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Type 7 LSA Cannot Change into the Type 5 LSA Because Its P Bit Is Cleared Due to the Improper Configurations on the NSSA Router

Publication Date:  2012-07-27 Views:  35 Downloads:  0
Issue Description
Networking topology:
For detailed networking diagram, refer to the attachment. This is a simplified topology for analysis: MGW----S1-----------R1----------R3--------SS (softx3000).
Networking description:
This network carries the signaling service for the intra-provincial toll calls of an operator In the detailed topology, R1 and R2 are the ABRs, S1 and S2 are the ASBRs, and direct routes are imported into them. In the simplified topology, the MGW is located on network 100.1.1.0/24; S1 and R1 are interconnected with each other through int vlan 10 192.168.1.0/30; S1 and R1 are interconnected through the NSSA area 4; R1 and R3 are interconnected through area 0.
Current configurations:
Create interfaces loopback 1 on both S1 and S2. Enable area 0 and add loopback 1 on both devices to area 0.
ospf
import-route direct
area 0.0.0.4
network 192.168.1.0 0.0.0.3
nssa no-summary
#
area 0.0.0.0
network 1.1.1.1 0.0.0.0 // IP address of loopback 1
Symptom:
The preceding configurations cause abnormal toll service for some users. 
 
Alarm Information
No alarm is displayed on the routers and switches. An alarm about the disconnection with the SS (softx3000) appears on the MGW on the core network. 
Handling Process
1. Ping SS (softx3000) and the MGW from each other. The ping is unsuccessful.
2. Check the state of the OSPF neighbors. The OSPF neighbors are normal. Check the routes learned by the OSPF neighbors. R3 and R4 do not have the MGW service routes, but R1 has a route to the NSSA.
3. Static routes are configured to temperately resume the service, because the configurations made by the user are unknown.
4. View the OSPF routes. R1 has a route to the NSSA. Because the P bit is cleared, R1 (ABR) cannot translate the Type 7 LSA to the Type 5 LSA, and therefore R3 does not have routes to the MGW.
[R1]dis ospf lsdb nssa 100.1.1.0
Link State Data Base
Area: 0.0.0.4
type : NSSA
ls id : 100.1.1.0 //Routes to the MGW service network
adv rtr : 100.1.1.1 //S1’s ID
ls age : 21
len : 36
seq# : 80000001
chksum : 0x9dd9
options : (No Type 7/5 translation, DC) //The P bit is cleared.
Net mask : 255.255.255.0
Tos 0 metric: 1
E type : 2
Forwarding Address :192.168.1.1
Tag: 1
[R1]dis ospf lsdb ase 100.1.1.0
Link State Data Base
//The ABR does not have information about the Type 5 LSA. Therefore, R3 cannot have a route.
5. Delete the redundant data. The problem is solved. In normal case, S1 only runs as the ASBR.
<S1>dis c c ospf
#
ospf
import-route direct
area 0.0.0.4
network 192.168.1.0 0.0.0.3
nssa no-summary
[R1]dis ospf lsdb nssa 100.1.1.0
Link State Data Base
Area: 0.0.0.4
type : NSSA
ls id : 100.1.1.0
adv rtr : 100.1.1.1
ls age : 47
len : 36
seq# : 80000002
chksum : 0x234b
options : (Type 7/5 translation, DC) //P bit is reset to permit Type 7 to Type 5 translation.
Net mask : 255.255.255.0
Tos 0 metric: 1
E type : 2
Forwarding Address :192.168.1.1
Tag: 1
[R1]dis ospf lsdb ase
Link State Data Base
type : ASE //type-5
ls id : 100.1.1.0 //R1 (ABR) has translated the Type 7 LSA to Type 5 LSA.
adv rtr : 192.168.1.2
ls age : 65
len : 36
seq# : 80000001
chksum : 0x373d
options : (DC)
Net mask : 255.255.255.0
Tos 0 metric: 1
E type : 2
Forwarding Address :192.168.1.1
Tag: 1 
 
Root Cause
1. According to the preceding configurations, S1 runs as both the ASBR and the ABR. S1 runs as the ASBR when area 4 is configured on it and external routes are imported into it. On the S3528, OSPF is enabled and area 0 is configured as the backbone area. ABR is not strictly defined in RFC 2328. Therefore, S1 runs as the ABR as long as it is configured with loopback 1 and OSPF area 0 that contains an interface that is Up. In the subsequent VRP 5.3, if a device has at least two areas including area 0, the generated LSA will have the ABR mark. If area 0 contains interfaces that are Up, area 0 is then distributed to non-zero areas. If area 0 contains neighbors that are full, the system then counts the summary LSA routes of non-zero areas.
2. When routes are imported into S1, if S1 is configured with multiple areas and thus functions as the ABR and the ASBR, then the routes will be imported into both areas. That is, S1 will generate Type7 LSA (with P bit cleared) in the NSSA, and Type 5 LSA in area 0.
<S1>dis c c ospf
ospf
import-route direct
area 0.0.0.4
network 192.168.1.0 0.0.0.3
nssa no-summary
#
area 0.0.0.0
network 1.1.1.1 0.0.0.0
<S1>dis ospf lsdb nssa 100.1.1.0
Link State Data Base
Area: 0.0.0.4
type : NSSA
ls id : 100.1.1.0
adv rtr : 100.1.1.1
ls age : 14
len : 36
seq# : 80000002
chksum : 0x234b
options : (No Type 7/5 translation, DC)
Net mask : 255.255.255.0
Tos 0 metric: 1
E type : 2
Forwarding Address :192.168.1.1
Tag: 1
<S1>dis ospf lsdb ase 100.1.1.0
Link State Data Base
type : ASE
ls id : 100.1.1.0
adv rtr : 100.1.1.1
ls age : 328
len : 36
seq# : 80000005
chksum : 0x667a
options : (DC)
Net mask : 255.255.255.0
Tos 0 metric: 1
E type : 2
Forwarding Address :0.0.0.0
Tag: 1
3. The P bit in the NSSA is cleared because S1 has double roles. To be more specific, because S1 runs as the ASBR and the ABR, the Type 7 LSA generated by the imported external routes in the NSSA has the P bit cleared.
[R1]dis ospf lsdb nssa 100.1.1.0
Link State Data Base
Area: 0.0.0.4
type : NSSA
ls id : 100.1.1.0 //Route to the MGW service network
adv rtr : 100.1.1.1 //S1’s ID
ls age : 21
len : 36
seq# : 80000001
chksum : 0x9dd9
options : (No Type 7/5 translation, DC) //The P bit is cleared.
Net mask : 255.255.255.0
Tos 0 metric: 1
E type : 2
Forwarding Address :192.168.1.1
Tag: 1
4. After the P bit is cleared, the routes cannot be normally learned. The Type 7 LSA packet sent from S1 has its option P bit cleared, and therefore R1 (ABR) can learn this route, but it cannot translate Type 7 LSA to Type 5 LSA and thus cannot send the Type 5 LSA to area 0. As a result, the service is interrupted.
[R1]dis ospf lsdb ase 100.1.1.0
Link State Data Base
//The ABR does not have the Type 5 LSA information. Therefore, R3 cannot have a route.
5. References
The part of RFC 1587 that describes how to import routes into an ABR/ASBR device is as follows:
If a router is attached to another AS and is also an NSSA area border router, it may originate a both a Type 5 and a Type 7 LSA for the same network. The Type 5 LSA will be flooded to the backbone (and all attached Type 5 capable areas) and the Type 7 will be flooded into the NSSA. If this is the case, the P-bit must be reset in the Type 7 NSSA so the Type 7 LSA isn’t again translated into a Type 5 LSA by another NSSA area border router.
The ambiguous definition of ABR in RFC 2328 is as follows:
Area border routers
A router that attaches to multiple areas. Area border routers run multiple copies of the basic algorithm, one copy for each attached area. Area border routers condense the topological information of their attached areas for distribution to the backbone. The backbone in turn distributes the information to the other areas. 
 
Suggestions
If you encounter a complex routing problem, first resume the service by configuring static routes. 

END