1

I am having trouble routing between 2 switches and 1 router. My network diagram is below.

enter image description here

I currently have both switches trunked to ports fa 0/0 and 0/1 of the router.

I created sub interfaces on the router as well. I also set the IP address default gateways for the end devices for these sub interfaces.

The problem I am facing is PC0 and PC1 can talk to each other without an issue, however PC 2 and 3 cannot. Also, PC 2 and 3 cannot communicate with PC 0 and 1 and vice versa.

When I check to see how the packet is moving through the network, for example from PC 2 to PC 3 it reaches switch 1, switch 1 sends it to the router, the router says there isn't a sub interface with this VLAN enabled and then drops the packet.

When I check the router's interfaces in show int , it clearly shows the interface up, and VLAN id being correct.

My logic is that PC2 sends the packet to the default gateway, the router sends it out the sub interface associated with PC3's subnet and the switch sends it to PC3.

I also tried setting up static routes but it still isn't working.

It's driving me nuts, is there a problem with my logic?

Update Still not working, are some of my configs for the router and switches.

(Switch 1)

interface FastEthernet0/1
 switchport access vlan 30
 switchport mode access
!
interface FastEthernet0/2
 switchport access vlan 40
 switchport mode access
!
interface FastEthernet0/3
!
interface FastEthernet0/4
!
interface FastEthernet0/5
!
interface FastEthernet0/6
!
interface FastEthernet0/24
 switchport mode trunk
!

router

hostname Router
!
!
!
!
!
!
!
!
ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
interface FastEthernet0/0
 no ip address
 duplex auto
 speed auto
!
interface FastEthernet0/0.10
 encapsulation dot1Q 10
 ip address 192.168.10.254 255.255.255.0
!
interface FastEthernet0/0.20
 encapsulation dot1Q 20
 ip address 192.168.20.254 255.255.255.0
!
interface FastEthernet0/0.30
 encapsulation dot1Q 30
 ip address 192.168.30.254 255.255.255.0

!
interface FastEthernet0/0.40
 encapsulation dot1Q 40
 ip address 192.168.40.254 255.255.255.0
!
interface FastEthernet0/1
 no ip address
 duplex auto
 speed auto
!
interface Vlan1
 no ip address
 shutdown
!
ip classless
!
ip flow-export version 9
!

(Switch 0)

interface FastEthernet0/1
 switchport access vlan 10
 switchport mode access
!
interface FastEthernet0/2
 switchport access vlan 20
 switchport mode access
!
interface FastEthernet0/24
 switchport mode trunk
!
interface Vlan1
 no ip address
 shutdown
!
  • 3
    Your logic is fine, which suggests there is an error in your configuration. Please edit your question to include both switches and the router configuration. – Ron Trunk Jul 7 '18 at 17:01
  • Please, can you share the configurations of switches and router? – Orlando Gaetano Jul 7 '18 at 20:08
  • 2
    Your drawing shows two router interfaces used, but you configuration shows only one router interface used. The Switch 1 network cannot be coming out of the Switch 0 interface. You can use the same VLAN numbers on the two switches, but they are going to be different VLANs because a VLAN is bounded by a router. Routers route between networks, so each router interface must have a different network. – Ron Maupin Jul 9 '18 at 0:17
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 9:00
2

A quite common pitfall on ISR G1 Models (the generation of your 1841) is that you create a SVI Interface (g.G. interface VLAN10) for Layer3 configuration, but didn't mention to create the corresponding vlan in the vlan database that is seperated from the configuration file (similar to the VLAN database in older Catalyst switches).

vlan dat
vlan 100
name FOO
apply

From the configuration perspective everything looks fine, but it isn't working.

maybe you stepped in this pitfall too

  • 1
    Your are correct with regards to switch modules or fixed switched ports in ISR routers (any generation, 8xx and 19xx included). To turn these into (not quite) routed ports, the internal switch needs a vlan, than vlan being active on at least one switchport (by virtue of switchport mode access or trunk allowed vlan nnn) and the classic SVI (interface vlan nnn); even then, these ports aren't quite as good as "real" router ports (for example w/regards to QoS capabilitites). Models 1801-1812 are of that type. The 1841 however has only two router ports, proper ones. – Marc 'netztier' Luethi Jul 7 '18 at 21:11
  • Ok I'll try it out, thanks everyone for the input! I'll post any updates – Shin Jul 7 '18 at 22:50

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