Problem:
Not able to learn any routes from the fully adjacent OSPF neighbor
Diagnostics:We have OSPF area 0, running on two Aruba 3810M switches named SW1 and SW2.
SW2 is acting as a probable ISP router wanting to establish OSPF adjacency with SW1.
After the required configuration we can see that the OSPF neighborship is in FULL state.
However we have encountered an Issue where we do not see the routes being populated in our routing table.
The ISP states that they are pretty much advertising all the necessary routes.
Here we have simulated SW2 having a loopback IP of 17.1.1.1/32 which is being advertised via OSPF to its neighbor, SW2 10.1.1.1
So to verify the same we take a look at the link-state database
SW1(config)# show ip ospf link-state detail
OSPF Link State Database for Area 0.0.0.0
LSA Age : 161
LSA Type : 0x1 (Router LSA)
Advertising Router : 10.1.1.1
Link State ID : 10.1.1.1
LSA Sequence : 0x8000000a
LSA Checksum : 0xbd38
LSA Option Bits : E=1 MC=0 N/P=0 EA=0 DC=1
Router Capability Bits : B=0 E=0 V=0
Number of links : 1
Interface Type : 2 (Connected to Transit Network)
LSA Metric : 1
Link Data : 10.1.1.1
Link ID : 10.1.1.1
Number of TOS Metrics : 0
LSA Age : 163
LSA Type : 0x1 (Router LSA)
Advertising Router : 10.1.1.2
Link State ID : 10.1.1.2
LSA Sequence : 0x80000010
LSA Checksum : 0x2487
LSA Option Bits : E=1 MC=0 N/P=0 EA=0 DC=1
Router Capability Bits : B=0 E=0 V=0
Number of links : 3
Interface Type : 3 (Connected to Stub Network)
LSA Metric : 1
Link Data : 255.255.255.0
Link ID : 10.1.1.0
Number of TOS Metrics : 0
Interface Type : 1 (Point-To-Point Link)
LSA Metric : 1
Link Data : 10.1.1.2
Link ID : 10.1.1.1
Number of TOS Metrics : 0
Interface Type : 3 (Connected to Stub Network)
LSA Metric : 1
Link Data : 255.255.255.255
Link ID : 17.1.1.1
Number of TOS Metrics : 0
LSA Age : 170
LSA Type : 0x2 (Network LSA)
Advertising Router : 10.1.1.1
Link State ID : 10.1.1.1
LSA Sequence : 0x80000004
LSA Checksum : 0x8b76
LSA Option Bits : E=1 MC=0 N/P=0 EA=0 DC=1
Network Mask : 255.255.255.0
Attached Router ID : 10.1.1.1
Attached Router ID : 10.1.1.2
Incase you have multiple OSPF neigbors and want to only check the LSA being advertised by the specific neighbor in question you can run the following command:
SW1(config)# sh ip ospf link-state router-id 10.1.1.2 detail
OSPF Link State Database for Area 0.0.0.0
LSA Age : 132
LSA Type : 0x1 (Router LSA)
Advertising Router : 10.1.1.2
Link State ID : 10.1.1.2
LSA Sequence : 0x80000011
LSA Checksum : 0x2288
LSA Option Bits : E=1 MC=0 N/P=0 EA=0 DC=1
Router Capability Bits : B=0 E=0 V=0
Number of links : 3
Interface Type : 3 (Connected to Stub Network)
LSA Metric : 1
Link Data : 255.255.255.0
Link ID : 10.1.1.0
Number of TOS Metrics : 0
Interface Type : 1 (Point-To-Point Link)
LSA Metric : 1
Link Data : 10.1.1.2
Link ID : 10.1.1.1
Number of TOS Metrics : 0
Interface Type : 3 (Connected to Stub Network)
LSA Metric : 1
Link Data : 255.255.255.255
Link ID : 17.1.1.1
Number of TOS Metrics : 0
From the output it is pretty clear that ISP neighbor router is in fact advertising a network , "17.1.1.1/31"
However the output also gives us a pretty clear idea about the issue at hand which is mismatched network types between SW1 and SW2.
This can be seen looking at the Router LSA advertised by SW1 ( 10.1.1.1) and SW2 (10.1.1.2)
SW1 has a interface-type of broadcast signified by the "Transit" and SW2 has an interface type of point-to-point signified by "Stub-Network"
The reason they end up having “FULL adjacency is that “network type” is not encoded into OSPF packets and does not have to match to achieve “FULL” adjacency state.
The reason the neighboring router and networks attached to that router actually end up being unreachable and not installed in the RIB is due to the discrepancy in the LSDB and SPF computation.
The following occurs when the routers exchange LSAs and become adjacent. The broadcast router will create a router LSA that contains entries for all its directly attached networks including the mismatched link.
Because the link on its side is set to broadcast mode it will create an entry in the LSA for it as a Transit network with its own interface address as the DR and router interface address.
This DR address in the LSA is a reference to the network LSA.
The problem occurs when the router configured for point-to-point creates its router LSA. This LSA will contain entries for all its directly attached networks including this link.
The problem is how this link is listed in the LSA. Instead of being listed as a Transit network, it will be listed as connected to another router (point-to-point) with the neighboring routers RID and interface address
and another entry as a stub network including the link prefix
Now even though the two routers have full link state information, when they go to compute SPF and rebuild the network from the LSDB the network will be partitioned on the link between the point-to-point router's router LSA and the (network LSA).
SolutionRESOLUTION: Make sure the network link-type matches on the connected interfaces /vlans facing each other on both the OSPF routing devices.
Aruba switches have the network-type of broadcast enabled by default when you configure OSPF on a vlan.
Here now we have made sure that the network type is same on both ends so on SW2, we configured it to broadcast.
So as a resolution you either change it to a point-to-point or both ends or a broadcast, on both ends.
SW2(vlan-1)# ip ospf network-type broadcast
Outputs from SW1 after the change:
SW1(config)# sh ip route ospf
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
17.1.1.1/32 10.1.1.2 1 ospf IntraArea 2 110
SW1(config)# sh ip ospf link-state router-id 10.1.1.2 detail
OSPF Link State Database for Area 0.0.0.0
LSA Age : 40
LSA Type : 0x1 (Router LSA)
Advertising Router : 10.1.1.2
Link State ID : 10.1.1.2
LSA Sequence : 0x80000015
LSA Checksum : 0xfec3
LSA Option Bits : E=1 MC=0 N/P=0 EA=0 DC=1
Router Capability Bits : B=0 E=0 V=0
Number of links : 2
Interface Type : 2 (Connected to Transit Network)
LSA Metric : 1
Link Data : 10.1.1.2
Link ID : 10.1.1.1
Number of TOS Metrics : 0
Interface Type : 3 (Connected to Stub Network)
LSA Metric : 1
Link Data : 255.255.255.255
Link ID : 17.1.1.1
Number of TOS Metrics : 0