Autumn 2010
MULTIPROTOCOL LABEL
SWITCHING (MPLS) AND MPLS
VPNS
How Routers Forward Packets
Process switching
Hardly ever used today
Router looking inside the packet, at the ip address, comparing it to its routing table. See what the next hop ip address is, eventually performing an arp lookup.
Fast switching
First packet will be process switched – chances are big that there will be more packets going to the same destination.
So, most recent destinations are entered in the cache
The router wont have to look at the routing table, combining ip with mac, for the subsequent packets.
The router wont have to look at the routing table, combining ip with mac, for the subsequent packets.
Had some fallbacks, didnt support ”per packet load sharing” (requires multiple cache entries).
Topology-driven switching
Cisco Express Forwarding (CEF)
Eliminates the ”first packet problem”
Prebuilds the cache by making ”a copy” of the routing table and creating the FIB – Forwarding Information
Moves all routes into the cache.
Also creates an adjacency table that premaps all next hops. The mac addresses are also added to this table by consulting the arp cache.
Adds the feature of per packet load sharing.
MPLS
What is Multi Protocol Label Switching?
CEF is the fundamental switching path for MPLS. Without CEF, MPLS forwarding does not
occur. MPLS forwarding relies heavily on the IP routing table and the CEF architecture.
Therefore, MPLS VPN relies on CEF because MPLS VPN depends on MPLS for successful
operation.
MPLS is a switching mechanism that assigns labels, or numbers, to packets and
then uses those labels to forward packets. The labels are assigned at the edge of
the MPLS network, and forwarding inside the MPLS network is based solely on
labels.
the MPLS network, and forwarding inside the MPLS network is based solely on
labels.
The content of the label may vary
Destination network
Level of Quality of Service
The Label Distribution Protocol (LDP) is often used to establish MPLS and
handle the labels. Tag Distribution Protocol (TDP) is a Cisco proprietary protocol
managing the same thing.
The Label
LABEL
EXP
BS
TTL
DATA
PR
IP
L2
L1
MAC (L2)
32 bits
Labels
MPLS Example
20.0.0.0 /8
MPLS DOMAIN
Exchanging routes
Assigning labels
Sharing labels
Building tables
20.0.0.0 /8
20.0.0.0 =25
MPLS DOMAIN
Router B
NON-MPLS
Router A
NON-MPLS
MPLS Example - Z-router
20.0.0.0 /8
20.0.0.0 =30
MPLS DOMAIN
Network
Next Hop
20.0.0.0
Y
Router Y
Network
LSR
Label
20.0.0.0
Local
35
20.0.0.0
Y
30
Label
Action
Next hop
Tables
Network
Next Hop
20.0.0.0
Y
15.0.0.0
H
Network
LSR
Label
20.0.0.0
Local
35
20.0.0.0
Y
30
Label
Action
Next
hop
35
30
Y
Routing Table
Label Information Base LIB
Label Forwarding
Information Base LFIB
15.0.0.0
H
16.0.0.0
O
20.0.0.0
Y
30
15.0.0.0
Local
36
15.0.0.0
Y
12
40
untagged
Y
50
pop
B
Network
Next Hop
Label
20.0.0.0
Y
-15.0.0.0
H
-16.0.0.0
O
LIB FIB LFIB... FBI?
Confused? ;-)
LIB – Label Information Base - whenever a
labeled packet comes this table will be referred
FIB – Forwarding Information Base -whenever a
nonlabeled packet comes this table will be
referred
nonlabeled packet comes this table will be
referred
LFIB- Label Forwarding Information Base - Any
route in the LFIB will also be in the LIB, but not
the other way around.
Functions of Label Switching
Routers (LSRs)
Control Plane
Controls the routing information exchange and the label
exchange between adjacent devices.
Exchanges routing information via “normal” routing protocols
Exchanges label information using Label Distribution Protocol
(LDP)
(LDP)
Sets up framework for how everything is going to be forwarded.
Data Plane – where the action occurs
Also known as the forwarding plane, this plane controls
forwarding based on either destination addresses or labels. – L3
or L2 information
Router becomes “almost like a switch”
If there’s no label, it will work as “normal”. (CEF)
Label Switching Routers
EDGE LSR
LSR
IP header MPLS header L2 header
IP header L2 header IP header L2 header
EDGE LSR
EDGE LSR
LSR
LSR
LSR – forwarding packets
”Core router” Primary
MPLS Terminology
MPLS, Multiprotocol Label Switching
LDP (Label Distribution Protocol)
Penultimate Hop Popping
<-15.0.0.0 = pop
15.0.0.0 = 20
Y
Label
Action
Next hop
20
Pop
Z
Y LFIB
Two traditional categories
of VPNs
Overlay VPNs
Point-to-Point
Circuits between customer sites
Virtual Links – Layer 1, 2
Becomes expensive to buy virtual circuits for many sites
Links / Virtual Circuits
Peer-to-Peer VPNs
Service Provider
becoming a part of your network
Managing routing between parts of the organization
”Bringing our tables into their”
Private addresses from different customers will be a problem
No NAT
Customers will be using the same private addresses sometimes.
MPLS VPNs - overview
Provider is forwarding routes between the sites.
Virtual Route Forwarding allows you to run Separate Routing tables and
forwarding tables per customer.
Eliminates the problem of using the same address-space since VRFs make them
look like they are different routing tables.
PE routers – Provider Edge, like Edge LSR.
P routers – doing ”core business”. Wont see any routes.
Routing information packets are encapsulated using ”tags”.
Routing information packets are encapsulated using ”tags”.
Performed by PE routers. Customer one may tag it with a ”1”.
PE routers remove tags and propagate routes out to the customer 1.
Route Distinguisher (tag)
and Route Target
Route distinguisher (RD)
64-bit tag identifies customer route advertisements
May be any number the service provider chooses to use.
Keeps customer routes unique
Route Target (RT)
Additional field to allow customers to participate in
multiple VPNs.
“VRFs use the route target attribute to control the
import and export of VPNv4 routes through iBGP. The
route target is an extended BGP community that
The problem with overlaping
customer addresses
BGP/MPLS VPN support a mechanism that
converts nonunique IP addresses into globally
unique addresses by combining the use of
VPN-IPV4 address family with the
VPN-IPV4 address family with the
VPN-IPv4
A VPN-IPv4 address is a 12-byte quantity
composed of an 8-byte Route Distinguisher
(RD) followed by a 4-byte IPv4 address prefix.
Multiprotocol BGP Extensions
(MP-BGP)
Conventional BGP4 was originally designed
to carry routing information only for the IPv4
address family.
Realizing this limitation, the IETF is
Realizing this limitation, the IETF is
standardizing the Multiprotocol Extensions
for BGP4.
The extensions allow BGP4 to carry routing
information for multiple Network Layer
BGP/MPLS VPN can use up to three different
types of BGP extended community attributes
The route target attribute identifies a collection of
sites (VRFs) to which a PE router distributes
sites (VRFs) to which a PE router distributes
routes. A PE router uses this attribute to constrain
the import of remote routes into its VRFs.
The VPN-of-origin attribute
The MPLS part
CE routers should not be MPLS
VPN-aware; they should run
standard IP routing software.
PE routers must support MPLS
VPN services and traditional
Internet services. To make the
MPLS VPN solution scalable,
Customer
EIGRPMPLS VPN solution scalable,
P routers must not carry VPN
routes.
Multi Protocol BGP within the SP
The MPLS part
The top label in the stack is the LDP label for normal frame forwarding in the MPLS network. This label guarantees that the packet will traverse the MPLS VPN backbone and arrive at the egress PE router.
The second label in the stack identifies the egress PE router. This label tells the router how to forward the incoming VPN packet. The second label can point directly toward an outgoing interface. In this case, the egress PE router performs label lookup only on the VPN packet. The second label can also point to a VRF table. For this case, the egress PE router first performs a label lookup to find the target VRF table and then performs an IP lookup within the VRF table.
When you are implementing MPLS VPN, you need to increase the MTU size to allow for two labels.
