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New Member

Connecting 2821 to 3750

Hello, I have not been able to connected my 3750 to my 2821 through the Gi0/1 fixed interface on the 2821.  The 2821 has 1 serial interface, 2 fixed Gigabit Ethernet Interfaces and a card with 9 Fastethernet interfaces.  The serial is used to connect to my WAN.  I can connected the 3750 to any one of the ports of the 9 FastEthernet card when i configure it as a trunk.  I set up the Gi0/1 port on the 2821 the same way, the router doesn't see the switch.  I can get the router's mac-address from the switch however.  The 2821 has VLANs on it.  The 3750 has similar VLANs on it.  How do i configure the Gi0/1 for it to see the switch?

here is the current config for desired port:

(2821)

interface GigabitEthernet0/1
no ip address
no ip redirects
no ip unreachables
no ip proxy-arp
ip route-cache flow
duplex auto
speed auto
no mop enabled

Here is the config for the working port

(2821)

interface FastEthernet0/1/3
switchport mode trunk
switchport voice vlan 6
spanning-tree portfast

I did try putting switchport mode trunk on int Gi0/1 but had no joy doing that.

I tried to give that port an ip address but would get overlap errors with VLAN1.

11 REPLIES
VIP Super Bronze

Re: Connecting 2821 to 3750

Hi,

Gi0/1 is routed port and not a switch port.  If you want trunk it with multiple vlans, you need to create a sub-interface for each vlan.

for example

int gi0/1.1

int gi0/1.2

and so on.

Then add the IP address to the sub-interface and use encapsulation dot1Q and the vlan id

HTH

Reza

New Member

Re: Connecting 2821 to 3750

Hello,

I did create the sub-interface.  I wasn't able to assign an IP address to it.

2821-1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
2821-1(config)#int gi0/1.1
2821-1(config-subif)#ip address 10.x.x.6 255.255.255.0

% Configuring IP routing on a LAN subinterface is only allowed if that
subinterface is already configured as part of an IEEE 802.10, IEEE 802.1Q,
or ISL vLAN
.

VIP Super Bronze

Re: Connecting 2821 to 3750

Hi,

You need to add 802.1q trunk before you can assign your IP

HTH

Reza

New Member

Re: Connecting 2821 to 3750

Thanks everyone for your assistance

Still no joy.  Not able to add an IP address on to the interface.  I want the VLAN1 traffic to pass back and forth through this interface and to the connected switch.

2821-1#sho run int gi0/1.1
Building configuration...

Current configuration : 109 bytes
!
interface GigabitEthernet0/1.1
encapsulation dot1Q 1 native
no snmp trap link-status
no cdp enable
end


2821-1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
DET-MDF-2821-1(config)#int gi0/1.1
DET-MDF-2821-1(config-subif)#ip address 10.x.x.6 255.255.255.0
% 10.7.0.0 overlaps with Vlan1

Hall of Fame Super Gold

Re: Connecting 2821 to 3750

G0/1 and G0/2 is a routed port so you need to configure "router-on-a-stick" but your 6-port card is a chop-down version of a 2950 or a 3750, so it's a switch.

Configuring InterVLAN Routing and ISL/802.1Q Trunking on a Catalyst 2900XL/3500XL/2950 Switch Using an External Router
http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a00800949fd.shtml

Re: Connecting 2821 to 3750

did you get the error "Command rejected: [interface] not a switching port" when you applied the trunk config to gi0/1? To change a L3 interface to a L2 switching port, use the command switchport e.g.

interface GigabitEthernet0/1
switchport

switchport mode trunk

hth

New Member

Re: Connecting 2821 to 3750

Hello,

No I did not get that error when I applied the switchport mode trunk.

New Member

Re: Connecting 2821 to 3750

More Information:

Vlan Config on 2821

interface Vlan1
ip address 10.x.x.1 255.255.255.0
no ip redirects
no ip unreachables
no ip proxy-arp
ip route-cache flow
no autostate

Port config on 3750

interface GigabitEthernet1/0/47
switchport trunk encapsulation dot1q
switchport mode trunk
switchport nonegotiate
!
interface Vlan1
ip address 10.x.x.5 255.255.255.0
ip helper-address 10.x.x.24

Re: Connecting 2821 to 3750

Hi,

As per your configuration which you have pasted here, already you have configured vlan 1 as L3 vlan with ip segment 10.x.x.0/24 and the same segment of ip range you trying to assign into fast ethernet sub-interface.

So, As you know,It is not possible to assign same segment of IP range into two different L3 interfaces.

your configuration in the router should be like this

no int vlan 1

int gi0/0

no ip address

int gi0/0.1

encapsulation dot1q 1 native

ip address 10.x.x.1 255.255.255.0 // As per your network range //

Finally you need to remove the ip address in vlan 1. You cannot make the router as switch 100 %. But we can configure router-on-stick method to simulate switch functinality to router.

For more information have a look into this URL, which will clarify more.

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a00800949fd.shtml

regards

Karuppu

New Member

Re: Connecting 2821 to 3750

Hello,

We were using the 2821 with the 9 port ethernet switch card as the branch office router and switch.  We had a need for more ports and aquired a 3750.  On the 2821 we have multiple VLANS set up.  Throughout our domain, the switches all have the same VLANS configured.  When I get this first one configured I will be creating more interfaces and putting VLANS on those interfaces. 

To clarify I need to remove the VLAN1 from the L3.  Do I assign the current IP address of the router to the Gi0/1.1 interface or a different one? Because the  rest of the network is looking for that router (and LAN segment) at the 10.x.x.1 address. 

A question I have here would be would the Gi0/1.1 have the full gigabit bandwidth or will it be reduced by the number of subinterfaces I configure?

Thanks.

Re: Connecting 2821 to 3750

Hi,

Do I assign the current IP address of the router to the Gi0/1.1 interface or a different one? -- Yes. you can assign the vlan 1 ip address into gi0/1.1

And if you want to permit more vlan over this physical interface, create the required number of interfaces and assign those sub-interfaces into the respective vlan.

A question I have here would be would the Gi0/1.1 have the full gigabit bandwidth or will it be reduced by the number of subinterfaces I configure?

Even if you are creating the 10 sub-interfaces in a single gi0/1, the maximum throughput will be 1Gbps. it will be shared into the multiple sub-interfaces.

Hope this clarified to your question. If you need more info, raise your questions.

regards

karuppu

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