Solved: 3750 stack - what if one dies?

tylerlucas · ‎12-29-2010

Suppose two 3750's are connected in a stack configuration - SwitchA and SwitchB.

If I have a router connected to g0/0 on SwitchA, and then SwitchA dies, I'm out of luck - connection to the router is lost. What's the best way to add redundancy here? Is it to have a second router, which connects to SwitchB, and a routing protocol or HSRP?

milan.kulik · ‎12-30-2010

Hi,

if they don't, why don't you configure an L3 interface on each 3750 in the stack and connect it to a separate L3 interface on your router?

(As you were hoping to keep all connections L3 (/30) originally.)

Running EIGRP or OSPF would enable load-balancing over that two parallel L3 connections.

If you want to avoid a single point of failure in your network, you can use two routers instead of one, running HSRP/VRRP/GLBP on interfaces connected to the client subnets/VLANs. But those details depend on your LAN details.

HTH,

Milan

View solution in original post

Reza Sharifi · ‎12-29-2010

tylerlucas wrote:

Suppose two 3750's are connected in a stack configuration - SwitchA and SwitchB.

If I have a router connected to g0/0 on SwitchA, and then SwitchA dies, I'm out of luck - connection to the router is lost. What's the best way to add redundancy here? Is it to have a second router, which connects to SwitchB, and a routing protocol or HSRP?

You can connect the 3750 stacked (one from each switch) to 2 different routers and run HSRP or VRRP. So the the 3750s are just layer-2 and the user gateway is on the router. VRRP or HSRP provide gateway redundancy for you

HTH

Reza

tylerlucas · ‎12-29-2010

Hi Raza,

Thanks for the reply.

The 3750's will be the core of our network at our datacenter. I originally was hoping to keep all connections L3 (/30). Would it make more sense to to have another pair of stacked switches between the 3750's and the routers, running a L3 interface to the 3750's, and L2 interfaces to the routers?

What is best practice here?

Edit: The reason I propose that is so that the router can still form an EIGRP neighbor relationship with the 3750 stack.

andtoth · ‎12-29-2010

Hi,

You can also connect the router via a cross-stack etherchannel (by connecting one member link to each member) to the 3750 switch stack.

The other option you have is to use the 3750 stack as the router. It can do IP routing in hardware (line rate) and it also supports routing protocols, like EIGRP. Also, it supports HSRP in case you have 2 different stacks or want to run HSRP between the stack and a router. Upon a hardware failure of one member, the stack will continue to operate with the remaining members.

However, the 3750 does not support NAT so you will need a router or a 6500 switch if required.

Best regards,

Andras

tylerlucas · ‎12-29-2010

Hi Andras,

I don't believe ISR series routers support etherchannel. Am I wrong? That seems like it would be a good solution.

milan.kulik · ‎12-30-2010

Hi,

if they don't, why don't you configure an L3 interface on each 3750 in the stack and connect it to a separate L3 interface on your router?

(As you were hoping to keep all connections L3 (/30) originally.)

Running EIGRP or OSPF would enable load-balancing over that two parallel L3 connections.

If you want to avoid a single point of failure in your network, you can use two routers instead of one, running HSRP/VRRP/GLBP on interfaces connected to the client subnets/VLANs. But those details depend on your LAN details.

HTH,

Milan

tylerlucas · ‎12-30-2010

Hi Milan,

Thanks for the response.

I like the idea of having multiple /30 connections to the core. Is this a typical setup?

As for the WAN side - it is connected via Ethernet to a private MPLS cloud. Currently, with the one router I have now, the interface is configured with a provided /30 IP. I would like to add a redundant connection here, as well. I'm aware that the likelyhood of the circuit going down entirely is much greater than the ethernet hand-off, but this way we have redundant routers and don't have to worry about one dying. Any suggestion here? I would do (MPLS facing) HSRP if I didn't have a /30, maybe that's something that can be changed?

milan.kulik · ‎12-31-2010

Hi,

I'd say having /30 connections in the core is not a typical solution.

You are saving your address space, but it's limiting possible future smooth redesign.

IMHO, a good compromise is using /28 for L3 connections inside DCs - if you need to put more devices to the subnet in future (an IPS, e.g.), it's still possible without readdressing.

Regarding the WAN connection:

While peering via Ethernet to an MPLS cloud, we are using following design in our network:

Two provider CE routers peering to two our CE routers (full mesh) via BGP in one subnet.

If you use two different switches for L2 connection, there's no single point of failure. And due to BGP attributes you can play with routing preferences a lot :-)

HTH,

Milan