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Explorer

Basic question on trunking between router and 2 distribution switches

Hello folks,

Imagine I am designing  a small network with a C2900 router running OSPF and in the future BGP with service provider. Please see attached diagram.

The router is connected to (2) C3750 Layer 3 distribution switches. Then one C3560 layer 2 switch to serve future IP phone users and desktops.

Question:

a)If I connect the router interface to the (2) 3750 switches, if I make the router interface fa0/0 and fa0/1 as a trunk to accomodate VLAN 200 and other future VLANs, don't I have a problem with overlapping networks between router interface fa0/0 and fa0/1? 

b)Alternatively, I could make the Router1 fa0/0 and fa0/1 configured with IP addresses and advertised in OSPF. Then the SW1 fa0/24 and and SW2 fa0/24 I could make as 'no switchport and create a routed interface port' with IP addresses, also running OSPF. Question is, from a scalable design perspectie, would you create 2 management networks and use those when assigning the IP addresses for Router1 fa0/1 and fa0/2? Because again Router1 fa0/1 and fa0/2 obviously need to be placed on different networks to avoid overlapping. So my question is more about proper network planning design to make this scalable to accomodate future VLANs in the future.


Using trunks between the Router1 and SW1 option:

Router1

int fa0/0

   description connection to SW1

   no ip add

   int fa0/0.200

     encapsulation dot1q 200

   ip add 10.0.0.1 255.255.255.0 (So don't I have a problem here with overlapping with fa0/1?)

int fa0/1

   description connecto to SW2

   no ip add

  int fa0/01.200

    encap dot1q 200

   ip add 10.0.0.2 255.255.255.0

Then on SW1 and SW2 port fa0/24 (This part I am OK).

  int fa0/24

   description connected to Router1

   switchport mode trunk... etc

3 REPLIES 3
VIP Expert

Basic question on trunking between router and 2 distribution swi

Hi,

a)If I connect the router interface to the (2) 3750 switches, if I make  the router interface fa0/0 and fa0/1 as a trunk to accomodate VLAN 200  and other future VLANs, don't I have a problem with overlapping networks  between router interface fa0/0 and fa0/1? 

As you already know you can't assign the same subnet to two different interfaces on the router.

b)So my question is more about proper network planning design to make this scalable to accomodate future VLANs in the future.

Since the switches are layer-3, you would need to run VRRP or HSRP on them for redundancy.  You would also need to use each link as a routed port connecting them to the router with OSPF.  So, in the feature if you need to add more vlans, you can simply add the layer-2 vlans to the bottom switch and SVIs with VRRP or HSRP to the 3750s and whatever OSPF area you pick.

HTH

Contributor

Re: Basic question on trunking between router and 2 distribution

Hi there,
The alternative b would be my favourite because you would have layer 3 control on the core-to-distribution links but the solution a is viable as well and you could simply port channelise fa0/0 and fa0/1 so to assign a unique IP address to your logic po1 interface. By the way remember that a small 2900 like the 2901 or the 2911 are not recommended routers for full BGP table and that in any case you should upgrade the ram memory before even think about BGPing with somebody else. Their default is 512 MB RAM while nowadays, to accommodate 400000 prefixes, 2 gb are recommended while 1gb is a prerequisite.

At last, you will never need two management networks unless security requirements or very particular design

Hope this helps
Alessio

Sent from Cisco Technical Support iPad App

Explorer

Re: Basic question on trunking between router and 2 distribution

Hey very good, the alternative b) seems to be the best.

Question:

The IP address between the Router1 and SW1 (routed port interface) would be better to belong to a Management VLAN range? Because if that is the case, I would actually need two "Management" subnets to accomodate the IP addresses for Router1-connection-to-SW1 and from another range to Router1-connection-to-SW2 so I don't have overlapping networks.

Please let me know what you think.

Appreciated.

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