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 10.30.30.1,
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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