OSPF Duplicate Router ID + Hub: Default Summary route generated by OSPF redistribution of an external protocol.

[Steph1963]

Hi Everybody,

Question 1) Duplicate OSPF Router ID:

Cisco documentation states that SPF algorithm may run as frequently as once every 10 seconds in a case of OSPF neighbor using duplicate OSPSF router ID. Nowehere in the documentation I was able to find why.

What is the main reason why duplicate OSPF router ID might cause LSA to come an go and frequent SPF re-calculation.

Question 2) Default summary route generated by ABR

I would like to understand the mechanism behind the generation of a default route by an ABR or ASBR/ABR router. Based on my observation of the following network,it seems that an ASBR router will generate a summary default route to all it`s neighbor but the neighbor does not always install it.

hat is the reason why the ABR pass this route to the neighbors (stub and backbone routerO but does not install it as a Gateway
of last resort while the stub install it as a default gateway. Do not understand why the stub also generates a summary default
route in this situation.

Is there any way to influence the metric of the summary default route generated by the ABR router?

R3#show ip route

Gateway of last resort is not set

     192.168.1.0/24 is variably subnetted, 3 subnets, 2 masks
R       192.168.1.0/28 [120/1] via 172.16.15.2, 00:00:01, FastEthernet1/0
O       192.168.1.0/24 is a summary, 00:13:49, Null0
R       192.168.1.16/28 [120/1] via 172.16.15.2, 00:00:01, FastEthernet1/

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

  Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         943         0x80000001 0x0057DA
0.0.0.0         4.4.4.4         842         0x80000001 0x0039F4
0.0.0.0         5.5.5.5         725         0x80000001 0x001B0F

R4#show ip route

Gateway of last resort is not set

O E2 192.168.1.0/24 [110/20] via 10.0.0.14, 00:12:28, FastEthernet0/0

R4#show ip ospf database

            OSPF Router with ID (4.4.4.4) (Process ID 1)

  Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         981         0x80000001 0x0057DA
0.0.0.0         4.4.4.4         877         0x80000001 0x0039F4
0.0.0.0         5.5.5.5         761         0x80000001 0x001B0F

R5# show ip route

Gateway of last resort is 10.0.0.25 to network 0.0.0.0

O*IA 0.0.0.0/0 [110/2] via 10.0.0.25, 00:13:03, FastEthernet0/1
               [110/2] via 10.0.0.21, 00:13:03, FastEthernet0/0

R5#show ip ospf database

            OSPF Router with ID (5.5.5.5) (Process ID 1)
  Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         3.3.3.3         1014        0x80000001 0x0057DA
0.0.0.0         4.4.4.4         911         0x80000001 0x0039F4
0.0.0.0         5.5.5.5         794         0x80000001 0x001B0F

Thanks for your help
Stephane

[Peter Paluch]

Hello Stephane,

Question 1) Duplicate OSPF Router ID:
Cisco
 documentation states that SPF algorithm may run as frequently as once 
every 10 seconds in a case of OSPF neighbor using duplicate OSPSF router
 ID. Nowehere in the documentation I was able to find why.

The reason is that if a router receives an LSA originated by a different router with the identical OSPF RID, it will recognize that it is not identical to the one it has originated itself, so it reoriginates its own LSA once again (with a higher revision number) and floods it into network. This will cause the receiving routers to re-run the SPF, and as the inter-SPF delay is set to 10 seconds by default, this neverending reflooding of an LSA with a conflicted OSPF RID may hold these routers in constant SPF recalculations.

Question 2) Default summary route generated by ABR

I do not entirely understand your description of the network topology. What I am missing are the individual configurations and clear description where are the area boundaries and what are the types of your individual areas.

As a matter of rule, an ABR generates a default route into an area only if the type of the area is stubby, totally stubby or NSSA totally stubby. Otherwise, the only way to inject a default route into the OSPF is to use the default-information originate command (the default route cannot be redistributed into OSPF in Cisco's implementation).

Best regards,

Peter

[Steph1963]

Hi Peter,

Thanks for the clafiication for SPF recalculation with duplicate Router ID.

Sorry if my question was not clear about OSPF boundaries. My question was base on am misunderstanding of OSPF default-route generation. I thought that the only case where a default route was when a area was defined as totally stub, totally NSSA stub or with the help of default-information originate.

Never ran into the case where an external route redistributed by an ASBR in the area cause an ABR to generate a default route in a stub area. If my understanding is correct, we can say that in order to reach the outside networks, the router in the stub area use a default route which is injected into the area by the ABR.

Many thanks for your help

Stéphane