Single Tier T0 Routing question

zeebahi — Mon, 01 Aug 2022 02:34:51 GMT

Hi everyone,

Consider the following example:

We have two transport hosts with over lay segments SEG-A, SEG-B . ESX1 and ESXI2 ,SEG-A: 172.172.172.0/24 and SEG-B 173.173.173.0/24 have been attached to T0 DR routers. To DR1 has two LIF-A: 172.172.172.1/24 and LIF-B: 173.173.1/24

VM-A on ESXI1, is attached to SEG-A ( 172.172.172.10), VM-B on ESXI2, is attached to SEG-B ( 173.173.173.10)

VM-A pings VM-2. 1723.173.173.10.which is received by T0 DR on ESXI1. T0 DR on ESXI1 performs routing look up against 173.173.173.10, and finds it is reachable via directly connected LIF-B.

Next T0DR on ESXI1 needs to find VM-2 MAC address before traffic can be forwarded.

1) Will T0 DR look up VM-B MAC in LIF-A's arp table? Or Will T0 DR look up VM-B MAC in arp table associated with logical switch of SEG-B?

Additional info:

I find conflicting info :

This VM's blog suggest ToDR performs MAC look up in logical switch's arp table:

https://blogs.vmware.com/networkvirtualization/2017/09/nsx-t-routing-where-you-need-it.html/

Distributed Routing for VMs hosted on the different Hypervisors (ESXi & KVM):

Destination MAC, i.e. MAC address of DB VM1 is needed to forward the frame. An ARP entry exists for DB VM1. MAC address of DB VM1 is learnt via remote TEP 192.168.150.152. Again, this MAC/TEP association table was published by NSX Controller to the hosts.

NSX-T guide on the other hand says,

https://nsx.techzone.vmware.com/resource/nsx-t-reference-design-guide-3-0#_Toc59008623

Page 51:

The routing lookup happens on the HV1 DR, which determines that the destination
subnet 172.16.20.0/24 is a directly connected subnet on “LIF2”. A lookup is performed in
the “LIF2” ARP table to determine the MAC address associated with the IP address for
“App2”. This destination MAC, “MAC2”, is learned via the remote HV2 TEP
20.20.20.20.

Thanks !!!

Re: Single Tier T0 Routing question

ShahabKhan — Mon, 01 Aug 2022 08:56:09 GMT

Hi,

ARP lookup always happens with the same subnet. So in this case, it will happen within LIF-B's ARP table.

The blog you are referring to, says the Routing lookup is performed on the DR & the DB1 VM entry already existed in the ARP table.

I hope this clarifies your doubt.

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