The Concept of PHP (Penultimate Hop Popping) – MPLS

PHP is penultimate hop popping which means remove the label one hop before its destination.It refers to the process whereby the outermost label of an MPLS tagged packet is removed by a Label Switch Router (LSR) before the packet is passed to an adjacent Label Edge Router.

The process is important in a Layer 3 MPLS VPN environment as it reduces the load on the LER. If this process didn’t happen, the LER would have to perform at least 2 label lookups:

  1. The outer label, identifying that the packet was destined to have its label stripped on this router.
  2. The inner label, to identify which Virtual Routing and Forwarding (VRF) instance to use for the subsequent IP routing lookup.

In a large network this can result in the CPU load on the LER reaching unacceptable levels. By having PHP for an LER done on the LSRs connected to it, the load is effectively distributed among its neighbor routers.

To avoid this extra work on the very last (ultimate) LSR, MPLS uses a feature called penultimate hop popping (PHP). (Penultimate simply means “1 less than the ultimate.”) So the penultimate hop is not the very last LSR to process a labeled packet, but the second-to-last LSR to process a labeled packet. PHP causes the penultimate-hop LSR to pop the outer label, so that the last LSR—the ultimate hop if you will—receives a packet that only has the VPN label in it. With only this single label, the egress PE needs to look up only one entry in the LFIB

Penultimate Hop Popping is used only for directly connected subnets or aggregate routes. In the case of a directly connected interface, Layer 3 lookup is necessary to obtain the correct next-hop information for a packet that is sent toward a directly connected destination. If the prefix is an aggregate, a Layer 3 lookup also is necessary to find a more specific route that then is used to route the packet toward its correct destination. In all other cases, the Layer 2 outbound packet information is available within the LFIB and, therefore, a Layer 3 lookup is not necessary and the packet can be label switched.

 

With penultimate hop popping, the Edge-LSR can request a label pop operation from its upstream neighbors. In the SuperNet network, the Washington router pops the label from the packet and sends a pure IP packet to the New York router. Then the New York router does a simple Layer 3 lookup and forwards the packet to its final destination
Penultimate hop popping is requested through TDP or LDP by using a special label value (1 for TDP, 3 for LDP) that also is called the implicit-null value.
When the egress LSR requests penultimate hop popping for an IP prefix, the local LIB entry in the egress LSR and the remote LIB entry in the upstream LSRs indicate the imp-null value and the LFIB entry in the penultimate LSR indicates a tag pop operation

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s