IP unnumbered ospf - SVI to subinterface

[cslacker]

Spoiler

I'm trying to configure ospf over ip unnumbered interfaces.  I'm seeing conflicting information as to whether or not this is possible, or under what conditions it's possible.  I'm about to wave the white flag and state that while the config might be possible, some scenarios and combinations (SVIs, subinterfaces, interfaces) are not configurable.

In this case, I'm trying to bring up an ospf neighborship between an SVI and a subinterface.  Here are relevant configs:

Switch1 (C3750)

!
interface Loopback3
 ip address 3.3.3.1 255.255.255.0
 ip ospf network point-to-point

!
interface Vlan16
 ip unnumbered Loopback3
 ip ospf network point-to-point

!
router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 network 3.3.3.0 0.0.0.255 area 0
 network 172.16.16.0 0.0.0.255 area 0
 network 172.20.20.0 0.0.0.255 area 0

Router1 (ISR4331/K9)

!

interface Loopback3
 ip address 3.3.3.2 255.255.255.0
 ip ospf network point-to-point

!
interface GigabitEthernet0/0/0.16
 encapsulation dot1Q 16
 ip unnumbered Loopback3

 ip ospf network point-to-point

!
router ospf 1
 router-id 43.43.43.43
 network 3.3.3.0 0.0.0.255 area 0
 network 172.16.16.0 0.0.0.255 area 0
 network 172.17.17.0 0.0.0.255 area 0
 network 172.20.20.0 0.0.0.255 area 0
 network 172.21.21.0 0.0.0.255 area 0

Is this possible?

[Richard Burts]

This Cisco documentation says that it is possible to configure a vlan sub interface as unnumbered and has an example.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr/command/ipaddr-cr-book/ipaddr-i4.html#wp2633875003

IP unnumbered was intended for point to point interfaces. It is not clear to me whether this is a point to point connection or not. Can you clarify?

If you are trying to run OSPF over these interfaces perhaps you could post the output of these commands

show ip ospf

show ip ospf interface

show ip ospf neighbor

HTH

Rick

[cslacker]

It appears as though it's not possible to enable ospf on an unnumbered SVI.  Note: sh ip ospf neigh return nothing in each case.

Switch1 (C3750)

#sh ip ospf

 Routing Process "ospf 1" with ID 1.1.1.1
 Start time: 4w3d, Time elapsed: 00:03:15.312
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 5000 msecs
 Minimum hold time between two consecutive SPFs 10000 msecs
 Maximum wait time between two consecutive SPFs 10000 msecs
 Incremental-SPF disabled
 Minimum LSA interval 5 secs
 Minimum LSA arrival 1000 msecs
 LSA group pacing timer 240 secs
 Interface flood pacing timer 33 msecs
 Retransmission pacing timer 66 msecs
 Number of external LSA 0. Checksum Sum 0x000000
 Number of opaque AS LSA 0. Checksum Sum 0x000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Number of areas transit capable is 0
 External flood list length 0
 IETF NSF helper support enabled
 Cisco NSF helper support enabled
 Reference bandwidth unit is 100 mbps
    Area BACKBONE(0) (Inactive)
        Number of interfaces in this area is 1
        Area has no authentication
        SPF algorithm last executed 00:03:16.378 ago
        SPF algorithm executed 1 times
        Area ranges are
        Number of LSA 1. Checksum Sum 0x00F62A
        Number of opaque link LSA 0. Checksum Sum 0x000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0

#sh ip ospf interface vlan 16

%OSPF: OSPF not enabled on Vlan16

#sh ip ospf neighbor

Router1 (ISR4331/K9)

#sh ip ospf 1

 Routing Process "ospf 1" with ID 43.43.43.43
 Start time: 1w0d, Time elapsed: 00:07:25.498
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 Supports Link-local Signaling (LLS)
 Supports area transit capability
 Supports NSSA (compatible with RFC 3101)
 Supports Database Exchange Summary List Optimization (RFC 5243)
 Event-log enabled, Maximum number of events: 1000, Mode: cyclic
 Router is not originating router-LSAs with maximum metric
 Initial SPF schedule delay 5000 msecs
 Minimum hold time between two consecutive SPFs 10000 msecs
 Maximum wait time between two consecutive SPFs 10000 msecs
 Incremental-SPF disabled
 Minimum LSA interval 5 secs
 Minimum LSA arrival 1000 msecs
 LSA group pacing timer 240 secs
 Interface flood pacing timer 33 msecs
 Retransmission pacing timer 66 msecs
 EXCHANGE/LOADING adjacency limit: initial 300, process maximum 300
 Number of external LSA 0. Checksum Sum 0x000000
 Number of opaque AS LSA 0. Checksum Sum 0x000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 Number of areas transit capable is 0
 External flood list length 0
 IETF NSF helper support enabled
 Cisco NSF helper support enabled
 Reference bandwidth unit is 100 mbps
    Area BACKBONE(0) (Inactive)
        Number of interfaces in this area is 2
        Area has no authentication
        SPF algorithm last executed 00:07:19.974 ago
        SPF algorithm executed 1 times
        Area ranges are
        Number of LSA 1. Checksum Sum 0x00725D
        Number of opaque link LSA 0. Checksum Sum 0x000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0

#sh ip ospf interface gi 0/0/0.16

 GigabitEthernet0/0/0.16 is up, line protocol is up
  Interface is unnumbered. Using address of Loopback3 (3.3.3.2), Area 0, Attached via Network Statement
  Process ID 1, Router ID 43.43.43.43, Network Type POINT_TO_POINT, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_POINT
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:06
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Can be protected by per-prefix Loop-Free FastReroute
  Can be used for per-prefix Loop-Free FastReroute repair paths
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 0, maximum is 0
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 0, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s)

#sh ip ospf neighbor

[Richard Burts · Accepted solution]

Thanks for the output which I believe is helpful. Here are a couple of things from the 3750

    Area BACKBONE(0) (Inactive)
        Number of interfaces in this area is 1

It is interesting that area 0 is inactive and that is probably because it has established no neighbor relationship. There is one interface on the 3750 running OSPF but it is not vlan 16.

Also this

OSPF not enabled on Vlan16

So it looks like OSPF might not be supported on unnumbered SVI.

And if we know that one end of the connection is not running OSPF then it is to be expected that neither side will show any OSPF neighbor.

HTH

Rick

[Richard Burts]

This has been an interesting discussion on a topic which is not clearly described in documentation. I am glad that my response was helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information.

HTH

Rick