explicit-null simple explanation

[dknov]

Hi,

Is this correct to say that explicit-null prevents PHP from happening?

I mean if edge LSR has network A connected (in global space), normally it would advertise implicit-null tag value to its LDP/TDP neighbors to achieve PHP.

However if you configure to advertise explicit-null tag value, penultimate neighbor will NOT pop this label (with explicit-null tag ID), thus delivering "experimental bits" value of the outer label to edge LSR.

Is this correct? Is the the standard use of explicit-null tag ID?

Thanks,

David

[Harold Ritter]

You are absolutely correct. When advertising an explicit null, the penultmate router will do a label swap rather than a label pop. It swaps the in label to a label value of 0 (explicit null) therefore preserving the exp bits value. This is indeed the only use I see for the explicit null label.

Note that with mpls vpn or l2vpn, you don't really need to use an explicit null since you have your service label that can be used for the purpose of carrying the mpls exp bits to the egress LSR.

Hope this helps,