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Subnet design

I have 6 Hub-sites where we currently have a flat network with PIX 525, Catalyst 6513(Backbone local switch) and a Cisco 3660 (WAN router) connected to the same subnet. All the routing is performed by 3600 at this point. We are installing two Sup720 into Cat 6513 and introducing L-3 switching on the core 6513. We are going to change the Def.Gw. from 3600 to 6513. In the near future we will introduce another WAN router for the failover. We will use EIGRP as an IGP between 6513 and 3600. Is it recommended to create a new subnet between 6513 and the 3660 to isolate the EIGRP traffic and use it as administrative subnet in the future?


Re: Subnet design

I would recommend connecting the 3600 to the 6513 as a routed link (configure 'no switchport'). This way a link or power failure on the 3660 will be recognized immediately by the 6513, rather than waiting for the EIGRP hold-time to expire. The second router you add can connect the same way ('no switchport'). With this configuration it will also be easy to tune the EIGRP metrics such that the 6513 sees the other router as a feasible-successor to the 3660, or vice versa.

With routed links and feasible-successors you can achieve sub-second convergence in a properly configured EIGRP network.

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New Member

Re: Subnet design

Thank you for the answer. Do you recommend to move this link (6513-3600) to a separate subnet as both 6513 and 3600 are on the same flat primary subnet as the servers and workstations today. If yes, what are the advantages of this solution?


Re: Subnet design

When you configure a port on the 6500 as routed with 'no switchport' it becomes a Layer3 interface. Therefore this individual port becomes its own IP subnet and you will need to configure a IP address on this interface.

See the example config and attached diagram:


interface gig 3/1

no switchport

ip address

description Link to 3660

interface gig 4/1

no switchport

ip address

description Link to Second Router

The advantage of this is that link/router failures will be seen at Layer3 immediately and the network will converge much faster. If the 3660 dies, int gig 3/1 will go down and immediatley invalidate the EIGRP neighbor and routes pointing out that interface. If the 6500 and 3660 were connected via a VLAN, and gig 3/1 was a regular switchport assigned to this VLAN, a failure would not be detected right away. The EIGRP hold time would have to expire before the 6500 recognized that the 3660 is no longer available.

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