Quick Question

[keithu]

You will have to trust me, I have sat and passed the Cisco exams for this.

 

I am not trying to be rude or anything, but are just going around in circles here with the same thing.

 

I'm not trying to be rude either, but you haven't explained anything. You've simply made statements. Saying things like The router will literally move traffic to the next layer 3 device is just hand waving.

 

Please look back at my message #20. I have tried to be very thorough and explain exactly what I think will happen, packet by packet.

 

Please tell me exactly where my logic is wrong and what would really happen - in detail.

[Blue_Cookeh]

I think this thread is talking about two different setups. The OP has an LEA provided router with an interface that has a 'directly connected' subnet (like your average home router), in which case yes they need to make changes to support VLANs. Either add sub interfaces with VLAN tags or to configure it with routing capability on the port and set a next-hop address that points to your L3 capable switch/own router.

 

Other people are assuming the LEA provided router is already providing a L3 route to a core switch on the internal interface, which it may not be.

 

I don't know anything about Schools Broadband but this sounds over complicated.

 

They don't just provide you with an interface on the 10.x.x.x/20 subnet then?

 

In our situation no. SchoolsBroadband provide a router with an interface on 10.0.0.1, they expect our core switch/router to be on 10.0.0.2 (next hop from their router is 10.0.0.2, next hop from our core switch is 10.0.0.1). They have a route setup on their router that says anything in the '10.0.16.0/20' subnet goes via our L3 core switch. I'd say this is a more typical setup in schools that need to route private addresses to their ISP for filtering and what not, so not that complicated.

Edited January 19, 2018 by Blue_Cookeh

[FN-GM]

On post 20 you seem to have merged the LA Router and the Core Switch.

 

This is a simplified version of what will happen.

 

1. The LA router will consult its routing table and discover that 192.168.1.42 is part of the 192.168.0.0/22 subnet which is directly connected to its LAN interface.

2. The Core switch will pick this up. It will consult its own routing table and know what VLAN 192.168.1.42 belongs to. If 192.168.1.42 is in the CAM (mac address) table it will send it to the PC via the correct vlan. If not it will do the whole arp process and send it to the pc via the correct vlan.

3. The PC receives the packet.

4. It now replies. It knows the destination IP isn't in the same broadcast domain and uses its default gateway to send it to the core switch.

5. Core switch picks it up, looks at the routing table and finds the packet isn't destined to any of its vlans and uses the gateway of last resort (ip route 0.0.0.0 0.0.0.0 ) and sends this to the LA router.

6. LA router receives this and then looks at its own routing table, finds it needs to go into the LA WAN.

7. We don't really know what happens in the LA WAN.

Edited January 19, 2018 by FN-GM

[keithu]

I think this thread is talking about two different setups. The OP has an LEA provided router with an interface that has a 'directly connected' subnet (like your average home router), in which case yes they need to make changes to support VLANs. Either add sub interfaces with VLAN tags or to configure it with routing capability on the port and set a next-hop address that points to your L3 capable switch/own router.

 

Other people are assuming the LEA provided router is already providing a L3 route to a core switch on the internal interface, which it may not be.

 

 

 

In our situation no. SchoolsBroadband provide a router with an interface on 10.0.0.1, they expect our core switch/router to be on 10.0.0.2 (next hop from their router is 10.0.0.2, next hop from our core switch is 10.0.0.1). They have a route setup on their router that says anything in the '10.0.16.0/20' subnet goes via our L3 core switch. I'd say this is a more typical setup in schools that need to route private addresses to their ISP for filtering and what not, so not that complicated.

 

Many thanks for this

 

If all traffic is forwarded to the customer's own L3 switch/router I agree that they can subnet and route however they like.

 

I'm not sure that's the setup here though, which is why I introduced a model network to talk about. In the model network there is no route to a customer router but people are still arguing that they can introduce one and traffic will be 'magically' routed to it.

[FN-GM]

I didn't realise there was 2 separate conversations going on here! Apologies.

[keithu]

On post 20 you seem to have merged the LA Router and the Core Switch.

 

This is a simplified version of what will happen.

 

1. The LA router will consult its routing table and discover that 192.168.1.42 is part of the 192.168.0.0/22 subnet which is directly connected to its LAN interface.

2. The Core switch will pick this up....

 

The Core switch will pick what up? And how will it reach the core switch?

 

BTW I haven't merged anything. I don't even mention the core switch because it's not relevant.

 

Thank you for sticking with this; I do appreciate it.

[FN-GM]

The Core switch will pick what up? And how will it reach the core switch?

 

It will pick the packet up. A better word would be recieve the packet.

 

The routing table on the LA router would have the physical interface on the schools side of the network as a directly connected route. The router will send it out of that physical port. This port is phyisically connected to the core switch. The the port on the core sitch would need an IP address assigning to it. Or you could do it via a VLAN but that is a little more complex.

 

 

I don't even mention the core switch because it's not relevant.

 

It is becuase its a Layer 3 device doing routing and sitting between the PC and the LA router

 

 

Just to note, anything I am talking about is in relation to the OP.

Edited January 19, 2018 by FN-GM

[keithu]

It will pick the packet up. A better word would be recieve the packet.

 

It won't receive the packet because it won't be sent in the first place.

The LA Router will see that the destination host (PC) is supposed to be on the directly connected segment so will arp for the mac address of the host. This will fail.

 

You seem to assume that the LA router will forward the packet to the IP address of the L3 Switch's interface for onward routing, but why would it? It's not in the routing table.

[FN-GM]

Like I said, I can demo that solution will work, but you didn't want to know. I have done it more than one time in real life. I am not going continue with this conversation anymore. Edited January 19, 2018 by FN-GM

[keithu]

Like I said, I can demo that solution will work, but you didn't want to know. I have done it more than one time in real life. I am not going continue with this conversation anymore.

 

I'm sorry you feel that way.

 

I'm not interested in a demo because I want to understand how things work.

I've tried to express precisely why I think there is a problem and I don't think you've addressed that.

[gh5000]

Looking back at the OP and then what I started, the OP doesn't mention having a L3 switch acting as the router.

 

In the following answers/discussion has everyone (including me) assumed that that L3 switch is already setup as the L3 router or even exists? If so, we may have just skipped ahead with a couple points and we haven't seen @MM1 since.

[keithu]

Looking back at the OP and then what I started, the OP doesn't mention having a L3 switch acting as the router.

 

In the following answers/discussion has everyone (including me) assumed that that L3 switch is already setup as the L3 router or even exists? If so, we may have just skipped ahead with a couple points and we haven't seen @MM1 since.

 

I have not assumed that there is a router other than the LA (gateway) router. If there was already a second router under the control of the OP with all traffic forwarded to it, obviously there would be no issue.

 

@MM1 - there is a definitive answer to your question (several in fact) but we need to know whether you already have a router of your own on the network. If you describe what you've got and how it's configured we will be able to help.

[MM1]

With everything being on the same subnet we don't have our own layer 3 device doing any routing at the moment.. all devices currently just have the the LA router as there default gateway.. What has now been suggested by the LA is they set up a point to point connection from there L3 switch to our new core switch with a separate /30 subnet. They will then route the entire subnet to our core switch (And from what I have been reading in this thread I assume that because they are routing the whole subnet back to our L3 core switch.. any subnetting I do within this range will route back ok?)

 

Thanks for all the responses so far

[MM1]

To respond to whether they need to add static routes back for each subnet.. I assume they can just put a summary static route that covers all of them right?

[FN-GM]

To respond to whether they need to add static routes back for each subnet.. I assume they can just put a summary static route that covers all of them right?

Correct. In fact this preferred as it will mean more optimal routing.