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OSPF

I have 3 routers connected to a switch. The routers have loopback addresses along with Ethernet addresses.

Why does the router with lowest loopback address and lowest Ethernet address shows up as the DR?

Thanks.

SanJose1#sh ip int br

Interface IP-Address OK? Method Status Protocol

Ethernet0 193.168.1.1 YES manual up up

Loopback0 193.168.31.11 YES manual up up

SanJose1#sh ip ospf neighbor detail

Neighbor 193.168.31.22, interface address 193.168.1.2

In the area 0 via interface Ethernet0

Neighbor priority is 1, State is FULL, 6 state changes

DR is 193.168.1.1 BDR is 193.168.1.3

Options 82

Dead timer due in 00:00:30

Neighbor 193.168.31.33, interface address 193.168.1.3

In the area 0 via interface Ethernet0

Neighbor priority is 1, State is FULL, 6 state changes

DR is 193.168.1.1 BDR is 193.168.1.3

Options 2

Dead timer due in 00:00:30

SanJose2#sh ip int br

Interface IP-Address OK? Method Status Protocol

Ethernet0/0 193.168.1.2 YES manual up up

TokenRing0/0 unassigned YES unset administratively down down

Serial1/0 unassigned YES unset administratively down down

Serial1/1 unassigned YES unset administratively down down

Serial1/2 unassigned YES unset administratively down down

Serial1/3 unassigned YES unset administratively down down

Loopback0 193.168.31.22 YES manual up up

SanJose2#sh ip ospf neighbor detail

Neighbor 193.168.31.11, interface address 193.168.1.1

In the area 0 via interface Ethernet0/0

Neighbor priority is 1, State is FULL, 6 state changes

DR is 193.168.1.1 BDR is 193.168.1.3

Options is 0x2

Dead timer due in 00:00:30

Neighbor is up for 00:10:19

Index 2/2, retransmission queue length 0, number of retransmission 1

First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)

Last retransmission scan length is 1, maximum is 1

Last retransmission scan time is 0 msec, maximum is 0 msec

Neighbor 193.168.31.33, interface address 193.168.1.3

In the area 0 via interface Ethernet0/0

Neighbor priority is 1, State is FULL, 6 state changes

DR is 193.168.1.1 BDR is 193.168.1.3

Options is 0x2

Dead timer due in 00:00:39

Neighbor is up for 00:10:20

Index 1/1, retransmission queue length 0, number of retransmission 1

First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0)

Last retransmission scan length is 1, maximum is 1

Last retransmission scan time is 0 msec, maximum is 0 msec

SanJose3#sh ip int br

Interface IP-Address OK? Method Status Protocol

Ethernet0 193.168.1.3 YES manual up up

Loopback0 193.168.31.33 YES manual up up

SanJose3#sh ip ospf neighbor detail

Neighbor 193.168.31.22, interface address 193.168.1.2

In the area 0 via interface Ethernet0

Neighbor priority is 1, State is FULL

Options 82

Dead timer due in 00:00:31

Neighbor 193.168.31.11, interface address 193.168.1.1

In the area 0 via interface Ethernet0

Neighbor priority is 1, State is FULL

Options 2

Dead timer due in 00:00:33

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

Re: OSPF

Hi,

OSPF is a bit funny in this regards. The first router that comes up on the LAN usually becomes the DR unless its OSPF priority on that interface has been set to zero.

Therefore, the fact that it has the lowest address is of not much consequence.

Even if a router with a better priority subsequently comes up, it will not pre-empt the existing DR. The reason for this is to provide stabbility so that you don't have the DRs changing continually.

Pls do rate the post if it helps.

Paresh

1 REPLY
Purple

Re: OSPF

Hi,

OSPF is a bit funny in this regards. The first router that comes up on the LAN usually becomes the DR unless its OSPF priority on that interface has been set to zero.

Therefore, the fact that it has the lowest address is of not much consequence.

Even if a router with a better priority subsequently comes up, it will not pre-empt the existing DR. The reason for this is to provide stabbility so that you don't have the DRs changing continually.

Pls do rate the post if it helps.

Paresh

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