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Switched network

lucad7846
Beginner
Beginner

Hi everyone.

I have this PKT file to troubleshoot. The first issue was that the switches trunk interfaces were missing the allowed vlan's ID which I corrected. Now the pc in VLAN 10 can ping the pc in VLAN 20. However pcs 30 cannot ping. I have compared the configuration for all three switches and they look exactly the same. The I checked the router and it looks like the sub interfaces are configured correctly. I am really lost now. I am not sure why the pc directly connected to S3 can't ping.

Many thanks in advance to anyone who is willing to help.

Cheers

1 Accepted Solution

Accepted Solutions

Martin L
VIP Advisor VIP Advisor
VIP Advisor

This one is a bit tricky;  Vlan 30 is missing; can u find it where?

To pass traffic in vlan x via switch a (or to / from), all switches in the path must have vlan x in their database; database is stored on switch internal flash (show flash) in file called vlan.dat.    this is true for switch that does not have any hosts in vlan x but vlan x must still exists on that switch!   Often, vlan x is not added if u do not have any hosts connected (no access ports in vlan x). Note that making a port in access vlan x, automatically adds that vlan x into database. 

Also, vlan x should be included in trunk allowed vlan list (which u did )

attached is fixed file but you should really try fix it yourself after reading this

Regards, ML
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8 Replies 8

Martin L
VIP Advisor VIP Advisor
VIP Advisor

This one is a bit tricky;  Vlan 30 is missing; can u find it where?

To pass traffic in vlan x via switch a (or to / from), all switches in the path must have vlan x in their database; database is stored on switch internal flash (show flash) in file called vlan.dat.    this is true for switch that does not have any hosts in vlan x but vlan x must still exists on that switch!   Often, vlan x is not added if u do not have any hosts connected (no access ports in vlan x). Note that making a port in access vlan x, automatically adds that vlan x into database. 

Also, vlan x should be included in trunk allowed vlan list (which u did )

attached is fixed file but you should really try fix it yourself after reading this

Regards, ML
**Please Rate All Helpful Responses **

Hi Martin

Thanks so much! And you are right I was able to fix it after reading your post. 

So tricky indeed. 

I have 2 questions if I may. I basically fixed it by adding VLAN 30 to Switch S2, now switch 2 has only VLAN 20 and 30, however VLAN 10 ping still works. How is that possible?

The other question is about show flash command. I displayed the files stored in flash memory but I am not exactly sure how to access them to read them. 

Cheers 

You need to use show vlan or show vlan brief command to see your vlans. You can transfer vlan.dat file to your PC and open it with Notepad++ or UltraEdit but it looks messed up.  On real device, you can read some files stored in flash memory with text reader (Linux text editor like cat/vi, or use more command) but I am not sure if you can open and read vlan.dat file; probably not;

True, by default, Vlans are stored in vlan.dat file on flash when switch is in VTP server (default) or VTP client mode.  However, Vlans are listed in the running config when switch is in VTP transparent mode. So, you can easily switch VTP to transparent mode and read vlans from show run then switch back to server/client mode.  Do vtp mod transparent, then show run to see list of vlans before lists of interfaces.

 

Regards, ML
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Thanks so very much for the detailed information. I am using the show VLAN brief command to get info in relation to the VLANs but I want to try and read the file to see what it looks like. 

I really appreciate your help cheers. 

well, try this old backup file from my old home c3750 switch lab. You must unzip it one to get vlan.dat

Hi thank you so much. I'll have a look. Cheers 

Basically vlan 10 is not passing thru S2 like vlan 30 because you have a router who does inter-vlan routing.  Router is changing vlan tag in each frame; replacing vlan 10 for 20 or vice versa.  For case of vlan 30, router replaces tag 10 for 30. So, technically you should count number of switches between router and PC ; 1 switch in case of vlan 10 to 20 (or vice versa) but not 2;  In case of PC vlan 10 to PC vlan 30, traffic should flow between 2 switches.  However, one of 2 switches has missing vlan in its database, so that frame will be dropped.  Note that there is only one vlan tag in each frame between switches or switch and router.  S1 adds tag of 10, so vlan tag is 10 between S1 and R1; then tag is 20 when frame leaves R1. Tag is still 20 between S1 and S2 in direction of PC20. Switch 2 removes tag and forwards frame to PC20.   

Switch has Rules to follow in order to build CAM/TAM tables which are used to forward frames; Use show mac address-table to see some details.  One of those rules is for switch to drop frames when it does not have the Vlan ID indicated by vlan tag in the frame.

I add those rules once i find them in my notes later .

Regards, ML
**Please Rate All Helpful Responses **

Wow that's really cool. It's a bit tricky but I'm slowly but surely starting to understand how it all works. 

Thanks so much for your help. I really appreciate it. 

Cheers

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