Wednesday, January 25, 2017

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TroubleShooting Vlan Issues

                         In our last topic, we talked about “following the path from an End-Station” that was having issues to a Destination somewhere on the network and as we were following the path we make sure that the Switch into which our devices was connecting had learned that devices mac-addresses on an appropriate Port, assuming it had something else we can do is following the path is to check for Vlan issue
                         Let’s take a look at some common Vlan Troubleshooting issues we have typically a single subnet associated with the single Vlan, it could be that the host has an incorrect IP Address and Subnet Mask, and you determined weather or not that IP Address belonged to a specific network, you might need to take a look at Network Topology and based on the IP Address and Subnet Mask of the interface in the Router or Multilayer Switch, make sure that device connect of that interface belongs to the same Subnet so.
                      Let’s use our network documentation to confirm that host is in the correct subnet, we could make an IP Address misconfiguration on the Switch or the Multilayer Switch if we have an SVI, a Switched Virtual Interface setup which can route for ports belonging to the Vlan, we assign an IP Addresses much like we would assign an IP Address to a Routed interface, may be that IP Addresses incorrect again we can check Network documentation to make sure that it’s in an appropriate IP Address for that Vlan also as we following the Path we coming into a particular Port on our Multilayer Switch is that Port a member of the appropriate Vlan if not that can be issue or maybe the Vlan that we belong to doesn’t exist on the Switch, we might want to check our Vlan Database to make sure that Vlan actually exists on the Switch
                         And with these things in mind let’s take a look at sample Trouble Ticket, let’s assume that we have a Trouble Ticket tells us that a PC is not able to connect out on the network to any other device
                        And as we following the path we might go to its next Hop Switch and check out the Vlan Configuration for the Port end to which this PC is connecting, now let’s assume we already gone to the PC and checked its IP Address and its IP Address is indeed a member of the Subnet associated with Vlan 300 in this case and now we moved on the Switch and one of the first thing we wanna do is verify the IP Address of the SVI the Virtual interface that’s gonna do Routing for Vlan 300, let’s do a
Ø  Sw1#show ip interface brief
                       Here is our Vlan 300 SVI and we got a IP Address of, based on our network documentation that looks good IP Address appears to be correct, there is a bit of concerning it looks like status is UP and DOWN
                         Let remind our self, what would prevent an SVI from being in the UP/UP State, in order to be in the UP/UP State and SVI has to have a Port on that Switch, that is a member of that Vlan also in the UP/UP state, now that Port can be a Trunk Port that’s carrying traffic for that Vlan but if we have at least 1 port on that Switch belonging to that Vlan and it’s the UP/UP state then SVI Should be UP and here it’s not, we are in the UP/DOWN State so, we have an issue let’s next check the Configuration of a fastetherent 1/0/1 the Port into which we think our PC is connected and let’s make sure that it’s assigned to Vlan 300
Ø  Sw1#show run
                     (Switchport access vlan 300) we are telling that Port to belong to Vlan 300, so what’s going on here the Port is not administratively shutdown, we saying you belong to Vlan 300 we can do visualize Inspection to see that the PC is indeed connected into that Port and its Powered on what could be going on, let’s check our Vlan Database to confirm that fastetherent 1/0/1 really does belong to Vlan 300, it looks like we added it to Vlan 300, let’s double check our Vlan Database
Ø  Sw1#show vlan brief
                        Oh! this doesn’t look good, we got Vlan 1 and 100,200 where have a Vlan 300, it seems to been deleted my Vlan Database, well in that case to what Vlan does fastetherent 1/0/1 belong, let’s take a look under are Ports can you find fastetherent1/0/1 i cannot, it doesn’t exist in any of our Vlans and as a result that Port is not gonna be passing any traffic, i think this is our issue somehow Vlan 300 got deleted, let’s re-add it and see our SVI interface status goes to UP/UP
Ø  Sw1(config)#vlan 300
Ø  Sw1(config-if)#end
It says that Vlan 300 changed its status to UP, let’s check it out
Ø  Sw1#show ip interface brief
                     Now Vlan 300 is indeed in the UP/UP state because there is Port belonging to that Vlan which is in the UP/UP state, we can do a
Ø  Sw1#show vlan brief
                            That’s the look at how we can troubleshoot some common Vlan issues, keep in mind things like
Ø  IP addressing, we say that Vlan corresponds to a Subnet does the SVI interface belong to the correct subnet?
Ø  Does the hosts IP Address belong to correct Subnet?
Ø  Is the Port assigned to the correct Vlan?
Ø  Does that Vlan exists?

And this Trouble Ticket the Vlan not exits but everything starting to work when we re-edit the Vlan.
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