Topology: user (n groups)－－－>Ethernet switch－－－>NE20－－－>NE40－－－>IP Core－－－>Internet.
Version: NE20 (3.30-0330.20), NE40 (3.10-2226).
Configuration: N groups of users connect NE20 through Ethernet switch. Each group uses one IP network segment. The gateway is L3 interface of NE20. NE20 and NE40 connect through 100M Ethernet interface. Operate OSPF. NE40 distributes default route to NE20.
Phenomenon: With the increase of service, 100M Ethernet interface between NE20 and NE40 expands 155M POS. After cutover, the traffic of NE20 and NE40 reaches 150M. Many packets are dropped and online speed is slow.
First check NE20 configuration. NE20 has the following three interfaces, A.B.C.1/27, A.B.C.129/28 and A.B.C.161/28. The IP of interconnected interface of NE20 and NE40 is D.E.F.1. OSPF of NE20 as follows:
network D.E.F.0 0.0.0.3
network A.B.C.0 0.0.0.31
network A.B.C.128 0.0.0.15
network A.B.C.160 0.0.0.7
OSPF configuration of NE40 as follows:
network D.E.F.0 0.0.0.3
ip route-static A.B.C.0 255.255.255.0 D.E.F.1
Check the configuration and it is easy to confirm whether there is loop between NE20 and NE40. Parts of IP have no detailed route on NE20. They distribute Ne40 through default route. NE40 distributes NE20 through static route. IP package vibrates between NE20 and NE40 and exhausts 155M bandwidth.
Why does Ethernet interface have no loop before cutover and POS interface has loop after cutover?
When importing static route to OSPF, the interface type will influence FA（Forwarding address）of TYPE 5 LSA. VRP realization handling is as follows:
If ASBR imports routes and OSPF does not enable next-hop interface of these routes, FA is set as 0.0.0.0. If the following conditions are met, FA can be set as non-0.0.0.0.
1. The next-hop interface of OSPF on ASBR is enabled.
2. The next-hop interface of ASBR is not set as silent interface.
3. The next-hop interface of ASBR is not OSPF P2P or P2MP.
4. The next-hop interface address of ASBR is within the range of network command of OSPF.
NE20 and NE40 connect through Ethernet interface. NE40 creates Type 5 LSA of A.B.C.0/24 and the Forwarding address is D.E.F.1. NE20 learns this LSA. When NE20 calculates the route, the next-hop is its interface address and the interface is not P2P or P2MP. The route is not valid and cannot be in routing table. When NE20 and NE40 connect through POS interface, NE40 creates Type 5 LSA of A.B.C.0/24 and the Forwarding address is 0. NE20 learns this LSA and creates routing list and points to NE40. Then there is route loop.
After cutover the traffic reaches 155M, possible reasons are as follows:
1. There is abnormal traffic attack.
2. The configuration results in loop.
1. Router learns type 5 LSA, but it may not create routing list and be in routing table.
2. OSPF is non-loop and dynamic routing protocol. Non-loop is for intra-area route and inter-area route and OSPF has no protection mechanism fot external route loop.