Speed 100 & auto functioned weird today...!!

manju.cisco · ‎07-08-2011

I have below config on a Router and a Switch which is directly connected to each other.

------------------ On Router -----------------------

Router_C2811#

!
interface fast 0/0

no ip address

speed auto

duplex auto

!

interface FastEthernet0/0.10

encapsulation dot1Q 10

ip address 1.1.1.1 255.255.255.248

!

interface FastEthernet0/0.20

encapsulation dot1Q 20

ip address 2.2.2.1 255.255.255.128

!

router bgp xxx

neighbor <neighbor ip> remote-as <remote-as>

network <WAN IP network> mask <subnet mask>

network 1.1.1.0 mask 255.255.255.248

!

----------------- On Switch ----------------------

Switch_C3750#

!

interface FastEthernet1/0/24

switchport trunk encapsulation dot1Q

switchport mode trunk

speed 100

duplex full

!

interface Vlan 10

ip address 1.1.1.4 255.255.255.248

!

The link and line protocol on both Fa 0/0 and Fa1/0/24 and up/up.

And i have this router connected to a cloud where a server at remote site can reach both the router and switch.

i.e ping to 1.1.1.1 , 1.1.1.4 are successfull.

Now i did these things:

(NOTE: Router's Fa0/0 is auto/auto and Switch's Fa 1/0/24 is 100/full )

1. I changed the speed and duplex of the interface Fa0/0 on the Router to 100/full from auto/auto, the link didnt go down/up and remained up/up without any flap and from the router i was able to ping 1.1.1.4, but i was not able to ping from the remote server.....(ping response failed from remote server..!!!)

2. Then i revert the fa0/0 configs, ie., from speed/duplex 100/full to auto/auto then still the interface remained up(no flap) and then the remote server was able to ping 1.1.1.4 , router was able to ping to 1.1.1.4.

3. Again i changed Fa 0/0 to 100/full with shut and no shut, then the link went down and up on no shut, the remote server ping response to ip 1.1.1.4 failed however the ping from router to 1.1.1.4 is success.

4. Again i changed Fa0/0 back to auto/auto, with shut and no shut, then i was able to ping from the remote server.

Qestion 1 :

Can someone please analyse my observation and help me to understand why i was unable to ping the lan switch ip 1.1.1.4 when the speed/duplex is auto/auto on the router Fa0/0 ?

Qestion 2 :

Also, till these days i was aware that when a speed/duplex is changed on a link, the link goes down and comes up agian. This didnt happen here. Can someone please help to understant this one too.

Thanks in advance,

Manjunath

Leo Laohoo · ‎07-08-2011

Before answering any of your question I have a question on my own: Why bother setting duplex and speed settings to the router?

manju.cisco · ‎07-13-2011

Hi leolaohoo,

i was bothering because i have been asked to hard code speed and duplex on the setting on live network.

Since hard coding made network down, i want to find answer why it happened the way as i have explained the scenario

Pls let me know the answers/behaviours at that scenario. Thanks.

Latchum Naidu · ‎07-13-2011

Hi Manjunath,

Remember alwasy the first point that the interface settings at both ends (your end and porovider end) must be same.
If you harde coded to speed 100 and duplex full at the same time your provider end also must be the same settings other wise your network will be tear down.

HTH
Please click on the correct answer if this answered your question.
Regards,
Naidu.

manju.cisco · ‎07-16-2011

Hi,

here i am the one who manages both devices having full access & authority......

The switch was newly connected to the router and while connecting the new switch to the router i faced this case.

Can u pls help me to understand the reason for this case in better way........

NOTE:

At present the port on router is auto/auto and at switch is 100/full. Even now if i keep the router port to 100/full (same as swithc port has) i wont be able to ping from cloud but can ping only from router......And I can ping from cloud only when the port on Router is set to auto/auto.

In short, in my case 100/full at both sides is not working, its working with one end as auto/auto and other end with 100/full ....

Pls help.

Leo Laohoo · ‎07-16-2011

Here's the deal: The router is an ISR G1 and the switch isn't old. It's a 3750. This is good because unlike previous generation of routers and switches auto-negotiation of speed and duplex are better. So leaving these two variables as "AUTO" is better then constantly setting/adjusting them.