Packet Tracer routing doesn't work

[ngaoiwnwa2]

Hello all, unfortunately I'm stuck and I don't know what to do.

I configured the switches:

• created VLAN 10,20
• trunk with vlan trunk allowed vlan 1,10,20 between the firstfloor and secondfloor switches
• trunk from secondfloor switch to router
• added clients to their vlan
• created subinterfaces on the router
• added encapsulation on the subinterfaces for g0/0/0.20 and .10 for VLAN 20 and VLAN 10
• created p2p interface between routers with /30 mask

I can ping from vlan 10 to vlan 20 on the left and right side of the topology. Unfortunately I can't ping to the other side and I'm pretty much sure there's a routing problem so the packet can't flow over from 10.10.10.1 -> 10.10.10.2

I also created a static route:

• Network: 192.168.1.0
  mask: 255.255.255.0

• next hop: 10.10.10.2 on router 10.10.10.1 on reverse on the other side

Do I somehow have to enable a trunk between both routers?

Help is really appreciated because I'm stuck and I've been trying this for about 2 hours now.

[marce1000]

              - FYI : Community group for Packet Tracer project questions

 M.

[Blue_Bird]

Hello ngaoiwnwa2,

Check once again with Ip addressing scheme for the topology... I think...It is not correct.

Each router should be configured with defferent network and different ip addresses...

Best regards
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[ngaoiwnwa2]

This does not seem to be correct. Both are connected and both are on a /30 subnet. If I give the other router another network, the router does not know where it goes AFAIK.

[M02@rt37 · Accepted solution]

Hello @ngaoiwnwa2 

Impossible to do that like this.

Routers route network/subnet. On Router0 you have the subnet 192.168.2.0/24  directly connected and you have a static route towards Router1 to reach this same subnet..... 

And if you do a #show ip route on that Router0 you will see that subnet 192.168.2.0/24 is know only as connected networks...

Same logic on Router1.

You could have same vlan id on that 2 sites, because VLAN is locally significant, but you have to configure different subnets on the 2 sites attached to these vlan.

As an example:

on the right you should have vlan10: 192.168.1.0/25 and vlan20: 192.168.2.0/25

on the left you should have vlan10: 192.168.1.128/25 and vlan20: 192.168.2.128/25

Adjust ip static routes on Routers 0 and 1 as concerned the mask and the different Gateways.

[ngaoiwnwa2]

Thanks for explaination. I figured it out myself that I can't have the same network on both sides because I assume the router gets confused by both the same default gateway. So AFAIK if I understand it correctly it routes from Router 0 over to Router 1 but sees that the destination has the same network / default gateway?

Could you please explain, if you don't mind, a bit deeper why this does not work? I still don't understand it 100%.

[M02@rt37]

@ngaoiwnwa2 

When a router receives a packet, it makes a routing decision based on the destination IP address. Router 0 looks into its routing table to determine the appropriate outgoing interface for the packet toward 192.168.1.15 (PC on right side as example).

Router 0 can not route a packet towards 192.169.1.15 since this same subnet is directly connected to him. Same subnet....administrative distance of connected subnet is 0 ; so the best !