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OSPF Stuck in EXSTART/BDR when using NAT

Hi All,

I am hoping someone can help me with this problem because I am stumped.

I have two routers, R1 which is a 2821 and R2 which is a 1841 in a router on a stick configuration.

I am trying to setup OSPF between the routers but both routers are stuck with the neighbor status EXSTART/BDR.

I have done some research on this issue and most of the time this issue is caused by a MTU mismatch on the router interfaces.

I have checked both routers interfaces and they both using an MTU of 1500 bytes and I am not getting any MTU mismatch messages when I do a debug ip ospf adj.

Further to this I have checked for MTU issues with ping x.x.x.x df-bit size 1500 on both routers.

I have narrowed down the issue to a problem with NAT. As soon as I remove "ip nat outside" from GigabitEthernet0/1 on R1 the issue disapears and OSPF neighbor status changes from EXSTART/BDR to FULL/DR

The strangest part about all this is I was using a 2801 previously for R2 and I had no issues at all. I have even tried a few diffrent IOS versions to rule out any IOS bugs.

R1 Config

interface GigabitEthernet0/1

ip address

no ip redirects

no ip unreachables

no ip proxy-arp

ip nat outside

ip virtual-reassembly in

duplex auto

speed auto


interface Vlan1

ip address

ip nat inside

ip virtual-reassembly in

ip nat inside source route-map INTERNET interface GigabitEthernet0/0 overload

ip nat inside source route-map LAB interface GigabitEthernet0/1 overload

access-list 1 permit

route-map INTERNET permit 10

match ip address 1

match interface GigabitEthernet0/0

route-map LAB permit 10

match ip address 1

match interface GigabitEthernet0/1

router ospf 1

passive-interface default

no passive-interface GigabitEthernet0/1

no passive-interface Vlan1

network area 0

network area 0

R2 Config

interface FastEthernet0/1.101

encapsulation dot1Q 101

ip address


router ospf 1



redistribute bgp 65133 subnets

passive-interface default

no passive-interface FastEthernet0/1.101

network area 0

Hall of Fame Super Blue

Re: OSPF Stuck in EXSTART/BDR when using NAT

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