Remote Client Communication Before Mobility Event

Remote Client Communication Before Mobility Event

Now here we will see how traffic or remote clients communicate to each other before a Mobility Event. To do so, we have to verify that both DC and remote xTR have successfully registered their EID with MS.

Here we see that these Global EID are registered every 60 sec by each xTR, this can be verified by sing the timer which will not exceed the 60 sec. As both xTR will send the registration message, so the message received lastly by MS, those IP address will be updated in “Who last registered” section and will continuously change.

Below figure describes the routing table of DC xTRs before workload Migration.

In the Routing table, we see that two /25 Null0 routes have been installed, and this happens as soon as we configure “lisp extended-subnet-mode” for SVI VLAN 100. These two routes are required, in order to punt the traffic to CPU as soon as Workload will generate the first packet so that xTR can dynamically discover it and register specific /32 prefix with Map-server. 

This happens in following Sequence:

• The local Work load generates the first packet and is received by xTR on SVI 100 enable for Host Mobility.
• LISP logic will check the IP address of received packet, and lookup will be successful if its Unicast Reverse path Forwarding (URPF) check is successful. But here it will be not because the route table has more specific route with Null0 with compare to /24 of SVI VLAN.
• As packet will match the /25 Null0 routes, the packet will be punted to CPU which further triggers EID dynamic discovery.

Even though Null0 routes are more specific, but traffic originated from Layer3 domain will not be routed to mobile subnet and will be dropped.

Rule: 

One Basic rule to be kept in mind that Communication to workload for mobile subnet can only be possible after that work load has been dynamically discovered by xTRs independently described in below Figure

Now to establish the communication successfully, first discovery of EID at West DC has to be achieved, below is the following sequence that will occur in order to discover EID.