Redundancy - adding 2nd inside router port.

Answered Question
Aug 5th, 2009

Hello,

We are upgrading our main router from a 3725 to a 3825. We've added a GBE HWIC to have 2 inside connections to our core switch stack (2 x 3750G). We are currently running 'router on a stick' with 2 vlan subinterfaces on the 3725 - one for voice and one for data.

In moving to 2 inside interfaces, I'm not sure how to configure the redundant ports.

Do they need to be set up as etherchannel?

Do vlan subinterfaces need to go on both inside ports?

Thanks,

Rob

I have this problem too.
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Correct Answer by Jon Marshall about 7 years 5 months ago

"it's my understanding that with 2 connections from the 3750's to the 3825, most routing protocols will automatically load balance connections with the same administrative distance, is that correct?"

Yes, the 3750's will see 2 equal cost paths to all destinations beyond the 3825 router and will use both paths, and the 3825 will see 2 equal cost paths to all the vlans on the 3750's and will use both.

I recommend using a dynamic routing protocol if possible but it does depend on your feature set on the 3750's. If you only have the IP Base image then EIGRP/OSFP are not supported but you can just add 2 static routes on each device and you will still use both links.

Jon

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bflseanny Thu, 08/06/2009 - 07:34

I'd like to try to help.

I am a little unsure of what you are attempting to accomplish. Perhaps you could provide a schematic.

Are you trying to create redundancy to the router from the swithces so that if one port on the router goes bad than traffic will still flow to the remaining port?

If that is the case, I think that etherchannel would work. Then, you'd configure the etherchannel "interface" the way you would a normal interface - with subinterfaces and all.

What are your thoughts?

rcoote5902_2 Tue, 08/11/2009 - 09:10

Yes, I'm trying to create some redundancy on the inside interfaces of the 3825. We purchased the 1GE-HWIC card for the 3825 to add a third GBE port and would like to do something like this:

3825

| |

| |

3750-3750

We'd previously been using our old router for all layer 3 functions - inter-vlan routing, etc. I will likely now switch to the 3750 stack as the gateway and have a default route out to the 3825 so I will no longer have subinterfaces on the router.

So is this accomplished only through etherchannel? I can't have 2 inside ports with the same IP, obviously so I'm unclear on how to set this up. (the design was recommended by a consultant)

kwillacey Tue, 08/11/2009 - 09:43

the only way i can see this working is if you have a switch card in the router. that way you can create a separate vlan/segment between the router and the switch and you will then have a vlan interface on the router that the switches would point to and you can connect the router to both switches, hth.

Jon Marshall Tue, 08/11/2009 - 10:33

If you are doing all the inter-vlan routing on the 3750's then i would simply use 2 P2P routed links from the 3825 to the 3750 switches eg.

3825

int gi0/1

ip address 192.168.5.1 255.255.255.252

int gi0/2

ip address 192.168.5.5 255.255.255.252

3750_1

int gi0/1 <-- (connected to int gi0/1 on 3825)

no switchport

ip address 192.168.5.2 255.255.255.252

3750_2

int gi0/1 <-- connected to gi0/2 on 3825)

no switchport

ip address 192.168.5.6 255.255.255.252

then you run a routing protocol between the 3750's and the 3825 or you can simply use static routes.

Jon

rcoote5902_2 Tue, 08/11/2009 - 11:37

Thanks Jon for the help.

It's my understanding that with 2 connections from the 3750's to the 3825, most routing protocols will automatically load balance connections with the same administrative distance, is that correct?

jfraasch Tue, 08/11/2009 - 12:06

Is there a reason HSRP or VRRP are not considered? Otherwise this would seem to be the most obvious solution.

James

Correct Answer
Jon Marshall Tue, 08/11/2009 - 16:23

"it's my understanding that with 2 connections from the 3750's to the 3825, most routing protocols will automatically load balance connections with the same administrative distance, is that correct?"

Yes, the 3750's will see 2 equal cost paths to all destinations beyond the 3825 router and will use both paths, and the 3825 will see 2 equal cost paths to all the vlans on the 3750's and will use both.

I recommend using a dynamic routing protocol if possible but it does depend on your feature set on the 3750's. If you only have the IP Base image then EIGRP/OSFP are not supported but you can just add 2 static routes on each device and you will still use both links.

Jon

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