Cisco Bug: CSCvs34056 - NCS reach prefix points to correct outgoing interface but wrong next hop mac address
Jun 19, 2020
- Cisco Network Convergence System 5000 Series
Known Affected Releases
6.1.45.BASE 6.1.45.FWDG 6.1.45.ROUT
ASR9006-1 PE8(Te0/0/0/17) ----------- (Te0/0/0/0) NCS5001 (Te0/0/0/1) ----------- (Te0/0/0/17) ASR9006-2 PE9 00c1.6440.a169 5061.bfa1.dd76 5061.bfa1.dd77 00c1.6400.4e21 The setup is quite simple: 1. PE8, NCS-1, and PE9 are under same ospf area 0. 2. All three routers have configured redistribute static route and connected interface into OSPF process. 3. NCS-1 is having MPLS LDP neiborship with PE8 and PE9 respectively. The repro process for this issue is below: 1. Add a new static route on PE8. The static route destination is overlap with the LP1 interface on PE9. On PE8: router static address-family ipv4 unicast 220.127.116.11/29 Null0 <---- This subnet overlap with the Loopback 1 on PE9 ! 2. Right after adding this route. I found PE9 can not ping any interface on PE8. Eventhough the routing table is normal. P/0/RSP1/CPU0:PE9#sh route ...snip.. O E2 18.104.22.168/32 [110/20] via 22.214.171.124, 00:02:26, TenGigE0/0/0/17 RP/0/RSP1/CPU0:SNMP-Lab#ping 126.96.36.199 Sending 5, 100-byte ICMP Echos to 188.8.131.52, timeout is 2 seconds: ..... I found the problem is that on NCS that all the the CEF next hop MAC for PE9 changed to PE8's MAC after adding the satic route on PE8. For example: 184.108.40.206 is the interface IP of PE9 towards NCS. Before adding the static route on PE8, the CEF on NCS is below: RP/0/RP0/CPU0:NCS-5001#sh cef 220.127.116.11 hardware egress | inc MAC Next Hop MAC: 00c1.6400.4e21 < ---- This is MAC address of PE9 After adding the static route on PE8, the CEF on NCS is below: RP/0/RP0/CPU0:NCS-5001#sh cef 18.104.22.168 hardware egress | inc MAC Next Hop MAC: 00c1.6440.a169 <----- This is MAC address of PE8 3. To recover from this issue, I need flap the link between NCS and PE9. After mannually flap the link, PE9 is able to ping PE8 normally and CEF entry on NCS is normal. RP/0/RP0/CPU0:NCS-5001#sh cef 22.214.171.124 hardware egress | inc MAC Next Hop MAC: 00c1.6400.4e21 <------ This is MAC of PE9, which is correct after flap interface. Symptom: CEF misprogramming to reach prefix NCS points to correct outgoing interface but wrong next hop mac address Conditions: Import an external route that overlap with another ospf routes into OSPF process.
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