VMware Networking Community
ChrisOk
Enthusiast
Enthusiast
‎02-13-2020 03:38 AM

[NSX-T 2.5.1] PathMTU black hole on Tier-0-Gateway?

So I started mtupath and mturoute from an VM on NSX-T to an external IP address:

- mturoute.exe -t 10.30.16.24

mturoute to 10.30.16.24, 30 hops max, variable sized packets

* ICMP Fragmentation is not permitted. *

* Speed optimization is enabled. *

* Maximum payload is 10000 bytes. *

1  +-  host: 10.30.48.24  max: 1500 bytes

2  ...-+++++.-+.-++.-  host: 100.64.80.0  max: 1442 bytes

3  +.-  host: 10.30.79.2  max: 1442 bytes

4  +.-  host: 10.30.80.158  max: 1442 bytes

5  +.-  host: 10.30.80.150  max: 1442 bytes

6  +.-  host: 10.30.252.75  max: 1442 bytes

7  +.-  host: 10.30.80.102  max: 1442 bytes

8  +.-  host: 10.30.253.78  max: 1442 bytes

9  +.-  host: 10.100.0.2  max: 1442 bytes

10  +.-  host: 10.100.0.5  max: 1442 bytes

11  +.-  host: 10.30.80.210  max: 1442 bytes

12  +.-  host: 10.30.80.214  max: 1442 bytes

13  +.-  host: 10.30.80.218  max: 1442 bytes

14  +.-  host: 10.30.16.24  max: 1442 bytes

- mtupath.exe 10.30.16.24

MTU path scan to 10.30.16.24, ttl=64, limit=48

# 21 best MSS 1414 (estimated MTU 1442) [pPPPPpPppP***p*p*****]

# 08 nearest minimum MTU on 100.64.80.0 (2 hops away)

        #1 MSS IN RANGE     1 <==  1413 ==>  1414

        #2 SCAN TIMEOUT  1415 <==    57 ==>  1472

        #3 MSS EXCEEDED  1473 <== 14911 ==> 16384

[WARNING] PMTU blackhole found in path from this host to 100.64.80.0 -----> Why?

- VM is connected to Tier-1-Gateway which is connected to Tier-0-Gateway:

100.64.80.1/31 belongs to the Tier-1-Gateway

100.64.80.0/31 belongs to the Tier-0-Gateway

Why is the MTU at Tier-0-Gateway limited to 1442 bytes? Does anyone have an idea? How to fix this situation?

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1 Reply
mauricioamorim
VMware Employee
VMware Employee
‎02-17-2020 10:01 AM

When going to the Tier0 router the SR that is running on the Edge Node will be used. This means there will be traffic from the host TEP to the Edge Node TEP and this goes through your physical network. Does the physical network have an MTU >1600? Another place to check is the regarding Edge Node. Typically a VM is used connected to a vDS. If this is the case the vDS needs to have MTU>1600. Is this the case? How have you deployed your edge VM?

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