From: Tom Lijnse (Tom.Lijnse@globalknowledge.nl)
Date: Thu Mar 17 2005 - 05:16:46 GMT-3
Hi Jonathan,
I just performed a final test to confirm the behavior. Look at this.
There are two ABRs in the NSSA Area R4 and R6 with RID 192.168.4.4 and
RID 192.168.6.6 respectively. Since R6 has the highest RID it becomes
the router doing the Type-7 to Type-5 translation. Have a look at the
most important snippets of output:
R6#sh ip ospf int brief
Interface PID Area IP Address/Mask Cost State Nbrs
F/C
Se0 1 0 172.16.0.6/24 64 P2MP 2/2
Lo0 1 0 192.168.6.6/24 1 LOOP 0/0
Et0 1 1 172.16.46.6/24 10 DR 1/1
R6#sh ip ospf data router 192.168.6.6 | i Area|Number of Links
Router Link States (Area 0)
Area Border Router
Number of Links: 4
Router Link States (Area 1)
Area Border Router
Number of Links: 1
R6#sh ip ospf | i translation|ID
Routing Process "ospf 1" with ID 192.168.6.6
Perform type-7/type-5 LSA translation
As you can see R6 has 2 interfaces in area 0: Serial 0 and Loopback 0.
Now let's start by shutting down the serial and see what happens:
R6#sh ip ospf int brief
Interface PID Area IP Address/Mask Cost State Nbrs
F/C
Se0 1 0 172.16.0.6/24 64 DOWN 0/0
Lo0 1 0 192.168.6.6/24 1 LOOP 0/0
Et0 1 1 172.16.46.6/24 10 DR 1/1
R6#sh ip ospf data router 192.168.6.6 | i Area|Number of Links
Router Link States (Area 0)
Area Border Router
Number of Links: 1
Router Link States (Area 1)
Area Border Router
Number of Links: 1
R6#sh ip ospf | i translation|ID
Routing Process "ospf 1" with ID 192.168.6.6
Perform type-7/type-5 LSA translation
As you can see this router still advertises itself as an Area Border
Router, even though it no longer has any neighbors in Area 0. It does
still have an interface in Area 0 (Loopback 0) and as long as it has
that it will announce itself as an ABR and take on the role of doing the
conversion. Now we have a problem as the Type-5 LSAs won't be propagated
to Area 0.
Now let's shut down the Loopback 0 interface and see what happens:
R6#sh ip ospf int brief
Interface PID Area IP Address/Mask Cost State Nbrs
F/C
Se0 1 0 172.16.0.6/24 64 DOWN 0/0
Lo0 1 0 192.168.6.6/24 1 DOWN 0/0
Et0 1 1 172.16.46.6/24 10 DR 1/1
R6#sh ip ospf data router 192.168.6.6 | i Area|Number of Links
Router Link States (Area 0)
Number of Links: 0
Router Link States (Area 1)
Number of Links: 1
R6#sh ip ospf | i translation|ID
Routing Process "ospf 1" with ID 192.168.6.6
As you can see R6 now lost all interfaces in Area 0 and therefore does
not consider itself an ABR anymore. This will also stop it from being a
candidate for Type-7 to Type-5 translation and R4 will take over:
R4#sh ip ospf | i translation|ID
Routing Process "ospf 1" with ID 192.168.4.4
Perform type-7/type-5 LSA translation
So how could we fix this issue? I think in this case it would be best to
assign the R6 loopback to Area 1 instead of Area 0. This means it would
only have one active interface in Area 0 and when it now loses that
interface it will stop being an ABR and hand over the translation to R4.
I hope this clears up the whole Type-7 to Type-5 conversion issue.
Regards,
Tom Lijnse
CCIE #11031
Global Knowledge
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Tom Lijnse
Sent: woensdag 16 maart 2005 18:00
To: Jonathan ZD; Pankaj Madhukar Kulkarni; alsontra; CCIE - GS
Subject: RE: ABR of Not-So-Stubby Area
Hi Jonathan,
Quoting from RFC 1587:
4.1 Translating Type-7 LSAs Into Type-5 LSAs
This step is performed as part of the NSSA's Dijkstra calculation
after type-5 and type-7 routes have been calculated. If the
calculating router is not an area border router this translation
algorithm should be skipped. All reachable area border routers in
the NSSA should now be examined noting the one with the highest
router ID. If this router has the highest router ID, it will be the
one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise
the translation algorithm should not be performed.
As you can see only Area Border Routers are candidates for doing the
Type-7 to Type-5 Conversion. So even if a non-ABR has a higher router-id
then still the ABR with the highest router-id will be doing the
conversion.
The only problem I could see here is what happens when the ABR doing the
conversion loses its connection to area 0. If it does not stop
advertising its ABR status then the other ABR will not take over in
doing the conversion and we have a problem.
So the question really comes down to: When do you advertise yourself as
an ABR?
When you have interfaces in Area 0? Or when you have neighbors in Area
0?
Unfortunately I can't really test this right now, so unless somebody
knows this of the top of his head:
To be continued...
Regards,
Tom Lijnse
CCIE #11031
Global Knowledge
-----Original Message-----
From: Jonathan ZD [mailto:Nuvo25@hotmail.com]
Sent: woensdag 16 maart 2005 17:40
To: Tom Lijnse; Pankaj Madhukar Kulkarni; alsontra; CCIE - GS
Subject: Re: ABR of Not-So-Stubby Area
Dear Tom,
Does this mean that eventhough there's non-ABR router with higher
router-id,
in the area, the actual ABR of NSSA will be the one and only that will
translate type-7 to type-5 into backbone? (if you confuse about my
question,
please scroll down to read the original question for this topic.)
Thanks,
Jonathan.
----- Original Message -----
From: "Tom Lijnse" <Tom.Lijnse@globalknowledge.nl>
To: "Pankaj Madhukar Kulkarni" <PankajKu@hclcomnet.co.in>; "alsontra"
<alsontra@gmail.com>; "Jonathan ZD" <Nuvo25@hotmail.com>; "CCIE - GS"
<ccielab@groupstudy.com>
Sent: Wednesday, March 16, 2005 2:09 AM
Subject: RE: ABR of Not-So-Stubby Area
Hi,
Sorry for the somewhat overdue reply, but I didn't have access to a lab
to get some show command output from until now.
Each router knows the Area Border Routers in its area because the ABRs
advertise their ABR status via their Type-1 Router LSA's.
Let's have a look at the following output:
This is taken from a border router between area 1 and area 0:
R6#sh ip ospf border-routers | i Area 1
i 192.168.4.4 [10] via 172.16.46.4, Ethernet0, ABR, Area 1, SPF 3
So it knows that router 192.168.4.4 is a border router as well.
Let's verify this via the Type-1 LSA:
R6#sh ip ospf database router 192.168.4.4 | b Area 1
Router Link States (Area 1)
Routing Bit Set on this LSA
LS age: 1990
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 192.168.4.4
Advertising Router: 192.168.4.4
LS Seq Number: 80000002
Checksum: 0x1B4A
Length: 36
Area Border Router
...
And as you can see here it has an entry saying that it's an ABR.
So this is how OSPF border routers know the other border routers in
their area.
Hope this helps,
Tom Lijnse
CCIE #11031
Global Knowledge
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Pankaj Madhukar Kulkarni
Sent: donderdag 10 maart 2005 5:45
To: alsontra; Jonathan ZD; CCIE - GS
Subject: RE: ABR of Not-So-Stubby Area
Al,
This is quite interesting...
How does an ABR know about the presence of the other ABR in the area?
Let's continue hunting...
Pankaj
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
alsontra
Sent: Thursday, March 10, 2005 9:24 AM
To: 'Jonathan ZD'; 'CCIE - GS'
Subject: RE: ABR of Not-So-Stubby Area
Search the archives, I had to learn this the hard way....
And the answer is: If you have two ABR to an NSSA, the ABR with the
highest
router-id will translate N2---to--->E2. If for some reason the ABR with
highest RID is not connected to area 0, no conversion will take place.
Easies solution is to change RIDs.
To paraphrase:
NSSA RFC 1587 states that before an NSSA ABR converts Type-7 to Type-5,
all
NSSA ABR must examine all other NSSA ABR to determine the highest
router-id.
The NSSA ABR with the highest router-id "must" the conversion from N2 to
E2.
(Troubleshooting IP protocols, Cisco Press -Page 444)
Happy hunting
Al
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Jonathan ZD
Sent: Wednesday, March 09, 2005 9:31 AM
To: CCIE - GS
Subject: ABR of Not-So-Stubby Area
I've followed the link (911networks.com) some one posted it under group
study
one time, I've come across many summary they posted and felt so unsure
about
it. There's one topic that I'd like to ask Group to confirm it.
Under the following link (about Not-So-Stubby Area):
http://911networks.com/node/234?PHPSESSID=38ceea92a6df4ad93d2fba9ecc6477
80
It said that "ONLY THE ROUTER WITH HIGHEST ROUTER-ID DOES THE CONVERSION
LSA
7
TO 5, MAKE SHURE THAT IT'S THE ABR".
Is this true?
Thanks,
Jonathan.
This archive was generated by hypermail 2.1.4 : Sun Apr 03 2005 - 17:56:46 GMT-3