Nexus OSPF

[hs08]

Hello,

I have cisco ISR and Nexus 7707 with ospf enabled. Nexus use 40Gbps for reference bandwidth and ISR use 100Mbps.

From R1 perspective cost to reach 10.103.248.1 is 41, and i have though that 40 is cost from R1 to R2 (40 div 1), and 1 is cost from R2 to the host. 

My question is, why the cost is not 80 (R1-R2 is 40 and R2-host=40)? Since from R1 to R2 and R2 to the host same using 40Gbps for the bandwidth reference?

[Pavel Tarakanov]

Because R2 calculate cost to the network on his own (40) and send it to R1.

R1 know nothing about reference bandwidth of R2 and speed of link from R2 to 10.103.248.1.

Next, from R1 perspective, cost to reach R2 is 1 (1 Gpbs/1 Gbps (reference bandwidth)). So total cost from R1 to 10.103.248.1 is 41 (40 + 1).

[hs08]

hello,

can you explain ore details about :

"Because R2 calculate cost to the network on his own (40) and send it to R1."

Is that mean 40 is cost from R2 to the host?

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[Pavel Tarakanov]

Can you share "show ip route 10.103.248.1" from R2?

[hs08]

Here the result, i change the destination from 10.103.248.1 to 10.103.248.55 because i have typo previously

[Pavel Tarakanov]

If I understand correctly, on R2 configured SVI VLAN 301 with subnet 10.103.248.0/24 (or some other mask).

Because speed of SVI in NX-OS is 1Gbps, then cost for this network will be 40 (ref BW 40G/1G = 40).

This cost will be passed to R1 and R1 will add own cost of the link to R2 (1).

You can play around with metric of SVI with "ip ospf cost" command.

[hs08]

Hello,

Correct, subnet 10.103.248.0/24 is SVI VLAN 301 on R2.

Noted if cost SVI is 40, but when i have different reference BW on R1 dan R2 why the cost is 1 and not use 40GB as reference BW? Is there any ospf term if 2 routers connected with different reference BW?

[Pavel Tarakanov]

>when i have different reference BW on R1 dan R2 why the cost is 1 and not use 40GB as reference BW?

Because R2 sends only cost to the network from own perspective (40 - cost of SVI).

Then R1 accept this cost and add own cost of the link toward R2, based on configured reference bandwidth.

Assume same situation in opposite direction - SVI on R1.

Then R1 will inform R2 that cost to the network from R1's perspective - 1 (1G ref / 1G SVI).

Then R2 add cost of the link (40G ref/1G link) - 40. And total cost will be 41.

> Is there any ospf term if 2 routers connected with different reference BW?

Can you rephrase your question?

[Giuseppe Larosa]

Hello @hs08 ,

the cost of 40 is reported inside the Router LSA originated by the Nexus node.

The IOS node adds the cost of the link between itself and the Nexus to that cost using its own point of view = outgoing cost to reach the shared subnet between the two nodes.

This is why the total cost is 41.

In production networks it would be good practice to have the same reference bandwidth on all nodes.

If you use 40 Gbps as reference bandwidth also on the IOS node the total cost will become 80

Hope to help

Giuseppe

[M02@rt37]

Hello @hs08 

Do you check LSAs on R2 regarding the prefix10.103.248.x ?

[hs08]

Which LSA?

[M02@rt37]

@hs08 

On R2 do show ip ospf database.

Check which type of LSA references this prefix.

[hs08]

The prefix inside R2 (Link ID 10.103.103.1)

[Giuseppe Larosa]

Hello @hs08 

use

show ip ospf router <Nexus-OSPF-RID> to see the detalis you should be able to see a cost of 40 for the subnet of interest

Hope to help

Giuseppe

[hs08]

This is what you mean?