OSPF Dual ABR and Type 4 LSA reports adv Router is not-reachable

[igrahamit]

I have the below topology set up in GNS3 running OSPF. R8 and R1 are ABRs, Area 0 is on on the left and Area 1 is on the right. R3, R4, and R5 are redistributing some loopback interfaces into Area 0. When I run show ip ospf database asbr-summary on Either R8 or R1, it lists the 3 ASBR router for Area 0, but lists states "Adv Router is not-reachable." However I have type 1 LSAs for both in each routers respective topology table. R1 and R8 are neighbors within Area 1 and R6 and R7 both do not report the same problem reaching either R1 or R8.

I have read some of the troubleshooting docs related to this error, and none of them seem applicable to this situation. Router configs and show commands attached.

[Giuseppe Larosa · Accepted solution]

Hello igrahamit,

you are manually configuring the ospf router-id on your routers.

for example on R8 config you have:

ospf 1
router-id 0.0.0.8

but the OSPF router-id is not equal to a Loopback address or other interface address and it is not advertised in OSPF.

the meaning of the message is simply to advise that OSPF Advertising router = OSPF Router-id of the router that originated the LSA is not advertised in OSPF.

This configuration is not wrong. OSPF routing should work, but you cannot telnet or ssh to the OSPF router-id of a device when performing troubleshooting.

>> Adv Router is not-reachable in topology Base with MTID 0
LS age: 1015
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(AS Boundary Router)
Link State ID: 0.0.0.3 (AS Boundary Router address)
Advertising Router: 0.0.0.8
LS Seq Number: 80000004
Checksum: 0x2C81
Length: 28
Network Mask: /0
MTID: 0 Metric: 128

0.0.0.8 is not advertised in OSPF so from router 1 when issuing the command you see the line telling you that.

However, OSPF does not check for OSPF RID reachability in order to accept an LSA and to install it in the IP routing table.

Hope to help

Giuseppe

[igrahamit]

Hi Giuseppe,

Thank you for that explanation. I have one more question regarding the reachability of the router based on RID. My understanding is that OSPF algorithm is recursive, so even though the RID of 0.0.0.8 is not a reachable IP, the router would be able to locate the advertising router via the Type 1 LSA, is that correct? Technically the Advertising router is advertised into OSPF.

[Giuseppe Larosa]

Hello igrahamit,

the OSPF router id is simply a node identifier that must be unique in the OSPF database to associate to a single router.

Each router uses the OSPF router-id in the advertising router field of each locally generated LSA starting from the router LSA for the area.

>> My understanding is that OSPF algorithm is recursive, so even though the RID of 0.0.0.8 is not a reachable IP, the router would be able to locate the advertising router via the Type 1 LSA, is that correct? Technically the Advertising router is advertised into OSPF.

OSPF performs some recursions however if the OSPF router-id is not advertised in OSPF you cannot ping it and you cannot telnet/SSH to it.

As explained above the OSPF RID is the node identifier in the link state database it must be unique, it it not necessary to be advertised / reachable in OSPF.

There is a special case when even if OSPF RID are not advertised recursion works well and it is the case of OSPF virtual links that are configured specifying the remote OSPF router-id.

I would expect that in this case OSPF RIDs need to be advertised in OSPF to have the virtual link to be setup successfully.

However, during lab tests I have seen that even if the OSPF RIDS are not advertised the virtual link is able to come up.

The explanation I have been given by  a Cisco expert is that the OSPF RID in this case is used to pick up the remote router LSA and the virtual link is setup with the nearest reachable IP address in the router LSA.

Hope to help

Giuseppe

[igrahamit]

Hi Giuseppe,

When I update the router-id to reflect the same IP as the interface in Area 1, I am still getting the same message. So even when the adv-router ID is an IP address in the same subnet as the other ABR, it is still listing the Adv-router not reachable.

Sorry for the continued question, I just don't understand why it's list this as not reachable, when it's an interface in the same area, with an IP address in the same subnet, and and a full neighbor relationship up.

Ian

[Giuseppe Larosa]

Hello igrahamit,

I have looked at the log file.

The log file demonstrates that local router R8 has put its own OSPF router-id to 192.168.11.8.

R8 is acting as ABR in area 0 and area 1.

More specifically R8 is generating one router LSA for area 0 and one router LSA for area 1.

We see that R8 OSPF RID is in area 1.

Router Link States (Area 1)

LS age: 188
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 192.168.11.8
Advertising Router: 192.168.11.8
LS Seq Number: 80000003
Checksum: 0x54E9
Length: 36
Area Border Router
Number of Links: 1

>>>>Link connected to: a Transit Network
(Link ID) Designated Router address: 192.168.11.8
(Link Data) Router Interface address: 192.168.11.8
Number of MTID metrics: 0
TOS 0 Metrics: 1

However, we don't know if R1 new OSPF RID 192.168.11.1 is part of OSPF in area 1.

So I would ask you to post the following:

from R8:

show ip ospf neighbor

show ip ospf database router 192.168.11.1

R8 is seeing at least one router out of Fas0/0 as the Link is declared a Transit Network, OSPF DR/BDR election has occurred and R8 is the DR for the segment.

From show cdp neighbors we see that R1 is a CDP neighbor of R8 on Fas0/0

R8#show cdp ne
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID Local Intrfce Holdtme Capability Platform Port ID
R2 Ser 1/1 173 R 7206VXR Ser 2/0
>>R1 Fas 0/0 130 R 7206VXR Fas 0/0
R6 Fas 0/0 132 R 7206VXR Fas 0/0
R7 Fas 0/0 164 R 7206VXR Fas 0/0

post also the following

show ip ospf database network 192.168.11.8

to see all known OSPF neighbors in area 1 .

Either R1 is not advertising its own OSPF router-id or R1 is advertising it in a different area and it is not an OSPF neighbor of R8 in area 1 on the shared subnet.

Note: you can ping R1 OSPF RID 192.168.11.1 from R8 because they are directly connected, this does not mean that R1 is advertising its own OSPF RID in OSPF.

Hope to help

Giuseppe

[igrahamit]

HI Giuseppe,

Thank you for your continued assistance. I have attached the .txt with the output from the show commands you requested. 

[Giuseppe Larosa]

Hello igrahamit,

your configuration is correct and also R1 has the LAN interface in area 1.

R8#show ip ospf data net 192.168.11.8

OSPF Router with ID (192.168.11.8) (Process ID 1)

>>>>>>Net Link States (Area 1)

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 23
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 192.168.11.8 (address of Designated Router)
Advertising Router: 192.168.11.8
LS Seq Number: 80000001
Checksum: 0x3505
Length: 40
Network Mask: /24
Attached Router: 192.168.11.8
Attached Router: 0.0.0.6
Attached Router: 0.0.0.7
>>>>>>Attached Router: 192.168.11.1

R8#

From the point of view of R8 it should not declare R1 OSPF RID 192.168.11.1 as unreachable anymore.

The output you have seen may be a transient from previous scenario

On R8 if the problem is still there try the following:

clear ip ospf

or

clear ip ospf process 1

Hopefully this should solve.

Edit:

As a best practice routers that are ABR connected to area 0 and to other areas should have their OSPF RID advertised as an OSPF area 0 link.

This is really useful in service provider environments, where some features apply only to area 0 (like MPLS traffic engineering MPLS TE that started as single area support)

Hope to help

Giuseppe

[igrahamit]

Thank you, I have cleared the ospf process and also fully rebooted all routers in this topology. I can only assume at this point that this may be a bug in the IOS, likely cosmetic. Since we can reach R1 from R8 within area 1, and also see it's Type 1 LSA in the database, the adv router not reachable message is clearly not accurate.

One more questions:

"As a best practice routers that are ABR connected to area 0 and to other areas should have their OSPF RID advertised as an OSPF area 0 link."

Is this the default behavior? What configuration command would manipulate this behavior?

[Giuseppe Larosa]

Hello igrahamit,

"As a best practice routers that are ABR connected to area 0 and to other areas should have their OSPF RID advertised as an OSPF area 0 link."

Is this the default behavior? What configuration command would manipulate this behavior?

No, it is not default behaviour it is just a design recommendation. However, if you like you can use loopback addresses as OSPF RID and advertise them in area 0 using network command.

I just wonder if this might be the root cause of your cosmetic bug having two routers that are ABR for area (0,1) areas and their OSPF RID are advertised in area 1.

Hope to help

Giuseppe