Configuring intervlan routing and help deciphering "show ip route" results

[Darshana Diyagama]

Hello,

Hopefully someone can help me with my novice query.

I'm using GNS3 to simulate a simple network topology and to test my basic cisco IOS configuration skills.  Please refer to the attached image for the network topology I'm simulating and a brief overview of my configuration.

I'm simulating five sites where layer 3 switches are installed.  I've allocated each site with the following IP address ranges:

Site A: 10.254.4.0/24 (Native VLAN: 104)

Site B: 10.254.6.0/24 (Native VLAN: 106)

Site C: 10.254.8.0/24 (Native VLAN: 108)

Site D: 10.254.10.0/24 (Native VLAN: 110)

Site E: 10.254.12.0/24 (Native VLAN: 112)

I want to be able to ping hosts on Site B from the hosts on Site A.  So I've configured SVIs on switch 1 and switch 2 (with an IP on each others subnet) to enable this.  Also I wanted to configure routing so if the direct connection between Switch 1 and Switch 2 is disconnected (shown in green) hosts on Site A can still ping hosts on Site B via the longer route.  So I configured SVIs on each switch with the neighbors subnet.  I enabled RIP version 2 to trial this configuration:

IP Routing

Router RIP

Version 2

No Auto Summary

Network: 10.254.4.0

Neighbour: 10.254.6.241

Neighbour: 10.254.12.241

Is this the correct way to configure routing on such a topology?

This configuration seems to work OK in some instances (sometimes the Pings from Host 1 to Host 3 seemed to timeout).

The show IP route results show multiple routes to a specific destination.  E.g. from Switch 1:

C       10.254.12.0 is directly connected, Vlan112

R       10.254.10.0 [120/1] via 10.254.12.242, 00:00:24, Vlan112

                   [120/1] via 10.254.12.241, 00:00:09, Vlan112

                   [120/1] via 10.254.6.243, 00:00:12, Vlan106

R       10.254.8.0 [120/1] via 10.254.12.242, 00:00:25, Vlan112

                  [120/1] via 10.254.6.243, 00:00:12, Vlan106

                  [120/1] via 10.254.6.241, 00:00:04, Vlan106

                  [120/1] via 10.254.4.242, 00:00:04, Vlan104

C       10.254.6.0 is directly connected, Vlan106

C       10.254.4.0 is directly connected, Vlan104

Can someone help me decipher these results please?

Thanks

Dash

[Jon Marshall]

Dash

From your routing table one of the next hops for 10.254.8.0/24 is 10.254.6.243 but i can't see that anywhere on your diagram ?

It's a bit difficult to read the diagram ie. i saved it but enlarging it makes it blurry.

In answer to your general question you can set it up this way but a more common approach would be -

1) each switch has a L3 vlan interface (SVI) for it's own local vlan(s).  So any vlans with clients in it on the switch would only be local to this switch.

2) then you use a separate routed connection between each switch. The way you have done it is to extend a local vlan between a pair of switches eg.

site A and site B both have an SVI for vlan 106 even though vlan 106 is really local to site B.

Using routed links would also make the routing table somewhat easier to read ie. each switch would have RIP learnt routes for all the non local vlans whereas at the moment they don't because each switch is directly connected to a pair of vlans (or three for the middle switches).

I'm not sure whether GNS3 will support routed links but if it does the way to connect the switches would be -

1) for each switch to switch connection you would need a /30 subnet. So as an example use a new 10.254.x.x0/24 subnet and break it down into /30s.

2) using site A and site B switches as an example -

site A

=====

int fa0/0

no switchport

ip address 10.254.x.1 255.255.255.252

site B

=====

int fa0/0

no switchport

ip address 10.254.x.2 255.255.255.252

note that there is no need for a vlan as this is a routed link.

You would then need to add that subnet to your RIP config.

If GNS3 does not support routed ports then you can emulate them by using a dedicated vlan (ie new vlan) just for the switch interconnect. You would create an SVI on each switch for that vlan and apply the addressing directly to the SVIs and again add to RIP configuration. The ports at each end of the interconnect would be placed into that new vlan ie. no trunk needed.

Each switch interconnect should use a different vlan for the interconnect to another switch.

With the above each switch peers with the other switch (or switches) using a separate L3 connection.

All that is not to say your config is wrong so please don't take it the wrong way and if you want we can stick with this setup.

So if you want to stick with the original setup can you answer the first question about one of the next hops ?

Jon

[Jon Marshall]

Dash

Just a followup point.

My original post was based around the fact that each switch represented a separate site so i recommended L3 interconnects.

But the title of your post implies you want specifically to test inter vlan routing and this is generally considered to be between SVIs on L3 switches. So really inter vlan routing is most often see on the same L3 (or a pair) switch for clients vlans local to that switch ie. not between sites.

However you may well want to stick with your setup. If so bear in mind you do not need trunk links between the switches only the shared vlan needs to be allowed across per switch interconnect.

Jon

[Darshana Diyagama]

Hi Jon,

Thank you for your reply.  With regards to your original reply, sorry about the quality and the accuracy of the image.  I had a couple of incorrect IP addresses on it.  I've attached a PDF version of the drawing.  Basically 10.254.6.243 was the SVI IP configured for VLAN106 on Switch 3.

I will try your intial suggestion on GNS3.

Thanks again,

Dash