Quick Question

[MM1]

Just a quick question currently have a flat network looking to subnet and implement some vlans.

 

Current allocated range provided by LA is 10.x.y.y/22. If i was to create seperate vlans and subnet these down into 10.x.x.y/24 would the lea need to change the route backs or would I be ok as long as I subnetted within my allocated range.

 

Currently seeing conflicting things so which is right?

 

the LA need to add a route to each vlan via your core switch otherwise traffic will not get back to the clients

 

No they don't as long as

 

1. He subnets within his exist assigned LA range.

 

Any help would be much appreciated. Thanks

 

[Norphy]

As long as your /24s are in the same range as your /22, your LEA shouldn’t need to change routing.

[gh5000]

As long as your /24s are in the same range as your /22, your LEA shouldn’t need to change routing.

 

I would say it depends on who controls your router. With us the LA controls the Cisco router and have to setup the IP routes for us back to our core switch which we then do the vlans and vlan routing on.

[gh5000]

Just reread your post.

 

No they don't as long as He subnets within his exist assigned LA range.

Is he possibly just talking about subnetting only and not assigning VLANs??? That shouldn't require the LA's knowledge and could be a solution if you don't need to restrict access.

 

For me if you're doing vlans and you have a router that is supplied by LA on 10.10.10.1 (current gateway) and you'll be configuring a core switch as your router - VLAN 10 will have to have to have 10.10.10.2 as its gateway and VLAN 11 will have 10.10.11.1 as a gateway etc for your 4 /24 vlans.

 

Presuming, for the ease of explanation, that you won't have access control between your vlans everything will be fine to communicate intra-vlan. You will also have a default route 0.0.0.0 0.0.0.0 10.10.10.1 so that everything that doesn't know where to go (ie can't find something by routing through the individual vlan gateways) for example trying to get to google.com will hit the LA router. It (LA router) will need to know all about your gateways. So IMO your LA will have to setup routes back.

 

Another thing to consider if you're looking to save IP addresses is that you can create other VLANs that don't have to communicate outside of your lan. eg printers could be VLAN'd to vlan 100 in the IP range 172.16.1.1 /24. Those printers shouldn't have to talk to the outside world (internal email relay if you do scan to email may be needed) but that's a bit of broadcast traffic separated and a bunch of IP addresses saved from your stingey LA. You get the idea anyway I hope.

Edited January 18, 2018 by gh5000

[FN-GM]

As long as your /24s are in the same range as your /22, your LEA shouldn’t need to change routing.

 

This is correct. As long as you don't go outside of that original /22 you will be fine.

 

Just remember to set the gateway of last resort / default route to point to your LA router.

 

In the Cisco world: IP route 0.0.0.0 0.0.0.0

Edited January 18, 2018 by FN-GM

[gh5000]

This is correct. As long as you don't go outside of that original /22 you will be fine.

 

Just remember to set the gateway of last resort / default route to point to your LA router.

 

In the Cisco world: IP route 0.0.0.0 0.0.0.0

 

More than happy to be wrong but can you explain why the LA router doesn't need to know about his VLAN setup on the core switch.

 

For me I've always had to set a equivalent route on the router.

ie

ip route 10.100.100.0 255.255.254.0 10.10.10.5 (vlan 100)

ip route 10.200.200.0 255.255.254.0 10.10.10.5 (vlan 200)

where 10.10.10.5 is the IP of the core switch that is handling the routing.

[FN-GM]

More than happy to be wrong but can you explain why the LA router doesn't need to know about his VLAN setup on the core switch.

 

For me I've always had to set a equivalent route on the router.

ie

ip route 10.100.100.0 255.255.254.0 10.10.10.5 (vlan 100)

ip route 10.200.200.0 255.255.254.0 10.10.10.5 (vlan 200)

where 10.10.10.5 is the IP of the core switch that is handling the routing.

 

 

Routers don't need to know how to VLANs are broken down. They just need to know how to get traffic onto the next hop (in this case the core switch). It will then be up to the core switch to place the correct traffic into the correct VLAN.

 

 

IP Route 10.0.0.0 255.255.255.0 192.168.1.1

IP Route 10.0.1.0 255.255.255.0 192.168.1.1

IP Route 10.0.2.0 255.255.255.0 192.168.1.1

IP Route 10.0.3.0 255.255.255.0 192.168.1.1

 

The above 4 x /24 routes will achieve the same as the 1 x /22 below. In fact the below makes the routing more efficient although that only really matters on very large networks, Cisco call the below a summary route.

 

IP Route 10.0.0.0 255.255.252.0 192.168.1.1

Edited January 18, 2018 by FN-GM

[keithu]

Routers don't need to know how to VLANs are broken down. They just need to know how to get traffic onto the next hop (in this case the core switch).

 

So how will the router know that hosts in the new subnets are on the other side of the routing switch?

[FN-GM]

So how will the router know that hosts in the new subnets are on the other side of the routing switch?

 

There are no new subnet. The OP is just breaking down his existing /22 subnet.

[Blue_Cookeh]

So how will the router know that hosts in the new subnets are on the other side of the routing switch?

 

Because the router knows that anything in the /22 subnet is behind the core switch (or 'next hop'), so it throws any traffic destined for any IP in the /22 at the core switch. The core switch then knows where each IP is on a more granular level (the VLANs) and does it's thing.

 

SchoolsBroadband give us a /20 - they don't care what we do with it, their router just needs to know that our core switch knows what to do next.

[keithu]

Because the router knows that anything in the /22 subnet is behind the core switch (or 'next hop'), so it throws any traffic destined for any IP in the /22 at the core switch. The core switch then knows where each IP is on a more granular level (the VLANs) and does it's thing.

 

SchoolsBroadband give us a /20 - they don't care what we do with it, their router just needs to know that our core switch knows what to do next.

 

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?

[keithu]

So how will the router know that hosts in the new subnets are on the other side of the routing switch?

 

Because the router knows that anything in the /22 subnet is behind the core switch (or 'next hop')

 

My question is HOW does it know? (And just to clarify, I'm talking about the /24 subnets in the original question.)

[Norphy]

The remote router doesn't know. All it knows is all addresses in that address block go to the core switch at the school. The core switch at the school then knows what to do with it and forwards stuff onto the appropriate place.

 

As far as the remote router is concerned, there is no difference between

 

192.168.0.0/22

 

and

 

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

Edited January 19, 2018 by Norphy

[FN-GM]

My question is HOW does it know? (And just to clarify, I'm talking about the /24 subnets in the original question.)

 

It doesn't need to know how the subnets are broken up. That is the job of the core switch.

[keithu]

The remote router doesn't know. All it knows is all addresses in that address block go to the core switch at the school. The core switch at the school then knows what to do with it and forwards stuff onto the appropriate place.

 

As far as the remote router is concerned, there is no difference between

 

192.168.0.0/22

 

and

 

192.168.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

 

I may be misunderstanding what you get with Schools Broadband.

In your example then, don't they provide you with an on-premises router sitting at 192.168.0.1 (for sake of argument)?

 

If so, what address would the switch interface be on (hypothetically)?

[Norphy]

Think of it as a postal sorting office in, say Glasgow. They get a load of mail for the SE and MK postcodes. The postal sorting office in Glasgow doesn't need to know where exactly the mail to those postcodes is going, it just needs to send it to the appropriate sorting office down south. It's then down to the sorting offices in SE London and Milton Keynes to sort it into SE1, SE2, SE3, MK1, MK2, MK45 etc then again to individual post offices to deliver the mail after that.

[Norphy]

I may be misunderstanding what you get with Schools Broadband.

In your example then, don't they provide you with an on-premises router sitting at 192.168.0.1 (for sake of argument)?

 

If so, what address would the switch interface be on (hypothetically)?

 

Assuming the core switch is a L3 switch, any IP address in that subnet. If the remote router then sends traffic to 192.168.1.42, the router still forwards it to the 192.168.0.x address and the core switch forwards it to the appropriate subnet.

[keithu]

Assuming the core switch is a L3 switch, any IP address in that subnet. If the remote router then sends traffic to 192.168.1.42, the router still forwards it to the 192.168.0.x address and the core switch forwards it to the appropriate subnet.

 

ok, let's look at a concrete example then:

 

ISP is routing 192.168.0.0/22

ISP router is at 192.168.0.1

Our L3 switch is at 192.168.0.2

PC is at 192.168.1.42

 

Router send a packet to PC - no problem.

 

 

Now we define a VLAN on a spare switch port and update the switches routing table to route 192.168.1.0/24 via that interface.

We plug PC into the newly configured port.

 

Router sends a packet to PC (192.168.1.42).

What do you think will happen?

[FN-GM]

The title of this thread is quite ironic now!

[keithu]

ok, let's look at a concrete example then:

 

ISP is routing 192.168.0.0/22

ISP router is at 192.168.0.1

Our L3 switch is at 192.168.0.2

PC is at 192.168.1.42

 

Router send a packet to PC - no problem.

 

 

Now we define a VLAN on a spare switch port and update the switches routing table to route 192.168.1.0/24 via that interface.

We plug PC into the newly configured port.

 

Router sends a packet to PC (192.168.1.42).

What do you think will happen?

 

Since I seem to have killed the thread, I'll answer my own question

 

The 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.

That means it can send a packet directly to the PC once it has found its MAC address.

It will broadcast an ARP request for 192.168.1.42 on the directly attached LAN segment in order to discover PC's MAC address.

The PC will not receive the broadcast because it's no longer on that LAN segment so it will not reply.

The router will wait for the ARP reply but won't receive it.

The router will give up and discard the packet.

 

That's what I think will happen.

As I seem to be in a minority of one I'd be interested to hear where you think I've gone wrong.

[FN-GM]

Since I seem to have killed the thread, I'll answer my own question

 

The 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.

That means it can send a packet directly to the PC once it has found its MAC address.

It will broadcast an ARP request for 192.168.1.42 on the directly attached LAN segment in order to discover PC's MAC address.

The PC will not receive the broadcast because it's no longer on that LAN segment so it will not reply.

The router will wait for the ARP reply but won't receive it.

The router will give up and discard the packet.

 

That's what I think will happen.

As I seem to be in a minority of one I'd be interested to hear where you think I've gone wrong.

 

I think you're getting mixed up between a router and a switch. They don't do the same thing. The router will literally move traffic to the next layer 3 device (that be a layer 3 switch, another router, firewall etc). The LA router in the OP Setup will direct network traffic between the schools L3 core switch and the LA network. The LA router will not talk directly to the PC, the core switch will be doing that.

 

Do you have Cisco packet tracer? I can knock something up to demo this for you.

[keithu]

I think you're getting mixed up between a router and a switch. They don't do the same thing. The router will literally move traffic to the next layer 3 device (that be a layer 3 switch, another router, firewall etc). The LA router in the OP Setup will direct network traffic between the schools L3 core switch and the LA network. The LA router will not talk directly to the PC, the core switch will be doing that.

 

Do you have Cisco packet tracer? I can knock something up to demo this for you.

 

I don't need a demo thanks, just an explanation.

 

Just answer this:

 

In the first part of my scenario there is just the ISP's router on the LAN. In the second part of the scenario we introduce another router by enabling L3 routing on the switch.

How do you suppose the ISP's router knows there is now a second router on the LAN?

[Norphy]

It doesn't need to know. All the ISP's router needs to know is where to send traffic for a specific IP address range. It's then down to the router or L3 switch at the next hop to determine what to do with that traffic.

[keithu]

It doesn't need to know.

 

If it doesn't know it exists how can it forward traffic to it?

 

All the ISP's router needs to know is where to send traffic for a specific IP address range.

 

Exactly. But it doesn't know we've changed anything on the network so why is it suddenly going to start routing packets via a router it doesn't know about to a host it thinks is on the same subnet and thus doesn't need routing?

Edited January 19, 2018 by keithu

[FN-GM]

Exactly. But it doesn't know we've changed anything on the network so why is it suddenly going to start routing packets via a router it doesn't know about to a host it thinks is on the same subnet and thus doesn't need routing?

 

You won't need to change anything and it will still work if:

 

1. You don't subnet can keep within the assigned IP range the router has provided.

2. Subnet within the assigned range (ie break it up). You cannot go outside of the provided range.

 

If it doesn't know it exists how can it forward traffic to it?

 

It does know it exists. For example the OP as a /22 assigned to them, so the router is already configured for this. Although they are sub-netting they are not going outside of this /22

 

 

I don't need a demo thanks, just an explanation.

 

 

Both me and @Norphy have given you one (in fact more than one). No matter how many time you ask the same question, the answer won't change. 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.

Edited January 19, 2018 by FN-GM