need terminology

[mikecoles]

I didn't believe it until I saw no route or default gateway in the "show run" on a 3750.

I have fumbled through search because I don't what the correct terminology is.

We have an odd setup and I'm sre that contributes to what I'm seeing.

We have a 3750 which is plugged into a generic switch. The management IP address on the switch is attached to VLAN1 (192.168.4.14/24) IOS version is 12.2(20)SE4

My workstation is plugged into a different generic switch. My ip address is 10.17.12.45/24. My default gateway is 10.17.12.1. My OS is SuSE 9.3

Both of the generic switches are plugged into a thrid generic switch

MY_WS <-> GS2 <-> GS3 <-> GS1 <-> 3750

I am able to telnet to the 3750, ping it, etc.

How am I able to do this w/o there being a route/gateway on the 3750? A Cisco guy I was talking w/ said it was because the switch had heard traffic coming from my workstation and already knew what my MAC was.

I had thought that if the 3750 and the workstation were on different subnets (by address, not 'wire-wise'), they would have to go through a router.

When I remove the default gw from my workstation, communcication stops. This would lead me to believe that this is something special w/ Cisco.

Could someone further explain this?

Thank you,

Mike Coles

[paddyxdoyle]

Mike,

I'm not sure how the 3750 learns the gateway either but it IS a feature of the 3750 so your are correct in your findings.

I have looked this up previously on Cisco and although the documentation confirmed the 3750 can detect the default gateway, it didn't say how....must be magic!

Rgds

PJD

[mikecoles]

Paddy,

Thanks for the verification. It's helped my sanity.

I've run across IRDP and proxy-arp. Both of which could possibly have been the culprit, except for the fact they'e both disabled.

Good research project for a Friday.

Mike Coles

[Richard Burts]

Mike

It would be interesting and possibly useful to see the output of show ip route on the 3750. Could you do it and post the output?

And it would be helpful to know how the 3750 is configured - especially how the vlan interface is configured. Could you post that?

I can think of a couple of explanations that might fit what you experienced. It certainly looks like your end station and the 3750 are in the same broadcast domain so physically their frames would get to each other. If your end station would ARP for the address of the 3750, the ARP request would certainly get to the 3750 and it would respond and they would talk to each other. The fact that communications stops when you remove your default gateway indicates that this is probably not the correct explanation for your symptoms.

The other thing that occurs to me has to do with your default gateway. Your post gave the address of your gateway but did not indicate whether the gateway is present in the network or not. If it is present it might be that you forward to the gateway and it might have logic that would forward to the 3750. If your gateway is physically not present in the network then I would think about proxy arp on the 3750. I know you said it was disabled but it sure sounds like a proxy arp behavior.

One other possibility also occurs to me. Most IP stacks will send a gratiutous ARP when the interface comes up (it is frequently part of the logic of checking for duplicate addresses). Is it possible that your end station and the 3750 learned each others IP to MAC mapping through the effects of gratiutous ARP?

HTH

Rick

[mikecoles]

Rick,

Info from the switch listed below. As for the gateway for the workstation, it is up. All equipment is in the same broadcast domain. ARP traffic is blown around the network freely. (Our campus is basically one big vlan. Yep, I know.) I could see them learning of each other's MAC addresses, but they don't show up in each other's tables.

#show ip route

Default gateway is not set

Host Gateway Last Use Total Uses Interface

ICMP redirect cache is empty

#sh run

Current configuration : 5185 bytes

!

version 12.2

no service pad

service timestamps debug uptime

service timestamps log uptime

service password-encryption

!

!

switch 1 provision ws-c3750g-24ts

switch 2 provision ws-c3750g-24ts

ip subnet-zero

ip dhcp ping packets 0

!

!

spanning-tree mode pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

!

interface GigabitEthernet1/0/1

description uplink

spanning-tree portfast

!

interface GigabitEthernet1/0/2

description user

spanning-tree portfast

!

interface GigabitEthernet1/0/3

description user

spanning-tree portfast

!

interface GigabitEthernet1/0/4

description user

spanning-tree portfast

!

interface GigabitEthernet1/0/5

description user

spanning-tree portfast

!

interface GigabitEthernet1/0/6

description user

spanning-tree portfast

!

!

interface GigabitEthernet2/0/27

description SFP

spanning-tree portfast

!

interface GigabitEthernet2/0/28

description SFP

spanning-tree portfast

!

interface Vlan1

description Main Uplink!!

ip address 192.168.4.14 255.255.255.0

no ip proxy-arp

no ip route-cache

!

ip classless

no ip http server

!

!

control-plane

!

!

!

!

monitor session 2 source vlan 1

monitor session 2 destination interface Gi1/0/2

end

#show ip int vlan1

Vlan1 is up, line protocol is up

Internet address is 192.168.4.14/24

Broadcast address is 255.255.255.255

Address determined by setup command

MTU is 1500 bytes

Helper address is not set

Directed broadcast forwarding is disabled

Outgoing access list is not set

Inbound access list is not set

Proxy ARP is disabled

Local Proxy ARP is disabled

Security level is default

Split horizon is enabled

ICMP redirects are always sent

ICMP unreachables are always sent

ICMP mask replies are never sent

IP fast switching is disabled

IP fast switching on the same interface is disabled

IP Flow switching is disabled

IP CEF switching is disabled

IP Fast switching turbo vector

IP multicast fast switching is disabled

IP multicast distributed fast switching is disabled

IP route-cache flags are None

Router Discovery is disabled

IP output packet accounting is disabled

IP access violation accounting is disabled

TCP/IP header compression is disabled

RTP/IP header compression is disabled

Probe proxy name replies are disabled

Policy routing is disabled

Network address translation is disabled

WCCP Redirect outbound is disabled

WCCP Redirect inbound is disabled

WCCP Redirect exclude is disabled

BGP Policy Mapping is disabled

#sh mac-address-table | include fa7e

(fa7e is part of my workstation's mac)

On my workstation, the only MAC's that show in the table are for my gateway (10.17.12.1), and a printer on the same network (10.17.12.44)

I didn't design the network, so I don't have all the answers as to why settings are the way they are. I would try to dig up the answers the best as I can if anyone needs more info. (Such as why `ip dhcp ping packets 0` is set.)

Thanks for taking the time to look at this.

Mike Coles

[Richard Burts]

Mike

Thanks for the additional information. It does reinforce what you had said about proxy arp being disabled and about having no default gateway on the 3750. I am a bit surprised that the show ip route is entirely empty. I would have expected to see at least a route for 192.168.4.0

Especially having seen this I believe as you said that they are not communicating directly (which was my first theory). So now I am assuming that your workstation must be forwarding to its default gateway. I must then assume that the default gateway must be forwarding to the 3750. Does the default gateway device for your workstation (10.17.12.1) happen to also have an address in 192.168.4.0? Does the default gateway device have a default route?

HTH

Rick