OSPF Peering Via IP Unnumbered Sourcing Loopback - Not Added In DB As A Type-7?

[Oh Drother]

Hi all,

I was hoping for some insight regarding an issue I have been experiencing when peering OSPF via IP unnumbered. As per the Nexus 9k documenation, I have added the ip router ospf 100 area 0.0.1.146 to both the physical interfaces on 7236/cores, as well as on the Loopback 101 for each. The problem I'm seeing, is that when I add the above command on Loopback101, then that Loopback IP does NOT get installed as a type-7 in the OSPF database.

Without the ip router ospf 100 area 0.0.1.146 command on Lo101:

mdfsw-7236# show ip ospf database nssa-external self-originated
OSPF Router with ID (10.3.8.48) (Process ID 100 VRF default)

Type-7 AS External Link States (Area 0.0.1.146)

Link ID ADV Router Age Seq# Checksum Tag
10.3.8.48 10.3.8.48 5 0x80000003 0x22c2 0

mdfsw-7236#
mdfsw-7236# show run int lo101

!Command: show running-config interface loopback101
!Time: Wed May 27 14:13:16 2020

version 7.0(3)I7(4)

interface loopback101
ip address 10.3.8.48/32

With the ip router ospf 100 area 0.0.1.146 command on Lo101:

mdfsw-7236# show ip ospf database nssa-external self-originated
OSPF Router with ID (10.3.8.48) (Process ID 100 VRF default)

mdfsw-7236#
mdfsw-7236# show run int lo101

!Command: show running-config interface loopback101
!Time: Wed May 27 14:15:24 2020

version 7.0(3)I7(4)

interface loopback101
ip address 10.3.8.48/32
ip router ospf 100 area 0.0.1.146

If the Lo101 does not have the ip router ospf 100 area 0.0.1.146, then the cores will not receive any OSPF routes - although the adjacency is fully formed.

Is this expected behavior? Or am I doing something incorrect within OSPF/loopbacks?

Thank you!

[Giuseppe Larosa]

Hello @Oh Drother ,

the loopback 101 must partecipate in OSPF in order to have the routes accepted by upstream devices.

This is reasonable, your physical links use ip unnumbered but the referenced interface loop101 has to take part in OSPF.

At that point if loop101 is an internal route it cannot be installed as a route type 7.

It is part of Router LSA in NSSA area and then in ABR is translated to OSPF LSA type 3.

This is normal and OSPF even without NSSA involved is not going to install an external route if the forwarding address is another external route.

This is part of the consistency checks performed by the protocol.

You have found a working configuration keep to run OSPF on loop101, the good sign is that all other routes are accepted by upstream devices.

Hope to help

Giuseppe

[efc_99]

This seems to be related to the NX-OS implementation of OSPF. In IOS-XE, you can use ip unnumbered and redistribute the loopback used on the unnumbered interface--without explicitly adding the interface to the OSPF process. All routes will show up in the neighbor's routing table and the database will show the redistributed subnets as Type-7.

Consider a very simple topology shown below.


The CSR routers have two loopbacks interface each. Loopback 101 is used for the unnumbered interfaces and router-id. The other interfaces are used for testing.

CSR-1	CSR-2
interface Loopback0 description Backbone interface needed to distribute 0/0 to nssa ip address 10.1.1.1 255.255.255.0 ip ospf 100 area 0 ! interface Loopback101 ip address 10.3.8.1 255.255.255.255 ! ! interface GigabitEthernet1 ip unnumbered Loopback101 ip ospf network point-to-point ip ospf 100 area 401 negotiation auto medium p2p ! ! router ospf 100 router-id 10.3.8.1 area 401 nssa no-summary	interface Loopback0 description Test Subnet ip address 10.2.2.1 255.255.255.0 ! interface Loopback101 ip address 10.3.8.2 255.255.255.255 ! ! interface GigabitEthernet1 ip unnumbered Loopback101 ip ospf network point-to-point ip ospf 100 area 401 negotiation auto medium p2p ! ! router ospf 100 router-id 10.3.8.2 area 401 nssa redistribute connected subnets

The subnets advertised by CSR-2 show up as Type-7 and get installed into CSR-1's routing table.

csr-1#sho ip ospf database

OSPF Router with ID (10.3.8.1) (Process ID 100)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
10.3.8.1 10.3.8.1 7 0x80000004 0x00A54D 1

Router Link States (Area 401)

Link ID ADV Router Age Seq# Checksum Link count
10.3.8.1 10.3.8.1 7 0x80000005 0x00E6F5 1
10.3.8.2 10.3.8.2 807 0x80000003 0x00CD11 1

Summary Net Link States (Area 401)

Link ID ADV Router Age Seq# Checksum
0.0.0.0 10.3.8.1 850 0x80000001 0x008897

Type-7 AS External Link States (Area 401)

Link ID ADV Router Age Seq# Checksum Tag
10.2.2.0 10.3.8.2 807 0x80000002 0x006216 0
10.3.8.1 10.3.8.1 6 0x80000001 0x001260 0
10.3.8.2 10.3.8.2 807 0x80000002 0x00FF6F 0

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
10.3.8.1 10.3.8.1 6 0x80000001 0x002E46 0

csr-1#sho ip route | b Gate
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
C 10.1.1.0/24 is directly connected, Loopback0
L 10.1.1.1/32 is directly connected, Loopback0
O N2 10.2.2.0/24 [110/20] via 10.3.8.2, 00:09:49, GigabitEthernet1
C 10.3.8.1/32 is directly connected, Loopback101
O N2 10.3.8.2/32 [110/20] via 10.3.8.2, 00:09:49, GigabitEthernet1