How to configure router to connect to two different stack switch belonging to the same broadcast domain?

[rrairineo1]

Hi all,

I have this design plan for redundancy for our stack switch. i have simulated it and works perfectly fine (using STP). Somehow I notice i cannot simulate the router to switch connection. The routers will be connected to the two stack switches but they all belong to the same broadcast domain. I believe for every interface of the router means a broadcast domain. I want to connect two interfaces of the router to each of the stack switch belonging to the same broadcast domain. how can i do it? or if not, are there any suggestions. For now, i am limited to do changes on the IP allocation that's why i thought of this. please see attached diagram for reference.

thanks and best regards, 

[Mark Malone]

Hi

Do the routers have a switching card hwic module ? if they do you can assign 2 ports in hwic to same vlan at layer 2 and just have the layer 3 vlan on each router and use a fhrp protocol between the 2 routers for failover

[rrairineo1]

Hi mark, unfortunately i dont have switching card hwic module available. if ever i will be buying the said module, can i use link aggregation to the two ports connected to router to different switches? 

[Mark Malone · Accepted solution]

Hi even if you had the card you cant use something like lacp or pgap between 2 routers , only if the devices were vpc or vss would they understand that but you could of had 1 blocking 1 open for redundancy through STP , pos need to be bundled to the same device if they don't have that technology

Another way is just have 1 link back to each switch  , cross connect the routers and run fhrp between them that way if either switch fails or link you still can break out the other router , just an option you could route between the routers and switches as well if there capable

[rrairineo1]

thank you ill go with what you suggested. for now ill have to consider first buying that module. thank you once again. 

[rrairineo1]

"Another way is just have 1 link back to each switch , cross connect the routers and run fhrp between them that way if either switch fails or link you still can break out the other router" - what do you mean here? i am confused. do the two interfaces of the routers belong to the same broadcast domain?

please see attached file.

PS

can't have the vss or vpc since our existing are only 3750s. 

[Mark Malone]

Yes you can have a router on a stick setup with sub interfaces over 1 link on say 2 routers connecting back to one switch with hsrp running on the sub ints , that would split the broadcast domain across 2 routers but only 1 path could be active at a time

[rrairineo1]

Hi, thanks for much needed reply. one more thing, where will i apply fhrp? (router or the 3750 stack switch) i can do hsrp on 3750 right? meaning i will have 2 sets of four hsrp group. (based on the diagram). is this right? 

[Mark Malone · Accepted solution]

If you were to do it with sub ints the hsrp would need to be configured under each sub-interface as that represents the layer 3 breakout broadcast domain for each layer 2 vlan , so the 2 switches would be purely layer 2 behind the routers just trunking and basically your pcs would have a default gateway of the virtual ip address of the hsrp so if either router failed they still have a breakout point , then just add extra redundant links at layer 2 switches use stp to manipulate them as required

[rrairineo1]

Thank you for your expert advice. I think i do get you now. once again, thank you. 

[rrairineo1]

hi sorry to bother you again. I thought i got it. i am trying to simulate this but i wonder since i will be connecting the router to (2) stacked switches i'll be using also 2 interfaces right? each interface will be have subinterfaces depending on how many vlans ill be using. in my example ill use only 2 vlans. 1 interface (router) - 2 subinterfaces for 2 vlans right? but for the other interface - 2 subinterfaces also for the said 2 vlans. but as i am applying ip addresses. i noticed that the two interfaces belonging to router will be of same broadcast domain. how can i do this one. i need to connect each router's two interfaces to each stacked switch for redundancy purposes (when ssw0 fails, ssw1 takeover). what am i missing here? please see attached file. thanks

PS 

can i use ip unnumbered for subinterfaces?

any suggestions aside from packet tracer, what software can simulate the above command?

thanks

[Mark Malone]

Hi sorry your right there's an ip subnet conflict there you can only have 1 link back per routers to the switches to avoid conflict and running an fhrp, are the switches L3 capable you could create point 2 point routed links between switches and routers use eigrp between them , have layer 3 vlans on the 2 switches running glbp and also advertised in eigrp that way the routers and switches are all active links as eigrp can load balance across multiple links and glbp can too and then users would have redundancy between 2 stacks as well and you can have multiple links per router back in the same igp eigrp domain

without a hwic on the router i cant see how you can have 2 routed links same BD back to switches same router as thats the L3 gateway so the router would need to have logical layer 3 interface with layer 2 ports as no ip conflict then, its a design conflict

gns3 with iou is better for lab testing as full images

[rrairineo1]

Hi again, i am planning to buy hwic module for the router. do you think it can work? ill be using hwic interfaces to connect to the two stack switches on the same broadcast domain. is ip unnumbered can be used here? or do you have any suggestions to correct design conflict? our stack switch i think is L3 capable because its 3750 but its primarily used as L2.

[Mark Malone]

hwic will work as ports will be layer 2 on router and there will only be 1 layer 3 interface the vlan interface instead of 2 routed physical ports, with hwic layer 2 ports assigned to trunks to carry vlans and then run hsrp or glbp between the routers under the vlan interface , that way theres 2 links trunk at layer 2 connected to the router and back to the switches and 1 broadcast domain vlan interface so no conflict in ips , uyou dont need ip unnumbered as layer 2 trunks

if its layer 3 you dont have to buy anything though but with that many links will make it bit more complicated to configure but it will work too

[rrairineo1]

hi again, the hwic layer 2 ports when i use it, will it act as switch ports? because i think i can use stp there configuring only the priority so that it will take care of the redundancy from stack switches to the routers? just a thought. please correct me if im wrong. thank you