MPLS reduces CPU-usage on routers, by allowing routers to make forwarding
decisions depending on the attached label rather than looking to routing table.
MPLS is not restricted to IP, or any specific Layer-2 technology, and thus is essentially
protocol-independent.
Introduction to terms used in MPLS
Provider Edge Routers(PE) / Edge LSR (Label Switch Router) : Routers on
the ISP side which is connected to Customer/Client side. Responsible for
adding/removing labels from packets. Cisco refers to this as a PE (provider edge) router.
Provider Router(P) / LSR (Label Switch Router) : Routers
used inside the ISP network which is not connected to customer side. Responsible
for forwarding packets through the
provider core based on the packet’s label
Non-Label Routers – Cisco refers to this as C
(customer) routers
LSRs perform the following functions:
• Control Plane – Logical plane where exchange
of routing and label information takes place
• Data Plane - forwards actual packets based
on label information
Label Information Base (LIB) – label information is exchanged between LSRs using label protocols, such as LDP
(Label Distribution Protocol) or TDP
(Tag Distribution Protocol).
LDP is now default on Cisco devices, and uses TCP port 646. TDP is
a Cisco-proprietary label protocol, and uses TCP port 711.
Label Forwarding Information Base (LFIB) – for
forwarding labeled packets.
The MPLS Label
MPLS label format is a 32-bit label that is injected between the
Layer-2 and Layer-3 header.Even MPLS is called as L2 service theoritically it
can be called as a 2.5 layer service
LABEL
|
Experimental
|
Bottom
of Stack
|
TTL
|
20 bit ` 3
bit 1 bit 8
bit
Label (20 bits) – 20 bits are exactly used for label
representation
Experimental (3 bits) – This field is officially undefined, but is
used by Cisco as an IP precedence value.
Bottom-of-Stack (1 bit) – This field indicates the last label, as multiple
labels are supported in the same packet. A value of 1 identifies the last label in the stack.
TTL (8 bits)[Time to Live] – This field indicates the number of router
this label can ‘live’ through.
Penultimate Hop Popping: Edge Routers(PE) need to check both
normal routing table and LIB table before forwarding packets to customer side.This
will slow down the process.To avoid this
PE router informs adjacent router to send packets after popping up(removing)
labels.This process is called Penultimate Hop Popping
From Wikipedia
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 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
Actually you are a great writer . I have read your full post and i seen here you present MPLS very creatively.I am impresses to read your article. Because i am a student of networking Cisco.And this post is very effective for me. Thanks for your great post.
ReplyDeleteI am also a network cisco profession, this is a very useful for me & very .simple language, thanks for this post.*
DeleteReally wonderful and informative material...thanx
ReplyDeleteEasy to understand for beginners, thanks for the post.
ReplyDeleteGood to recollect
ReplyDeleteVery well written notes.....really appreciate your hard work!!!
ReplyDelete