Penultimate Hop Popping
The LSR directly connected to a destination network (typically the PE router)
is referred to as the ultimate hop for that network. The router right before it
is the penultimate hop for that network. The directly connected LSR can
advertise a label value of 3 for that destination network. Label 3 tells the
neighbor router to pop the label before forwarding the packet—referred to
as penultimate hop popping (PHP). This is recorded in the neighbor’s FIBs
as a null label.
PHP saves time and work for the PE LSR. Without it, the PE router would
have to do an LFIB lookup, remove the top label, and then do a FIB lookup.
With PHP, it only needs to do the FIB lookup, which helps optimize MPLS
performance.
Figure 3-3 puts all these concepts together to show how MPLS routers use
labels and each of the tables to forward traffic.
MPLS Packet Forwarding
Customer IP = X Label = 18 Label = 37 IP = X
Edge (CE)
Router
Customer
Edge (CE)
Router
Network
X
MPLS Network
Provider LSR LSR
Edge (PE)
LSR
Provider
Edge (PE)
LSR
CE-1 A B C D CE-2
FIB
LIB
LFIB
18
5
18
B
B
local
B
18
X
XX
52
FIB
LIB
LFIB
37
18
37
C
C
local
C
37
X
XX
18
FIB
LIB
LFIB
37
null
D
D
local
D
pop
X
XX
37
FIB
LIB
LFIB
20
conn.
local
untagged
X
XX
20
In Figure 3-3, router CE-1 forwards a packet destined for network X, which
is directly connected to LSR D. LSR A receives it as an IP packet, does a FIB
lookup, and imposes the label of 18. This is the value that LSR B advertised
to it for network X. It forwards the labeled packet to LSR B.
LSR B receives the labeled packet, does and an LFIB lookup, and swaps the
labels. It forwards the packet to LSR C with a label of 37, the label value
advertised by C for network X.
LSR C receives the labeled packet and does an LFIB lookup. It sees that it
must pop the label before forwarding the packet to the next hop, router D.
When the label is popped, there are no other labels, so the packet is
forwarded to LSR D as an IP packet.
The packet is addressed to the WAN interface of CE-2, so LSR D forwards
it as an IP packet to CE-2.