<s>
An	O
"	O
ingress	B-General_Concept
router	I-General_Concept
"	O
is	O
a	O
Label	B-Protocol
Switch	I-Protocol
Router	I-Protocol
that	O
is	O
a	O
starting	O
point	O
(	O
source	O
)	O
for	O
a	O
given	O
Label	O
Switched	O
Path	O
(	O
LSP	O
)	O
.	O
</s>
<s>
An	O
ingress	B-General_Concept
router	I-General_Concept
may	O
be	O
an	O
egress	B-General_Concept
router	I-General_Concept
or	O
an	O
intermediate	O
router	O
for	O
any	O
other	O
LSP(s )	O
.	O
</s>
<s>
Hence	O
the	O
role	O
of	O
ingress	O
and	O
egress	B-General_Concept
routers	I-General_Concept
is	O
LSP	O
specific	O
.	O
</s>
<s>
Usually	O
,	O
the	O
MPLS	B-Protocol
label	O
is	O
attached	O
with	O
an	O
IP	O
packet	O
at	O
the	O
ingress	B-General_Concept
router	I-General_Concept
and	O
removed	O
at	O
the	O
egress	B-General_Concept
router	I-General_Concept
,	O
whereas	O
label	O
swapping	O
is	O
performed	O
on	O
the	O
intermediate	O
routers	O
.	O
</s>
<s>
However	O
,	O
in	O
special	O
cases	O
(	O
such	O
as	O
LSP	O
Hierarchy	O
in	O
RFC	O
4206	O
,	O
LSP	O
Stitching	O
and	O
MPLS	B-General_Concept
local	I-General_Concept
protection	I-General_Concept
)	O
the	O
ingress	B-General_Concept
router	I-General_Concept
could	O
be	O
pushing	O
label	O
in	O
label	O
stack	O
of	O
an	O
already	O
existing	O
MPLS	B-Protocol
packet	O
(	O
instead	O
of	O
an	O
IP	O
packet	O
)	O
.	O
</s>
<s>
Note	O
that	O
,	O
although	O
the	O
ingress	B-General_Concept
router	I-General_Concept
is	O
the	O
starting	O
point	O
of	O
an	O
LSP	O
,	O
it	O
may	O
or	O
may	O
not	O
be	O
the	O
source	O
of	O
the	O
under-lying	O
IP	O
packets	O
.	O
</s>
