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Connect 2821 Router to two different switches on the same network

I have a Cisco2821 Router and 2 cisco 2960G switches.  I would like to connect the router to the two switches (1 connection each) on the same LAN Segment/Subnet.  I am wanting this to provide fault tolerance on my LAN side so if one switch goes down my devices can still hit the router via the second switch.  Is this possible and if so how should i go about configuring this.
Avatar of achapman26

You can connect both switches to the router as it will have 2 FA ports. But anything connected to a switch that goes down(physically), will go down no matter what. You can connect the 2 switches together via a TRUNK though.
achapman26 is right anything connected to the downed switch will not be able to connect. To resolve this  connect the devices to both switches and use a trunk between the switches. This will allow for the most failover.

Avatar of fluidiqsit


Physically connecting of course is not an issue.  What we are trying to accomplish is redundancy from the router to the switches. So if any one of the two switches go down - a device (i.e. server w/ dual nics) can still hit the gateway (router).  

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Avatar of achapman26

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On the 2821 your two ethernet ports will be have "switchport mode on". You want your two FA ports to act as switchports, not router ports. Have the two FA ports be members of VLAN1 and assign an IP address to VLAN1. Like, or whatever subnet you have on that LAN.

The two FA ports on the 2821 would each connect to one of the switches. Likewise on the server one of the FA ports would each connect to one of the switches.

The default gateway of your server would be
Regardless of whether a switch or port fails as long as the other switch is running correctly switching should update its CAM table and you should be able to reach the gateway.

There will probably be some configuration required on your server, I couldn't tell you what because I don't know what kind of software your running on it.

That's how I would probably setup what your trying to do. Hope this helps.
If the switches are only being used for connectivity to that server then I wouldn't use them at all, it would be wasting two very powerful peices of hardware. If you just need to ensure reliable connectivity to the server only, then I would just connect the server directly into your 2821 using the method I specified earlier completely bypassing the switch. If you are actually running a LAN on those switches and have devices connected to them then you may not want to have an entire switch in a "standby mode". And to be honest, I'm not even sure how you would place an entire catalyst switch into a standby mode. Such a command is news to me. But honestly it seems like a waste of 1600 bucks.

If the LAN segment is being used and devices are plugged into those switches then you would want to do what achapman said and have the two switches be connected via crossover cable. I don't like to say "trunking" because were not dealing with VLAN's. You arn't trunking anything, your just connecting to devices. If you wanted to use port aggregation you could setup etherchannels and have several crossver cables connect each switch. This will increase the bandwidth between the two and allow for fault tolerancy.

You would still want to setup your 2821 in the manner I stated. The two FA ports on your 2821 router need to be in a switchport mode and they need to be assigned to a VLAN. Probably VLAN1 by default. VLAN needs to have an IP addressed assigned to it. You access it just like any other interface in the CLI.

"router#config t"
"(router config)interface vlan1"

Any how, the VLAN1 ipaddress will be the default gateway for all devices on your LAN. This allows you to have 1 default gateway but your router would provide 2 ports to access that gateway.

Normal layer 2 switching will do the rest of the work for you over the LAN and over the other switches. The CAM tables in your switches will auto update. If you do use my method and have your two switches connected for extra redundancy (which I recommend) then you should make sure STP is running, I believe STP(Spanning Tree Protocol) runs by default on the catalyst. You would have created a loop between the two router switch ports and your two switches. For those of you who don't see the loop, the two switch ports on the router would be the second point of connection between the two switches.

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There are ton's of ways a catalyst can offer excellent fault tolerance, STP and RSTP or port aggregation and more. I  would do something that justifies the cost of a 1600 dollar piece of hardware or don't use it at all. Save it for another place in your network.

Having an actual switch processor fail is fairly rare in my experience, typically what you see is a failed ports, which is solved again with the solutions I Just mentioned. RSTP, STP, Port aggregation, ect....
Question would the router see each interface as a switch port then only one port would be active via spanning tree?