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Incorrect Configuration of the FA in the OSPF Type 5 LSA Leads to the Abnormality of the Routes on the Attached Router

Publication Date:  2012-07-27 Views:  36 Downloads:  0
Issue Description
The networking diagram is as follows:

   \   /                 \   /

 NE80E-1---NE80E-2

           \            /

             \         /

            S8512

The fault symptom is as follows:

The NE80E-1 and NE80E-2 have two upstream GE interfaces, each interface configured with two static routes as follows:

NE80E-1 (192.168.1.253)

ip route-static 0.0.0.0 0.0.0.0 192.168.0.69

ip route-static 0.0.0.0 0.0.0.0 192.168.0.65

NE80E-2 (192.168.1.254)

ip route-static 0.0.0.0 0.0.0.0 192.168.0.5

ip route-static 0.0.0.0 0.0.0.0 192.168.0.1

The two NE80Es send these static routes to the S8512. During the test, the following fault occurs on the S8512:

In the normal situation, the S8512 has two OSPF default external routes to the two NE80Es. But in the following situation, the S8512 has only one OSPF default external route to one of the two NE80Es.

Run the following command on NE80E-1(192.168.1.253). NE80E-2 still sends two default static routes to the uplink.

[NE80E-1]undo ip route-static 0.0.0.0 0.0.0.0 192.168.0.65

// ip route-static 0.0.0.0 0.0.0.0 192.168.0.69 is reserved. If ip route-static 0.0.0.0 0.0.0.0 192.168.0.69 is removed and ip route-static 0.0.0.0 0.0.0.0 192.168.0.65 is reserved, the following fault does not occur.

The corresponding LSA information on the S8512 is as follows:

[jian-ar1]disp ospf lsdb ase 0.0.0.0 verbose

                OSPF Process 1 with Router ID 192.168.1.248

                         Link State Database

Type      : ASE

Ls id     : 0.0.0.0

Adv rtr   : 192.168.1.254 

Ls age    : 1038

Len       : 36

Seq#      : 0x8000000d

Chksum    : 0xd80e

Options   : (Non DC)

Net mask  : 0.0.0.0

   Tos 0 metric: 1

   E type    : 2

   Forwarding Address :0.0.0.0

   Tag: 1

Type      : ASE

Ls id     : 0.0.0.0

Adv rtr   : 192.168.1.253 

Ls age    : 34

Len       : 36                           

Seq#      : 0x80000007

Chksum    : 0xfccd

Options   : (Non DC)

Net mask  : 0.0.0.0

   Tos 0 metric: 1

   E type    : 2

   Forwarding Address :192.168.0.69

   Tag: 1

[jian-ar1]

As the preceding information shows, if the FA is set incorrectly on NE80E-1, only one default OSPF route on the S8512 leads to NE80E-2 (192.168.1.254). This is because the SPF calculation of OSPF on the S8512 shows that the 192.168.0.69 is inaccessible.

 

Run the following command on NE80E-2 (192.168.1.254). NE80E-1 still sends two default static routes to the uplink.

[NE80E-2]undo ip route-static 0.0.0.0 0.0.0.0 192.168.0.1 

// ip route-static 0.0.0.0 0.0.0.0 192.168.0.5 is researved. If ip route-static 0.0.0.0 0.0.0.0 192.168.0.5 is removed and ip route-static 0.0.0.0 0.0.0.0 192.168.0.1 is reserved, the following fault does not occur.

The corresponding LSA information on the S8512 is as follows:

[S8512]disp ospf lsdb ase 0.0.0.0 ver

                OSPF Process 1 with Router ID 192.168.1.248

                         Link State Database

Type      : ASE

Ls id     : 0.0.0.0

Adv rtr   : 192.168.1.254 

Ls age    : 37

Len       : 36

Seq#      : 0x8000000f

Chksum    : 0x639e

Options   : (Non DC)

Net mask  : 0.0.0.0

   Tos 0 metric: 1

   E type    : 2

   Forwarding Address :192.168.0.5

   Tag: 1

Type      : ASE

Ls id     : 0.0.0.0

Adv rtr   : 192.168.1.253 

Ls age    : 1394

Len       : 36                           

Seq#      : 0x80000008

Chksum    : 0xe804

Options   : (Non DC)

Net mask  : 0.0.0.0

   Tos 0 metric: 1

   E type    : 2

   Forwarding Address :0.0.0.0

   Tag: 1

[S8512]

As the preceding information shows, if the FA is set incorrectly on NE80E-2, only one default OSPF route on the S8512 leads to NE80E-1 (192.168.1.253). This is because the SPF calculation of OSPF on the S8512 shows that the 192.168.0.5 is inaccessible.  
Alarm Information
Null
Handling Process
As the reason why the OSPF route learned by the S8512 is not the expected result and the rules for configuration of the Forwarding Address (FA) in the VRP Type 5 LSA are identified, an easy solution to fix the fault is to violate the conditions for setting the FA to an address other than 0.0.0.0. Do as follows:

1. Check the data configuration on the two NE80Es:

OSPF on NE80E-1 (192.168.1.253) advertises 192.168.0.68/30 rather than 192.168.0.64/30. OSPF on NE80E-2 (192.168.1.254) advertises 192.168.0.4/30 rather than 192.168.0.0/30. Therefore, undo the network configuration of the next hop of the corresponding static route in OSPF on the two NE80Es.

2. You may not undo the network configuration or modify the configuration in the OSPF view. You can configure the ospf network-type as p2p command at the interface of the two NE80Es where OSPF advertises the routes. Do the same on the peer interface.

3. Set the corresponding interface on the NE80E to silence, or make upstream IP addresses of the two NE80Es accessible to the S8512 attached to the NE80E.

After the NE80E is reconfigured such, the OSPF default route calculation on the S8512 will work normally. 
Root Cause
The preceding fault description shows that the incorrect setting of the FA on the NE80E is the root reason why the OSPF route learned by the S8512 is not the expected result. The rules for configuration of the FA in the VRP Type 5 LSA and its route calculation are specified as follows:

When the FA is set to 0.0.0.0:

When the FA in the Type 5 LSA is set to 0.0.0.0, the router receiving the LSA calculates the next-hop address according to the Adv Rtr (that is, ASBR).

When the FA is not set to 0.0.0.0:

If the following conditions are satisfied, the ASBR does not set the FA in the Type 5 LSA to 0.0.0.0. The router receiving the LSA calculates the next-hop address according to this address set by the ASBR.

1. OSPF is enabled at the next-hop interface that connects the ASBR to the extranet.

2. The next-hop interface that connects the ASBR to the extranet is not set to a passive interface.

3. The next-hop interface that connects the ASBR to the extranet does not belong to the OSPF P2P type or the P2MP type.

4. The address of the next-hop interface that connects the ASBR to the extranet belongs to the network scope released by OSPF.

If any of the above four conditions is not met, the FA is set to 0.0.0.0. 
Suggestions
Null

END