Source IP of PINGs from PE router in MPLS

[CCIE28]

Hi,

I wasn't able to find any documentation from Cisco on this.

When doing a ping on PE device thru the MPLS network (ping vrf custA 1.1.1.1) how does the router choose the source address of the pings? 

Please don't tell me that it uses the outgoing interface's address!

I'm pinging thru the MPLS network, there is only one outgoing interface (towards the P router), and for sure that address won't be used.

I did lab it, and it appears that the router is choosing an arbitrary address that is part of custA vrf. When I shut down that interface, the router uses any other available address. If no interfaces are up in that vrf, the router can no longer generate the pings.

My question is this, is the address chosen really arbitrary? or do Cisco routers have an ordered selection of it (i.e. start with the highest loopback Ip, followed by the highest physical int IP...)

[Elvin Arias Soto · Accepted solution]

There is an internal order which can be checked with the “show ip vrf interface” command. The first interface in the list would be used as a source, then the next in that table and so on.

Elvin

[Patrick.Williams]

So i did some simulation in my Lab containing 4 C9300  switches to gain some additional insights as to the question you have regarding the source IP chosen for a  vrf containing several interface / source IP address.

The initial or first interface created will always be chosen as the source address for all communication within a vrf containing multiple interfaces, if that interface is removed or disabled for some reason the second interface created or added to the vrf will be the one chosen as the  source address for all communication once a specific network or source  interface is not specified.

Also to note is that these source IP address is not chosen by the highest interface or address that is associated locally within the vrf

Cheers.