.0.0/16) and (11.0.0.0/8). A Cisco router route the traffic between these networks.

I have a problem on one switch (catalyst 2950) located on the 11.0.0.0/8 network

From this switch, I can ping the machine 10.100.1.50 and 10.100.1.52. Arp table shows :

Internet 10.100.1.52 0 1111.fde7.ac01 ARPA Vlan1

Internet 10.100.1.50 2 1111.fde7.ac01 ARPA Vlan1

1111.fde7.ac01 is the mac address of the router interface.

If I unplug the router and plug it back, arp tables shows :

Internet 10.100.1.52 4 1111.fde7.ac01 ARPA Vlan1

Internet 10.100.1.50 3 2222.0fff.939b ARPA Vlan1

2222.0fff.939b is the mac address of the 10.100.1.50 machine.

Then I cannot ping 10.100.1.50. I have to clear the arp cache to be able to ping the machine again.

What can I conclude ?

I don't have this problem with all the other switch located on the 11.0.0.0/8 network.

Thanks for your help

Chris.

10 REPLIES
Hall of Fame Super Silver

Re: Can someone help me to understand ?

Hello Chris,

it looks like the switch is relying on proxy-ARP provided from router.

verify if it has an interface in net 10.100.0.0/16

if only ip address is in 11.0.0.0 it shouldn't have ARP entries for two IP hosts like 10.100.1.50.

when you remove the router and the switch performs new ARP requests for these IP addresses somebody else answers.

check if it has a L2 interface on 10.100.1.0 subnet.

the root cause is that it shouldn't perform these ARP requests if its management IP address is in net 11.0.0.0/8.

verify if its network mask is 254.0.0.0 instead of 255.0.0.0 that would make it think those IP hosts are in the same network and would lead it to perform ARP requests for them

Hope to help

Giuseppe

Community Member

Re: Can someone help me to understand ?

Thank you Giuseppe,

Management IP and netmask are OK.

As soon as I remove the router the switch performs a new arp request for all the IP in the arp cache ?

Is this a default behavior for any cisco switch ?

Chris

Hall of Fame Super Silver

Re: Can someone help me to understand ?

Hello Chris,

there is a line like

ip default-gateway x.x.x.x?

I mean is the switch configured with ip routing off+ ip default-gateway or is ip routing enabled?

this happens if you see a line like

ip routing

By comparing configurations of two different devices you can see if there are differences.

As I've noted in my first post the device shouldn't make ARP requests for IP addresses that are not in its major network.

But if ip routing is off and no default-gateway is configured it should not be able to reach other subnets.

if ip routing is enabled and the netmask is correct it shouldn't make those ARP requests.

Hope to help

Giuseppe

Community Member

Re: Can someone help me to understand ?

No there isn't a default-gateway.

On the other switchs, at the beginning of the configuration files, there are three command.

spanning-tree mode pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

I don't have these three lines on the switch with problems. Can it be related ?

I try to enter these lines on the switch but I can't. If I do "a spanning tree ?" I only receive :

backbonefast Enable BackboneFast Feature

portfast Spanning tree portfast options

uplinkfast Enable UplinkFast Feature

vlan VLAN Switch Spanning Tree

No mode choice.

Strange.

Chris

Hall of Fame Super Silver

Re: Can someone help me to understand ?

Hello Chris,

post a sh ip route of the switch

if there is no default gateway and ip routing is turned off this can be an explanation of the different behaviour of this switch.

the other commands are related to spanning-tree protocol and cannot influence TCP/IP stack behaviour.

clearly this switch should support only pvst and so doesn't support the mode option.

Hope to help

Giuseppe

Community Member

Re: Can someone help me to understand ?

10.x is private ip

11.x is internet

are these just examples?

Community Member

Re: Can someone help me to understand ?

Yes they are.

Community Member

Re: Can someone help me to understand ?

Hi Guiseppe,

There is no sh ip route command.

I have another 2950 switch that support the mode option, why ? Here are the result of show version for both switches.

The one that doesn't support the mode option :

Cisco Internetwork Operating System Software

IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(6)EA2a, RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2001 by cisco Systems, Inc.

Compiled Thu 27-Dec-01 15:01 by antonino

Image text-base: 0x80010000, data-base: 0x8042A000

ROM: Bootstrap program is CALHOUN boot loader

SW01.M.ICM46.ORG uptime is 3 weeks, 6 days, 18 hours, 25 minutes

System returned to ROM by power-on

System restarted at 14:07:24 CET Tue Oct 20 2009

System image file is "flash:c2950-i6q4l2-mz.121-6.EA2a.bin"

cisco WS-C2950G-24-EI (RC32300) processor (revision B0) with 21299K bytes of memory.

Processor board ID FOC0623W0WE

Last reset from system-reset

24 FastEthernet/IEEE 802.3 interface(s)

2 Gigabit Ethernet/IEEE 802.3 interface(s)

32K bytes of flash-simulated non-volatile configuration memory.

Base ethernet MAC Address: 00:09:E8:6D:DB:C0

Motherboard assembly number: 73-7280-04

Power supply part number: 34-0965-01

Motherboard serial number: FOC062305DV

Power supply serial number: DAB062145EZ

Model revision number: B0

Motherboard revision number: B0

Model number: WS-C2950G-24-EI

System serial number: FOC0623W0WE

Configuration register is 0xF

The one that supports the mode option

Cisco Internetwork Operating System Software

IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(22)EA2, RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2004 by cisco Systems, Inc.

Compiled Sun 07-Nov-04 23:14 by antonino

Image text-base: 0x80010000, data-base: 0x8055E000

ROM: Bootstrap program is C2950 boot loader

SW01.A.ICM46.ORG uptime is 2 weeks, 3 days, 18 hours, 4 minutes

System returned to ROM by power-on

System restarted at 14:31:02 CET Fri Oct 30 2009

System image file is "flash:/c2950-i6q4l2-mz.121-22.EA2.bin"

cisco WS-C2950T-24 (RC32300) processor (revision R0) with 21055K bytes of memory.

Processor board ID FCZ0909Y0JD

Last reset from system-reset

Running Enhanced Image

24 FastEthernet/IEEE 802.3 interface(s)

2 Gigabit Ethernet/IEEE 802.3 interface(s)

32K bytes of flash-simulated non-volatile configuration memory.

Base ethernet MAC Address: 00:13:1A:FD:40:C0

Motherboard assembly number: 73-6114-10

Power supply part number: 34-0965-01

Motherboard serial number: FOC090438UQ

Power supply serial number: DAB09010EE6

Model revision number: R0

Motherboard revision number: A0

Model number: WS-C2950T-24

System serial number: FCZ0909Y0JD

Configuration register is 0xF

Thanks again for your help.

Chris

Hall of Fame Super Silver

Re: Can someone help me to understand ?

Hello Chris,

the IOS version is quite older on first device

System image file is "flash:c2950-i6q4l2-mz.121-6.EA2a.bin"

second

System image file is "flash:/c2950-i6q4l2-mz.121-22.EA2.bin"

this is newer

this makes the difference

Hope to help

Giuseppe

Community Member

Re: Can someone help me to understand ?

I update the IOS, it's working now, thanks

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