How to check OSPF details on SVIs ?

[hichemguenfaf]

Hello,

As the title suggests, I would like to know to check OSPF information on an SVI interface. Info such as [Network type, timers...].

Command such as

`show ip ospf interface 

[INTERFACE]` where INTERFACE=vlan10 for example.

Thank you,

[MHM Cisco World · Accepted solution]

Yes the command you mention is what you need 

[hichemguenfaf]

Thanks for the confirmation, could you please explain what cost is used on the SVIs it the cost calculated based on the forwarding interface ?

Assuming that reference bandwidth is set to 1000Mbps, and say for example interface G0/0 is on VLAN10 so the traffic destined to 10.10.10.1 passes through G0/0 to the SVI, another interface F0/1 is on VLAN20 with SVI VLAN20 20.20.20.1, both Interfaces provide routes to some network say 30.30.30.1.

Which route will be on the routing table ? Will they have costs of 1, 10 respectively based on the physical interface speed ? or they'll both have a cost of 1 since that's the default SVI cost ?

Thank you, 

[paul driver]

Hello

show ip ospf interface brief

-  will show you the current calculated cost of each ospf enabled interface.

sh  ip ospf | in reference

 - will show you the current set reference bandwidth.

As for the reference bandwidth that is calculated at 10’8 or 100.000.000 bits (100mbs) / line bw

So a 1gb link with a ref BW of 1000mb would be 1000000000/100000000O  =1 

100mb link would have a cost of 10

1000000000/100000000 = 10

[Richard Burts]

The original poster asks "Will they have costs of 1, 10 respectively based on the physical interface speed". The answer is that no the calculated cost is not based on what physical interface was used. The physical interface used was a layer 2 interface (and a member of some vlan). What OSPF evaluates is the layer 3 interface used, and in this case it is the SVI (a virtual not physical interface).

[hichemguenfaf]

Hi Richard,

Yeah, that's exactly what I'm asking. But why is the SVI's cost more important than the physical interface which is the deciding factor for the link's throughput (100M, 1G, 10G...), if I had to guess I'd say that if SVI is used then the network engineer is supposed to manually specify the interface's cost based on the physical interface used in conjunction with the SVI (1 for Gig, 10 for Fast...), but that's just my guess!

[Richard Burts]

The original poster asks " isn't the traffic physically handled by the physical interface which makes the interfaces speed the real deal here ?". There are several ways of looking at this. One way is to recognize that the vlan may very well have several physical interfaces in it and some of those interfaces might have different speeds. OSPF can not know which of the several physical interfaces to use to calculate the cost and so it chooses to use the one thing that is common to all of them, which is the SVI. Another way to think about it is in terms of the layered model of the network stack. OSPF is an IP protocol. IP is a layer 3 protocol and deals with layer 3 entities. The physical interfaces are layer 2 entities and do not have any layer 3 properties. So OSPF can not consider the physical interfaces in determining the cost. OSPF must consider the layer 3 interface, which is the SVI and not the physical interface.

[hichemguenfaf]

Thank you @Richard Burts, the terminology you've used resonated with me so well,

I truly appreciate your help, thank you kindly,

[Richard Burts]

I am glad that our explanations have been helpful.  This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[Giuseppe Larosa]

Hello @hichemguenfaf ,

as noted by @Richard Burts in multilayer switch implementation the SVIs have a bandwidth associated with them that is not related to the current L2 interfaces in forwarding state for the same broadcast domain (L2 VLAN)

The reason is that multiple L2 interfaces of different speeds can be active and in forwarding state at the same time so it is not possible to track all of them.

In some cases when changing the OSPF auto reference bandwidth you may need to set the ip ospf cost under each SVI to achieve the desired result / value.

Hope to help

Giuseppe

[hichemguenfaf]

Got it, thank you for explaining. It makes much more sense now, I'll set the costs manually then.

Thank you Giuseppe,

[hichemguenfaf]

Hi Paul,

Thank you for the provided details on how the interface cost is calculated and sorry if my question wasn't that clear, but I asked for what cost the OSFP algorithm will associate to the interface is it the Phyical Interfaces's cost (1 for Gig & 10 for Fast) or the default cost of the L3 SVI which is 1.

If the latter is the the case, why is that, isn't the traffic physically handled by the physical interface which makes the interfaces speed the real deal here ?

[MHM Cisco World]

""There is no set recommended value of bandwidth on a VLAN interface (SVI). The default is BW 1000000 Kbit (1 Gigabit), because the route processor internal inband is only 1 Gigabit by design.""
from cisco doc. 

https://www.cisco.com/c/en/us/support/docs/lan-switching/inter-vlan-routing/41860-howto-L3-intervlanrouting.html#proc

[hichemguenfaf]

The link is really helpful, so the cost must be set manually depending on the physical interface used.

Thank you for helping @MHM Cisco World, I truly appreciate it,