3Com MSR 50 Series, Configuration Manual

Page: 1314 / 2443

1314 C
HAPTER
76: MPLS B
ASICS
C
ONFIGURATION
■
The existing protocols that are extended to support label distribution, such as
border gateway protocol (BGP) and resource reservation protocol (RSVP).
In addition, you can configure static LSPs.
n
■
For information about CR-LDP and RSVP, refer to “MPLS TE Configuration” on
page 1345 .
■
For information about BGP, refer to “BGP Configuration” on page 825 .
LSP tunneling
MPLS supports LSP tunneling.
An LSR of an LSP and its downstream LSR are not necessarily on a path provided
by the routing protocol. That is, MPLS allows an LSP to be established between
two LSRs that are not on a path established by the routing protocol. In this case,
the two LSRs are respectively the start point and end point of the LSP, and the LSP
is an LSP tunnel, which does not use the traditional network layer encapsulation
tunneling technology. For example, the LSP <R2
→
R21
→
R22
→
R3> in Figure 379 is
a tunnel between R2 and R3.
If the path that a tunnel traverses is exactly the hop-by-hop route established by
the routing protocol, the tunnel is called a hop-by-hop routed tunnel. Otherwise,
the tunnel is called an explicitly routed tunnel.
Multi-level label stack
MPLS allows a packet to carry a number of labels organized as a last-in first-out
(LIFO) stack, which is called a label stack. A packet with a label stack can travel
along more than one level of LSP tunnel. At the ingress and egress of each tunnel,
these operations can be performed on the top of a stack: PUSH and POP.
MPLS has no limit to the depth of a label stack. For a label stack with a depth of
m, the label at the bottom is of level 1, while the label at the top has a level of m.
An unlabeled packet can be considered as a packet with an empty label stack, that
is, a label stack whose depth is 0.
Architecture of MPLS Structure of the MPLS network
As shown in Figure 380 , the element of an MPLS network is LSR. LSRs in the same
routing or administrative domain form an MPLS domain.
In an MPLS domain, LSRs residing at the domain border to connect with other
networks are label edge routers (LERs), while those within the MPLS domain are
core LSRs. All core LSRs, which can be routers running MPLS or ATM-LSRs
upgraded from ATM switches, use MPLS to communicate, while LERs interact with
devices outside the domain that use traditional IP technologies.
Each packet entering an MPLS network is labeled on the ingress LER and then
forwarded along an LSP to the egress LER. All the intermediate LSRs are called
transit LSRs.