Connecting 3 buildings

[hmc2500]

I need to connect 3 buildings with redundancy but want to isolate the routing within each building as much as possible. Each building has a switch however only 2 buildings have layer 3 switches. THe L2 switch (R2) is a 2960 switch. I want to create an L3 link between R1 and R3 but Layer 2 trunks between R1 and R2 and R3 and R2. I guess an L2 trunk link between R1 and R2 should not be a problem since they share the same vlans. Not sure what to do between R2 and R3. How do I get this to work?

R1 (L3 switch)

vlans 1,2,3

interface fa0/1 ip 192.168.1.1 (L3 link beteen R1 and R3)

R2 (2960 L2 switch):

vlans 1,2,3

R3 (L3 switch):

vlans 1,4,5,6

interface fa0/1 ip 192.168.1.2 (L3 link beteen R3 and R1)

[ngkin2010 · Accepted solution]

Hi,

Technical speaking, you can still build the trunk between R3 (VL 1,4,5,6) and R2 (VL 1,2,3) even through they use different VLAN. At a result, only VLAN 1 would use the trunk between R3 & R2. Therefore, VLAN 2,3 cannot have the benefit of network redundancy (E.g. when R1 is down, the VLAN 2,3 on 2960 would also down). So, you could create VLAN2,3 (not SVI, just VLAN) on R3 as well to solve this single point of failure.

Beside, I see you emphasized the "layer-3", may I know if your L3 switch is acting as network gateway for those VLANs? If true, you should consider to use HSRP / GLBP / VRRP. After that, you need to check if there is any single point of failure. (E.g. enabled with GLBP, and if the link between R1&R3 has broken, is there any connectivity issue to outside network? ) Or to play safe, you could consider to setup trunk between R1 & R3 as well.

[hmc2500]

Thanks. Yes, R1 and R3 act as gateway for their own vlans. R1 and R3 do have SVI's created already for their vlans. The initial goal was to have L3 switches in all 3 buildings to contain routing or the local vlans within each building, however due to financial reasons this has changed. 

HSRP / GLBP / VRRP. would require us to host all the vlans and both R1 and R3. And it does not really confine routing within one building. 

Confiuring a trunk between R1 & R3 would make it a layer 2 domain again.

[ngkin2010]

Hi,

When you designing the network topology, you should try to avoid the single point for failure on your network, which you should think about:
- What will happen if your gateway (R1) is down?
- What will happen if your inter-site connection is down?

The VLAN 2-3 on 2960 switch have no routing ability, then they must choose either R1 and R3 to be network gateway.
With the current configuration and:
- if you choose R1, that's fine because as you said R1 contains the VLANs' SVI which could be used as the gateway.
- if you choose R3, that's not work:
---- a) the trunk link between R2 and R3 would not allowed to carry VLAN2-3 traffic because VLAN2-3 are not created on R3.
---- b) the VLAN2-3's SVI are not created on R3.
---- you need to the above 2 things before you choose R3 to become your network gateway for VLAN2-3 on 2960.

At this moment, if you choose R1 and if it down, your routing domain of VLAN 2-3 are down. Vice versa, if you choose R3 and if it down, your routing domain of VLAN 2-3 are down. There is no redundancy! That's why you need HSRP/GLBP for VLAN 2-3.

After you configured HSRP/GLBP on R1 & R3, either one of it is down, the other one would take over the gateway responsibility.

Now, you should think what will happen if the inter-site connection is down. For example, if the link between R1 & R3 is down, and R3 is your primary gateway for VLAN 2-3. Then, VLAN 2-3 would lost the connectivity to the subnets on/behind R1.

[Leo Laohoo]

I believe this is school work/assignment. 

If this is the case, please state so because you need to accept that answers provided may not be acceptable to your class instructor.

[hmc2500]

This is not a school assignment and not high priority.