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The NE80E Fails to Generate a Route After Receiving the LSA

Publication Date:  2012-07-27 Views:  47 Downloads:  0
Issue Description
The NE80E at a certain office fails to calculate the route after learning an LSA from the CDMA-RAN, which functioning as a VPN instance. 
Alarm Information
View detailed route information on the NE80E:
<SD-JN-SLLH-CE-4.CDMA>display ospf 2008 lsdb ase 6.29.0.13

OSPF Process 2008 with Router ID 6.29.0.6
Link State Database


Type : External
Ls id : 6.29.0.13
Adv rtr : 6.29.0.14
Ls age : 1110
Len : 36
Options : E
seq# : 8000005e
chksum : 0x6a38
Net mask : 255.255.255.255
TOS 0 Metric: 1
E type : 2
Forwarding Address : 6.29.0.77
Tag : 0
<SD-JN-SLLH-CE-4.CDMA>dis ospf 2008 routing 6.29.0.13

OSPF Process 2008 with Router ID 6.29.0.6

Destination : 6.29.0.0/19
AdverRouter : 6.29.0.8 Tag : 0
Cost : 1 Type : Type2
NextHop : 6.29.0.201 Interface : Eth-Trunk2.2008
<SD-JN-SLLH-CE-4.CDMA> 
 
Handling Process
1. Check the NE80E. It is found that the NE80E can receive the LSA of the destination network segment normally.
2. View the FA address of the LSA.
3. View the iterative route of the FA address.
<SD-JN-SLLH-CE-4.CDMA>dis ip routing-table vpn-instance CDMA-RAN 6.29.0.77

Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Table : CDMA-RAN
Summary Count : 1

Destination/Mask Proto Pre Cost Flags NextHop Interface

6.29.0.76/30 O_ASE 35 21 D 6.29.0.62 Eth-Trunk11.2008
The available iterative route of the FA is an OSPF external route. According to the relevant RFC documents, the FA needs to be covered by an inter-area or intra-area route to iteratively generate the LSA route.
4. Solution: Disable OSPF on the next-hop interface of ASBR or set the interface type to P2P to prevent the generation of the FA. 
 
Root Cause
If the forwarding address is non-zero, the system searches the forwarding address in the routing table. The matching routing entry must specify an intra-area or inter-area path; if no such path exists, the system does not process the LSA and considers the next in the list. 
Suggestions
According to relevant RFC documents, the route that covers the FA must be an inter-area or intra-area route.
OSPF is a loop-free link state protocol. The loop-free state in an area relies on the shortest path, but the loop-free state between areas relies on the connection of the non-backbone area to the backbone area. The reliability of route calculation within the AS is thus guaranteed.
But OSPF cannot control the reliability of the routes out of the AS. The FA is an address used for forwarding packets to external networks. If an external route is used to iteratively search for the FA and perform route calculation, routing loops may occur. 
 

END