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Cisco Bug: CSCvs34056 - NCS reach prefix points to correct outgoing interface but wrong next hop mac address

Last Modified

Jun 19, 2020

Products (1)

  • Cisco Network Convergence System 5000 Series

Known Affected Releases

6.1.45.BASE 6.1.45.FWDG 6.1.45.ROUT

Description (partial)

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
120.29.1.48/29 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 200.200.1.1/32 [110/20] via 121.50.212.198, 00:02:26, TenGigE0/0/0/17

 

 

RP/0/RSP1/CPU0:SNMP-Lab#ping 200.200.1.1
Sending 5, 100-byte ICMP Echos to 200.200.1.1, 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:

 

121.50.212.199 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 121.50.212.199 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 121.50.212.199 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 121.50.212.42 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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