Can't access cisco switch from computers, that are not directly connected to the switch

Profitgeier · ‎05-09-2019

I have a Cisco SG350-10 10-Port switch and a AVM FritzBox router.

On the ports of the CISCO switch 3 PCs and the AVM FritzBox are connected to.

For easier explanation i am calling these 3 PCs here "Group A" PCs.

On the ports of the FritzBOX i have connected the CISCO switch and 2 other PCs, i am calling them here "Group B" PCs.

On the CISCO switch DHCP server is turned off because this DHCP server is in the same network 192.168.178.x/24 , as the DHCP server of the FritzBox router.

On the FritzBox router, the DHCP server is turned on.

The CISCO Switch gets its IP configuration via DHCP from the FritzBox router. The CISCO switch's hostname is preconfigured there.

Let's call the hostname of the CISCO switch "joe".

Now here comes the problems:

1. After a while, i can't reach the switch via its hostname "joe" from PCs in Group A. Accessing the switch via its IP works, but not via its hostname. Also traffic over the switch works.

2. From PCs in Group B the switch isn't reachable. Neither over its hostname nor via its IP address.

But PCs from Group A can reach PCs from Group B and vice versa.

Can anyone tell me, what could be the problem and how to fix it?

EDIT:

All computers and the fritzbox router and the Cisco switch are in the same network.

This is: 192.168.178.x/24

Mark Malone · ‎05-09-2019

Hi
1 whos controlling the DNS the router ? if the DHCP is coming off the router its probably controlling DNS too , so hes the suspect , switch is operating at layer 2 and are open by default unless you blocked udp 53 to the devices by an acl , if not the router maybe the issue , have you tried reboot it if its just a GUI based device , i dont know much about that brand , as a workround you could take the DHCP/DNS off the fritzbox and let the cisco switch do it and see if it resolves it , or change what the DNS server is currently if external there are many to use , 8.8.8.8,1.1.1.1 etc

2 Again if these are on multiple subnets and controlled by the router the issue could be there , what IP address does the switch get compared to the PCs in A and B , some more detailed info may help , but intervlan routing if there different subnets would again be controlled by the layer 3 device in this setup the router not the switch , if switch s layer 2 all its doing is forwarding

Profitgeier · ‎05-10-2019

First, thank you for your answer.

The DNS is coming from the router.

The Router knows the hostname of the layer 3 switch, but the computers that are connected to the switch don't know it.

They receive all DNS informations from the router.

2. It's only one subnet.

The switch's IP is 192.168.178.100 but on the switch there is also another vlan 2 running for 2 PCs. One of them is a NAS and does have an IPMI BMC controller, the other one is the administration computer for this IPMI access. All other PCs are in vlan 1.

The router's IP is 192.168.178.1

The PCs have IPs between 192.168.10 and 192.168.178.90

The PCs and the switch get their IP from the Router in vlan 1.

For the computer's separate NICs that are on vlan 2 i am using static routes at the moment.

danjmcintyre · ‎05-10-2019

Can you access the switch via it's hostname from Group A when it first gets a DHCP address?

Can you access the switch at all from Group B at any time?

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Profitgeier · ‎05-10-2019

@danjmcintyre wrote:
Can you access the switch via it's hostname from Group A when it first gets a DHCP address?
Can you access the switch at all from Group B at any time?

To be able to access the switch by its hostname i have to enter the webinterface of the switch and click the save button there.

Then when doing so the switch is surprisingly accessible via its hostname for a couple of hours, even after power off.

But this information seems to vanish after a couple of hours or days.

After a couple of days, this stops to work and the host is no more accessible via it's IP address from computers in group B, but only from computers in group A. At that time, the switch is also no more accessible via its hostname, neither from computers in group A nor in group B.

Only the router seems to still know the hostname of the switch, but this is also stored in the routers configuration and the router is the only machine, that is running all the time.

The switch and the computers are switched off daily for a couple of hours and bootet the next day.

paul driver · ‎05-10-2019

Hello

@Profitgeier wrote:
After a couple of days, At that time, the switch is also no more accessible via its hostname, neither from computers in group A nor in group B.

Only the router seems to still know the hostname of the switch, but this is also stored in the routers configuration and the router is the only machine, that is running all the time.

The switch and the computers are switched off daily for a couple of hours and bootet the next day.

Simple solution, give the switch a hardcoded ip address/subnet/default gateway, and exclude that address from the dhcp scope and add a A host record for that switch in your DNS server

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Kind Regards
Paul