OSPF Point-to-Point Question

Answered Question
Apr 22nd, 2010

Hello,

I am trying to understand the following configuration, any help would be highly appreciated.

From the destribution block we have uplinks to core , where we used point-to-point ( not sure why ) also priority and cost was configured ....will it work ?

DIST1A#sh run int t5/5

Building configuration...

Current configuration : 194 bytes

!

interface TenGigabitEthernet5/5

description Connection to CORE1B T1/5

ip address 10.8.8.61 255.255.255.252

ip ospf network point-to-point

ip ospf priority 255

arp timeout 1200

end

DIST1A#sh run int t5/4

Building configuration...

Current configuration : 194 bytes

!

interface TenGigabitEthernet5/4

description Connection to CORE1A T1/5

ip address 10.8.8.57 255.255.255.252

ip ospf network point-to-point

ip ospf priority 255

arp timeout 1200

end

DIST1B#sh run int t5/5

Building configuration...

Current configuration : 190 bytes

!

interface TenGigabitEthernet5/5

description Connection to CORE1B T2/5

ip address 10.8.8.69 255.255.255.252

ip ospf network point-to-point

ip ospf cost 255

arp timeout 1200

end

DIST1B#sh run int t5/4

Building configuration...

Current configuration : 190 bytes

!

interface TenGigabitEthernet5/4

description Connection to CORE1A T2/5

ip address 10.8.8.65 255.255.255.252

ip ospf network point-to-point

ip ospf cost 255

arp timeout 1200

end

Thanks,

josh

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 6 years 9 months ago

Hello Josh,

>> ip ospf network point-to-point

having set the OSPF network type to point-to-point no OSPF DR election occurs on segment and so the ospf priority  is not effective on this link.

the use of OSPF  network point-to-point on LAN interfaces is used for fiber based point-to-point links where you know that there are only two devices and you want to save some resources by avoiding to elect one router OSPF DR and one OSPF BDR.

I think that the OSPF priority has been set as a safety measure should someone remove on both sides the ip ospf network point-to-point command it would be clear who would be elected OSPF DR on segment.

Hope to help

Giuseppe

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Jon Marshall Thu, 04/22/2010 - 10:10

Josh

Not sure exactly what you query is. Basically from each distribution switch you have 2 L3 P2P uplinks to the core switches. So yoy have a routed L3 distro/core setup which is quite common. Each distro switch will see 2 equal cost paths to destinations reached via the core.

Is there something specific in the config you think might not work ?

Jon

josh94950 Thu, 04/22/2010 - 11:47

Jon,

Thanks and you are right on distribution blocks to core, however I am confusing with the priority and cost on point-to-point interface  .... is that a reason to configure in this way or ..

from Dist 1A   ------------------>    Core 1A            Priority is setup to 255  , default cost

       Dist 1A   ------------------->   Core 1B            Priority is setup to 255 ,  dafault cost

From Dist B -------------------->   Core 1A             Cost is 255 , default  priority

         Dist B -------------------->   Core 1B            Cost is 255 , default priority

Thanks,

Correct Answer
Giuseppe Larosa Thu, 04/22/2010 - 12:53

Hello Josh,

>> ip ospf network point-to-point

having set the OSPF network type to point-to-point no OSPF DR election occurs on segment and so the ospf priority  is not effective on this link.

the use of OSPF  network point-to-point on LAN interfaces is used for fiber based point-to-point links where you know that there are only two devices and you want to save some resources by avoiding to elect one router OSPF DR and one OSPF BDR.

I think that the OSPF priority has been set as a safety measure should someone remove on both sides the ip ospf network point-to-point command it would be clear who would be elected OSPF DR on segment.

Hope to help

Giuseppe

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