Monday, February 27, 2006

 

NSSA ABR also automatically an ASBR?

While playing with OSPF over the weekend, I noticed something for which I could not find any references or explanations on the web. I have a lab OSPF network in which there is a NSSA area (area 2) with just two routers. The NSSA area ABR is connected to another router completely in the NSSA area which is also connected to a RIP network.

Everything works fine, but when I was checking the NSSA ABR I noticed that it was also claiming to be an ASBR. There are no routing protocols other than OSPF on the NSSA ABR and so this is a bit surprising. It sorta makes sense because after all the ABR of a NSSA area is indeed translating the type-7 LSAs to type-5 LSAs and so does behave like an ASBR since it is the source of type-5 external LSAs. However,
I hunted the web and checked some books I have access to and could find no reference to this behavior that the NSSA ABRs that do the type-7 to type-5 translation automatically get reported as ASBRs even though they are not themselves directly redistributing any routes into OSPF.

As an experiment, I changed area 2 into a normal area and as expected the ABR was reported as just an ABR. So, this automatic reporting of the NSSA ABR as also an ASBR seems to be intentional behavior. I just find it strange that nobody has mentioned it since it is odd at least at first glance that sucg would be the case since the router is running just OSPF and no other routing protocol.

If anyone has any references regarding this, please leave a comment. I tried a couple of different IOS releases and found that the behavior was the same.

R2#sh ip ospf
Routing Process "ospf 100" with ID 2.2.2.2
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
It is an area border and autonomous system boundary router
Redistributing External Routes from,
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 3. Checksum Sum 0x017003
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 2. 1 normal 0 stub 1 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm last executed 04:54:12.652 ago
SPF algorithm executed 12 times
Area ranges are
Number of LSA 10. Checksum Sum 0x0566E8
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
Area 2
Number of interfaces in this area is 1
It is a NSSA area
Perform type-7/type-5 LSA translation
Area has no authentication
SPF algorithm last executed 04:47:18.292 ago
SPF algorithm executed 13 times
Area ranges are
Number of LSA 8. Checksum Sum 0x0411B7
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

R2#sh ip protocols
Routing Protocol is "ospf 100"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 2.2.2.2
It is an area border and autonomous system boundary router
Redistributing External Routes from,
Number of areas in this router is 2. 1 normal 0 stub 1 nssa
Maximum path: 4
Routing for Networks:
10.10.10.0 0.0.0.255 area 0
20.20.20.0 0.0.0.255 area 0
50.50.50.0 0.0.0.255 area 2
Routing Information Sources:
Gateway Distance Last Update
6.6.6.6 110 04:47:30
8.8.8.8 110 04:47:30
1.1.1.1 110 04:47:30
2.2.2.2 110 04:47:30
Distance: (default is 110)

R2#

Comments:
I've encountered the same behaviour on the OSPF implementation of BladeSwitches (http://bladeconnect.com/), to which I am working on.
Not the same behaviour on Quagga, though!

Searched and searched and this page is the only one commenting on this. I'm happy at least I'm not the only one who wonders.
I presume it's a fix to some bug that leaked off the rfc.

Ion Petrescu
 
Actually, I've just found something:
http://archives.neohapsis.com/archives/microsoft/various/ospf/2002-q1/0125.html
http://archives.neohapsis.com/archives/microsoft/various/ospf/2002-q1/0126.html

Ion Petrescu
 
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