Routing Issues

[Mikey John]

Hi,

Not sure If i should be posting my query out here as it is a very basic one.

Iam using GNS3 simulator to connect the devices as shown in the diag attached. Iam using a Router on a stick configuration between R1 (3620) and R2(Cisco 3725 with Ether switch module).

On R1, I have created 2 sub-interfaces 200 and 300 on Fa0/0 towards R2.

interface FastEthernet0/0.200

encapsulation dot1Q 200

ip address 10.1.1.1 / 24

!

interface FastEthernet0/0.300

encapsulation dot1Q 300

ip address 11.1.1.1 / 24

On R2 switch, Fa1/1 connecting to R1 is a trunk port and Fa1/2 connecting to R3 is also a trunk port.

interface FastEthernet1/1 --->towards R1

switchport mode trunk

duplex full

speed 100

!

interface FastEthernet1/2 ---> towards R2

switchport mode trunk

duplex full

speed 100

interface Vlan200

ip address 10.1.1.10 255.255.255.0

no ip route-cache

ip route 0.0.0.0 0.0.0.0 10.1.1.1

On R3 router, Fa0/0 interface towards R2 is configured

interface FastEthernet0/0

ip address 11.1.1.5 255.255.255.0

duplex auto

speed auto

From R3, I need to reach the 11.1.1.1 IP address of R1, but Iam unable to ping, What should be the static route on R3 to reach R1? Do I have to make R2 a L3 switch?

Appreciate your help.

Thanks

Mikey

[Mikey John]

Any help on this would be appreciated.

Thanks

Mikey

[cadet alain]

Hi,

you don't need any static route on R3 to reach 11.1.1.1 as this IP address is on the same subnet.

If you are using a RoAS design then R2 should be a L2 switch only so disable ip routing on R2.

on R3 you must also be in the same VLAN as R1 f0/0.300 subinterface as you are in the same subnet so just make f1/2on R2 an access port in VLAN 300. For R3 to ping f0/0.200 then use a static route on R3 pointing towards R1 f0/0.300 IP address.

Regards

Alain

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[Mikey John]

Thanks. Also, would have a 2nd uplink between R3 and R2 make sense? R2 being a L2 switch would block the link anyways. Right?

[cadet alain]

Hi,

yes,you're correct that STP on R2 would block one of these links.But you also couldn't have the second link on R3 in the same subnet(every interface on a router must be in its own subnet) so unless you were doing a L3 port-channel between R2 and R3 which you won't as R2 is now a L2 switch then this  second link will have no purpose.

Regards

Alain

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[Mikey John]

So if I have a /24 subnet to be used between R1 and R3, I can further divide that and use as 2x/29 for both the uplinks.

The 2nd blocked uplink could act as a standby in case fa0/0 on R3 goes down.

But is it worth doing this? What would be the best practice?

Thanks

Mikey

[cadet alain]

Hi,

in that case you would have 2 subnets on R3 and from a /24 you can get 2 /25s but if R2 is still a L2 switch it means you either have 2 access links going to R3 each in a different VLAN and going to each routed interface on R3 which are in different subnets or you use a trunk on R2 for these 2 VLANs and on R3 you have 2 subinterfaces each in a different subnet and VLAN.if you have 2 access links in 2 VLANs then there won't be no blocking( I answered incorrectly about this in my latest reply) as they are on 2 different STP instances.

You'll also need the same /25s on a subinterface on R1 for intervlan routing.

Regards

Alain

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[Mikey John]

Thanks Cadet. Pretty helpful.