Routing my network

[AlexTottman2015]

Hi all,

Overe the past week or so I have been building a network. The network represents two buildings with a number of different departments. Each department has its own VLAN and there are two routers to join the two buildings. I had the whole network working (all departments could communicate) but I had only one router.

I have now added another router and have implemented fiber connection between. I have set up the sub interfaces on each router but now the two sides of the network can't communicte.

Can anyone help with diagnosing why I can't send packets to host on other side of the network?

I have attached my PKT file so you can see what I am talking about.

Any help would be greatly appreciated.

Thank you!!!!

(I have now attached pkt file in new packet tracer if it helps?)

[Giuseppe Larosa]

Hello Alex,

the title of your thread is appropriate !

After having added the second router, you have added on the second router subinterfaces for the Vlans in the second building?

(You need to shut down the subinterfaces now used in RB on RA.)

Now that you have two routers and a routed link between, you need to inform each router of what networks are reachable at the other end of the inter-router link.

Before, with a single router all was happening in a single box.

Now you have two options:

a) use static routes describing what subnets are reachabled on the other end of the inter router link

Example

RA - RB link 10.10.12.0/30

RA uses 10.10.12.1 address RB uses 10.10.12.2 address

on RA you need to configure:

ip route 10.20.10.0 255.255.255.0 10.10.12.2    (we suppose network 10.20.10.0/24 is in second building served by RB)

repeat the command for each subnet

on RB you need

ip route 10.30.10.0 255.255.255.0 10.10.12.1

repeat for each subnet defined in building1

b)  use a dynamic routing protocol like OSPF

RA

router ospf 10

router-id 1.1.1.1

network 0.0.0.0 255.255.255.255 area 0

RB

router ospf 10

router-id 2.2.2.2

network 0.0.0.0 255.255.255.255 area 0

note: default route for internet requires a different handling.

I recommend using option B because it will be able to cover future new subnets with no changes.

Hope to help

Giuseppe

[AlexTottman2015]

Thank you so much for your reply - can you please clarify what you mean by RB and RA? Just want to ensure I make the correct changes so not to break anything else :)

[Giuseppe Larosa]

Hello Alex,

in my terms RA is the already present router, the one that was able to manage both sites alone.

RB is the new router that you have added.

I haven't access to your PKT file so I made some assumptions.

If anything is not clear feel free to ask.

I provided some examples and the config solution is not complete regarding Internet access.

My assumptions are :

before you had a L2 trunk between the two buildings all Vlans were served at L3 by router RA with its subinterfaces.

Now you have placed the new router RB in second building and connected it to RA over fiber using an IP subnet (10.10.12.0/30 in my example)

You have created on RB subinterfaces for all Vlans used in second buillding  and you have moved configuration of those subinterfaces to RB. On RA corresponding subinterfaces have been shut down or even deleted (this is a key point to avoid RA to think those subinterfaces  are still alive and directly connected to it)

You now deal with two routers and  you need to perform routing between them.

Hope to help

Giuseppe

[AlexTottman2015]

I have attached screenshot of my network. Does it make a difference that I am using multilayer swicth for both sides? This is where vlans are created for each side. For the new router I have created the sub interfaces and removed them from previous router. I have assigned an ip for right router as 172.16.1.254 (gig0/1/0) and ip of 192.16.1.254 for left router (192.16.1.254) (I am aware this is public address:-)) :

[Giuseppe Larosa]

Hello Alex,

the two routers need to share a common IP subnet in order to be able to communicate and to route.

>> Does it make a difference that I am using multilayer swicth for both sides? 

It depends on how you use them if you do not define interface Vlan over them they act like L2 only switches.

This was my understanding.

>> I have assigned an ip for right router as 172.16.1.254 (gig0/1/0) and ip of 192.16.1.254 for left router (192.16.1.254) (I am aware this is public address:-)) :

As I have written before the two routers need to share a common subnet on the link between them otherwise nothing will work.

So for example if right router uses 172.16.1.254/24 on gig0/1/0 the left router can use 172.16.1.253/24 on gi0/1/0.

You have a LAN segment how can ARP work if the two routers are not in the same IP subnet?

Hope to help

Giuseppe