Layer 3: Switching

Unanswered Question
Mar 9th, 2009

I'm proposing the following layer 3 LAN topology. All connection link will be point to point routed ports (L3). EIGRP routing will run between all LAN switches using a /30 subnet mask.

The problem is the customer want to dual home all servers (AD & IPT servers) to the two core switches over VLAN 60 (6509-1 & 6509-2). Should I use a layer 2 link to carry HSRP traffic between the two switches. (please see diagram).

Will this cause problems with Eigrp routing. Please advise on network design.

I have this problem too.
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Giuseppe Larosa Mon, 03/09/2009 - 06:51

Hello Colm,

yes you need a l2 link to carry the shared vlan

the servers need to use an active/standby strategy because they connect to two different switches.

note: this in part violates the hierarchical network design you should connect the servers to theh server farm functional block however this is possible.

EIGRP will not be a problem it will advertise the shared vlan/subnet to the core.

Traffic coming from the core can also need to go through the interswitch link.

Hope to help


Jon Marshall Mon, 03/09/2009 - 06:51


If you want to connect servers to the core switches and run HSRP then you will have to have a L2 link between the 2 core switches. Ideally you would have your servers connected to 2 access-layer switches which are then uplinked with L3 but i appreciate there is an additional cost to that.

So yes you need a L2 link either an access link in the server vlan or a trunk if there are multiple server vlans.

It won't affect the routing but i would

1) make the vlan interfaces passive under eigrp then no neighborships will be formed over the L2 link ie.

router eigrp 1

passive-interface vlan20

where vlan 20 is the server vlan.

2) If it is a trunk link you will need to

a) only allow the relevant vlans across it

b) make passive all the vlan interfaces for vlans that are allowed on the trunk



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