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DHCP, VLANs, and NAT

Hi all,

I'm working with an 1811 router, only using one of the WAN interfaces, and have the switchports set to trunk the VLANs coming from two switches. The router is setup to provide DHCP to the devices connecting through the switches and appears to be working; in a test, my laptop got an address (172.16.6.2) in the correct VLAN subnet (172.16.6.0 /24) of the port on the switch that I connected to.

The problem seems to be the NAT; hopefully one of you can tell me for sure, because I can't get out past the router.

The router's VLAN 1 is setup as a 172.16.1.0 /24 network; each subsequent VLAN number increments the network by 1 - VLAN 2 is 172.16.2.0 /24. This is setup on the router and on each switch. Switch one is trunked to switch two, trunked to the one of the router switchports.

Although I can get an address from the router, NAT debugging shows an error and one output I'm not familiar with:

001120: *Nov 4 18:29:19.304 PCTime: %IP-4-DUPADDR: Duplicate address 172.16.6.1 on Vlan6, sourced by 0019.e7b5.5e45

AND

001113: *Nov 4 18:29:04.208 PCTime: NAT: expiring 75.xx.xx.17 (172.16.6.2) udp 1513 (1513)

75.xx.xx.17 is the router's WAN interface.

The DNS server is the ISPs; we're connected to a cable modem that's bridging - it's the default route address (xx.xx.xx.22) you'll see in the router config.

The router and one of the switch configs are attached.

Appreciate your comments.

Laner

4 REPLIES

Re: DHCP, VLANs, and NAT

Laner,

The vlan 6 interface on both the switch and router is configured with the same address (172.16.6.1). Can you change the address or shutdown int vlan 6 on the switch and check the connectivity.

HTH

Sundar

New Member

Re: DHCP, VLANs, and NAT

Sure, so I set the switch's VLAN 6 address to 6.2 - and so on for each VLAN?

Re: DHCP, VLANs, and NAT

Correct.

New Member

Re: DHCP, VLANs, and NAT

This may have helped to some degree, but now my NAT translations seem to be broken:

000166: *Nov 10 10:31:53.814 PCTime: NAT*: Can't create new inside entry - forced_punt_flags: 0

000167: *Nov 10 10:31:53.814 PCTime: NAT: translation failed (A), dropping packet s=172.16.7.4 d=75.xx.xx.xx

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