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Beginner

Destination host unreachable and request timed out

Hello,

This question is regarding a behavior of 3750X.

 

I have Cisco Catalyst 3750X.

PC A is on vlan 10 which is 10.1.10.10/24.

PC B is on vlan 20 which is 10.1.20.10/24.

They are connected to 3750X .

PC A can ping PC B.

When I turn off PC B, PC A gets mixture of destination host unreachable and request timed out.

Destination host unreachable is generated by 3750X which is a default gateway of PC A.

 

Here is a question.

If I get only destination host unreachable or only request timed out, then it make sense.

But I get mixture of those.

Why 3750X doesn't generate destination host unreachable against all pings?

Is it something like to prevent from consuming all CPU resource for that?

Thanks in advance.

 

1 ACCEPTED SOLUTION

Accepted Solutions
Highlighted

 

Thank you for your comment.

I think I have found the reason.

There is a icmp rate-limit command to adjust how often the device react against ping.

The page below is talking about a router, but I think it applies to a switch ,too.

http://www.itcertnotes.com/2011/03/icmp-destination-unreachable-rate.html

"sh icmp rate-limit" does not seem to be supported on 3750X.

I don't know if you can change it on a switch though.

Hi,

Normally By default, the input interface sends ICMP Unreachable messages whenever a packet is discarded, regardless of whether the packet was discarded because of an ACL on the input interface or because of an ACL on the output interface.

ICMP Unreachables are normally limited to no more than one every one-half second per input interface, but this can be changed by using the ip icmp rate-limit unreachable global configuration command.

Hope that Helps..

-GI

Rate if that Helps.

View solution in original post

8 REPLIES 8
Highlighted

Hi

If i'm not wrong..,

As per your post i understand that you are using switch gateway for VLAN10 i.e PCA.

But a VLAN10 interface IP should be the gateway of PCA because it is part of VLAN10.

 

Regards

Nagaraj

Highlighted

Thanks for the comment.

You are correct.

3750X is the default gateway for both PC A (VLAN10) and PC B (VLAN20).

3750X is simply working as a router.

PC A -----VLAN10----3750X----VLAN20-----PC B

Is my question unclear?

 

 

Highlighted

Taro,

 

can you please share the output that you are getting when you try to ping the PC B from PCA while PC B in turn off mode. Infact like you know that 3750 is layer 3 switch which has the capability to support routing between different vlans just like router.

Highlighted

Thank you for your comment.

I think I have found the reason.

There is a icmp rate-limit command to adjust how often the device react against ping.

The page below is talking about a router, but I think it applies to a switch ,too.

http://www.itcertnotes.com/2011/03/icmp-destination-unreachable-rate.html

"sh icmp rate-limit" does not seem to be supported on 3750X.

I don't know if you can change it on a switch though.

Highlighted
Beginner

I think I have found the reason.

There is a icmp rate-limit command to adjust how often the device react against ping.

The page below is talking about a router, but I think it applies to a switch ,too.

http://www.itcertnotes.com/2011/03/icmp-destination-unreachable-rate.html

"sh icmp rate-limit" does not seem to be supported on 3750X.

I don't know if you can change it on a switch though.

 

 

Highlighted

Hello,

This question is regarding a behavior of 3750X.


I have Cisco Catalyst 3750X.

PC A is on vlan 10 which is 10.1.10.10/24.

PC B is on vlan 20 which is 10.1.20.10/24.

They are connected to 3750X .

PC A can ping PC B.

When I turn off PC B, PC A gets mixture of destination host unreachable and request timed out.

Destination host unreachable is generated by 3750X which is a default gateway of PC A.


Here is a question.

If I get only destination host unreachable or only request timed out, then it make sense.

But I get mixture of those.

Why 3750X doesn't generate destination host unreachable against all pings?

Is it something like to prevent from consuming all CPU resource for that?

Thanks in advance.

Hi,

Is this a lab router where you doing testing, if yes try debug ip detail on cisco 3750 switch and see what happen when you give icmp response from PC A to PC B.

Ideally when you turned off the PCB it should get vanished from arp table once timer expired and gives you output of RTO , AS SVI are up in switch so it is connected interface in cisco 3750 switch.

Hope that Helps..

-GI

Rate if it Helps

Highlighted

Thank you for your comment.

I think I have found the reason.

There is a icmp rate-limit command to adjust how often the device react against ping.

The page below is talking about a router, but I think it applies to a switch ,too.

http://www.itcertnotes.com/2011/03/icmp-destination-unreachable-rate.html

"sh icmp rate-limit" does not seem to be supported on 3750X.

I don't know if you can change it on a switch though.

Highlighted

 

Thank you for your comment.

I think I have found the reason.

There is a icmp rate-limit command to adjust how often the device react against ping.

The page below is talking about a router, but I think it applies to a switch ,too.

http://www.itcertnotes.com/2011/03/icmp-destination-unreachable-rate.html

"sh icmp rate-limit" does not seem to be supported on 3750X.

I don't know if you can change it on a switch though.

Hi,

Normally By default, the input interface sends ICMP Unreachable messages whenever a packet is discarded, regardless of whether the packet was discarded because of an ACL on the input interface or because of an ACL on the output interface.

ICMP Unreachables are normally limited to no more than one every one-half second per input interface, but this can be changed by using the ip icmp rate-limit unreachable global configuration command.

Hope that Helps..

-GI

Rate if that Helps.

View solution in original post

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