Ospf terminology

[Ganesh Devarshetty]

Is there a distinction between an

interface and a link in ospf

An

interface describes a physical or logical interface

on the router (i.e., loopback

interfaces

ethernet ports). A link describes a relationship on the graph between two nodes.

or both have the same meaning?

[MHM Cisco World]

for other see post 

p2p 

interface is same as link 

p2mp 

interface is different than link 

[Flavio Miranda]

Hi

   If you were a bit "thorough" yes, there will be differences but the overall idea is almost the same. On Cisco docs we can read:

"What Do We Mean by Link-States?

OSPF is a link-state protocol. Think of a link as an interface on the router. The state of the link is a description of that interface and of its relationship to its neighbor routers.

A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the routers connected to that network and so on.

The collection of all these link-states would form a link-state database."

In short, there´s no point to distinguish between interface and link but it will be very important for OSPF the information gathered from interfaces/links for the link-state database.

[paul driver]

Hello

Interface(s) =  physical/logical end point of a link
Link =  connection between interface(s)

[Joseph W. Doherty]

With regard to OP, for a high level and succinct description, Paul says it very well.

[M02@rt37]

Hello @Ganesh Devarshetty,

An interface in OSPF refers to a physical or logical interface on a router. It represents the point at which a router connects to a network or another router. Examples of interfaces include Ethernet ports, loopback interfaces, and serial interfaces. Each interface has its own IP address and associated parameters.

A link, on the other hand, describes the relationship between two adjacent routers. It represents the connection or link between two interfaces on different routers. The link is used to exchange routing information and establish neighbor relationships between OSPF routers. The link in OSPF is typically represented by the IP network address that is shared by the connected interfaces of adjacent routers.

[Joseph W. Doherty · Accepted solution]

No, they are not the same.

Counter question, is there a distinction between a

physical interface and a logical interface or between a serial interface and an Ethernet interface

or between one of the four types of

Ethernet frames

?

Hopefully you agree there are distinctions in the prior question, but much overlap too.  Ditto with OSPF which abstacts networks in similar and/or different ways using its own terminology.

An example of an interface vs. OSPF difference if I connect two OSPF neighbors using

a /24

on a Cisco device, if an Ethernet interface is used OSPF will be set link as

multi-point

broadcast but if a serial interface is used link will be set as

p2p

If I know there will only be two OSPF neighbors on the Ethernet interface, I can set OSPF's link type to

p2p

As in this example, OSPF link type can differ on exactly the same

interface interface and link

are distinctly different.

[Ganesh Devarshetty]

Thanks for crystal clear explanation.

[MHM Cisco World]

FYI 

physical interface and logical interface is different than interface/link in OSPF 
logical interface appear in show ip int brief where link not appear.
link is router link appear in show ip ospf database router. 
in p2p you ca see one interface and one link 
in p2mp you can see one interface but multi link.

[Joseph W. Doherty]

physical interface and logical interface is different than interface/link in OSPF

Agree, but OP question was are

interface and link

really the same in OSPF.  They can be the same, but they might not be.  Hence the question, I suspect.  Really depends on context.  Notice how you made a distinction by using the terms

physical interface and logical interface different from interface/link OSPF

logical interface appear in show ip int brief where link not appear.
link is router link appear in show ip ospf database router. 

Correct, but then there's the

show ip ospf interface brief

command, which, sort of, mixes the two.  ; )

in p2p you ca see one interface and one link 
in p2mp you can see one interface but multi link.

True for

p2p

True for NBMA P2MP if running in that mode.  OSPF also defines another method for NBMA networks, which, if physical network can support, the RFC recommends over

P2MP

When trying to truly understand definitions, I find it best to go to the source documentation.  Let's look at RFC 2328, defining OSPFv2.

Within it, we find this section:

1.2. Definitions of commonly used terms

This section provides definitions for terms that have a specific meaning to the OSPF protocol and that are used throughout the text. The reader unfamiliar with the Internet Protocol Suite is referred to [Ref13] for an introduction to IP.

Actually, "link" is not used, generally, without the following word "state".  But we do find, in that section:

Interface
The connection between a router and one of its attached networks. An interface has state information associated with it, which is obtained from the underlying lower level protocols and the routing protocol itself. An interface to a network has associated with it a single IP address and mask (unless the network is an unnumbered

point-to-point

network). An interface is sometimes also referred to as a link.

I've bolded an "interesting" sentence, and underlined an "interesting" word of that sentence.

At first glance, the bolded sentence appears to make

interface and link

synonymous, yet, there's that word "sometimes".  Which, at least to me, implies the two terms might not be 100% synonymous, or in other words, there's some distinction between

link and interface

and whatever such distinction might be, depending on usage/context, it might not be significant "sometimes".

[MHM Cisco World]

show ip ospf interface brief <<- one interface 
show ip ospf database router <<- many router link 

[Joseph W. Doherty]

show ip ospf interface brief <<- one interface 
show ip ospf database router <<- many router link

@MHM Cisco World you're very emphatic about this.  But as I don't disagree, nor believe I've written anything to the contrary, unclear what's the point you're trying to make.

Is this in response to my writing "Correct, but then there's the

show ip ospf interface brief

command, which, sort of, mixes the two.  ; )"?

If so that was in jest (the wink and the "sort of") as those two show commands are telling us two different things, although not quite as different as your earlier

show ip int brief and show ip ospf database router

For example, your

show ip ospf database router

doesn't mention/show physical interface descriptions at all.

To me, your earlier example would be somewhat like comparing static routes, defined on a router with what a route table might show when there's other routers running, and sharing, the same dynamic routing protocol.  I.e. the route table has routes not known by just the same device's local routes.  So, again in jest, when you compare

sh ip int brief

which also only has local device information with the

show ip ospf database router

which has much more information, to me, you were comparing apples to oranges, so (wink) I mentioned the command

show ip ospf int brief

to include bananas too.

If you want a (sort of) interface command to show more information about connectivity, you might try

show cdp neighbors

In your second display, this command (if CDP running on the devices - and a "dumb" switch or hub) should also show the one local interface with two neighbors, and their interfaces.

e.g.:

R0#sh cdp n
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID    Local Intrfce   Holdtme    Capability   Platform    Port ID
R2           Gig 0/0/0        158            R       ISR4300     Gig 0/0/0
R1           Gig 0/0/0        154            R       ISR4300     Gig 0/0/0

If you ask, what's CDP have to do with this?  Exactly!

However, if you don't want to go beyond the bound of interfaces and OSPF, I would likely just use the command

show ip ospf neighbor

E.g.:

R0#sh ip os ne


Neighbor ID     Pri   State           Dead Time   Address         Interface
10.1.1.2          1   FULL/DROTHER    00:00:38    1.1.1.2         GigabitEthernet0/0/0
10.1.2.2          1   FULL/BDR        00:00:38    1.1.1.3         GigabitEthernet0/0/0

That command might be considered a more serious replacement for using the commands 

show ip int brief and show ip ospf database router

[MHM Cisco World]

Friend I want to clear points that all 

link 
interface 
neighbor 

the

link not meaning neighbor

below lab same topology I share before but this time I add LO
you can see

LO interface appear in

show ip ospf

interface

brief 
appear in

show ip ospf

db router 
not appear in

show ip ospf

nei 

[Joseph W. Doherty]

Friend I want to clear points that all

link 
interface 
neighbor 

the

link not meaning neighbor

below lab same topology I share before but this time I add LO
you can see

LO interface appear in show ip ospf interface

brief 
appear in show ip ospf db router 
not appear in show ip ospf nei

But understand, my prior reply, to yours, which I noted I was unclear about, addressed your apparent concern that

show ip ospf int brief

did not show the two routers known to OSPF through one interface, which showing the OSPF DB does.  Again, don't disagree.  I only suggested a show command that would show, in one command, both the interfaces and the neighbors, i.e. you can ascertain the additional OSPF logical connections on the single interface (in a multipoint network).

Yes, I can see your latest example shows OSPF defined on loopback interfaces both with the

show ip ospf int brief and show ip ospf database router

not shown with

show ip ospf nei

No surprise there.  Again, my last suggested command was for a more brief display to avoid the need to use those two other show commands in that particular instance.  Unfortunately, I didn't make tha clear, as I thought it was implied by the context of your prior replies.

Note, in my prior reply, I also wrote ". . .

show ip ospf

database router which has much more information . . ."

Also in that reply, I wrote "If you want a (sort of)

interface

command to show more information about connectivity . . .", and what connectivity do the loopback interfaces have?

In other words, my suggestion was NOT for an

end-all single

command, that addresses all information you may want about OSPF in some actual situation.

So if your point is,

show ip ospf db router

can be useful - 100% agree!  Certainly didn't mean to imply otherwise.  Did mean to imply, it might NOT be the most

optimal

(avoiding extraneous information) way to obtain some OSPF

factoid

(Consider, we've been both using the

brief

forms of the

two show interface

commands, but there are times to use the

non-brief version

of the command.  Do you believe I'm also suggesting we only need the

brief

variants?)

The above, though, runs a bit far outside of the OP question.

Friend I want to clear points that all 

link 
interface

Okay, so be clear - in regard to OP question, "Is there a distinction between an

interface and a link in ospf.

are

link and interface

distinct or not?

And if they are distinct, why then does OSPF RFC have "An

interface

is sometimes also referred to as a link."?

[MHM Cisco World]

Can I know exactly

rfc of ospf

I Need to read it 

Thanks 

MHM