Penultimate Hop Popping
Encyclopedia
Penultimate Hop Popping (PHP) is a function performed by certain routers in an MPLS
enabled network. 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
(LER).
The process is important in a Layer 3 MPLS VPN
(RFC2547) 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:
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 neighbour routers.
PHP functionality is achieved by the LER advertising a label with a value of 3 to its neighbours. This label is defined as implicit-null and informs the neighbouring LSR(s) to perform PHP.
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
enabled network. It refers to the process whereby the outermost label of an MPLS tagged packet is removed by a Label Switch Router
Label Switch Router
A label switch router , is a type of a router located in the middle of a Multiprotocol Label Switching network...
(LSR) before the packet is passed to an adjacent Label Edge Router
Label Edge Router
A label edge router is a router that operates at the edge of an Multiprotocol Label Switching network....
(LER).
The process is important in a Layer 3 MPLS VPN
MPLS VPN
MPLS VPN is a family of methods for harnessing the power of Multiprotocol Label Switching to create virtual private networks . MPLS is well suited to the task as it provides traffic isolation and differentiation without substantial overhead....
(RFC2547) 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:
- The outer label, identifying that the packet was destined to have its label stripped on this router.
- The inner label, to identify which Virtual Routing/ForwardingVRFIn IP-based computer networks, Virtual Routing and Forwarding is a technology that allows multiple instances of a routing table to co-exist within the same router at the same time...
(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 neighbour routers.
PHP functionality is achieved by the LER advertising a label with a value of 3 to its neighbours. This label is defined as implicit-null and informs the neighbouring LSR(s) to perform PHP.