Archive for June, 2011

MPLS Notes

MPLS is popular with service providers

Types of MPLS

– Cell mode MPLS
– Frame mode MPLS

With frame mode mpls the routing table will determine the outgoing interface by
looking at its routing table. The routing protocol such as eigrp or ospf will
determine this factor. MPLS will work over the routing protocol.

Once the packet enteres the mpls network the router will insert a label on
the packet.

It will be inserted between the layer 2 and layer 3 header.

MPLS is a layer 2.5 technology because the label is attached between the
layer 2 header and the layer 3 header.

The router will then make fowarding decisons based soley on the label and
not the destination IP address.

This will make fowarding more efficent as the router will no longer need
to make IP routing table lookups as the path is pre determined based on the

MPLS offers us many other beniefts such as traffic engineering and QOS. MPLS
is suited for VPN, which can add security for our networks.

The control plane will associate labels with the IP routes. The control plane
controls the protocol which does the label distruibtion. Every IP prefix has
its own label mapping. The control plane uses the LDP to populate the LFIB, the label fowarding information base.

CEF is required to run MPLS.

The data plane maintains the LFIB (label fowarding information base) and is used by mpls to send frames outside the apporiate interface.

inside a mpls network you have a:

LSR – Label swithc router. An LSR is responisble for swaping labels. Edge LSR can look at the destination address and apply a label based on the destination. This can happen when a packet enters a MPLS network. When the packet exits the mpls network the Edge LSR router is responsible for removing the label.

To enable MPLS we must have CEF confifured first.
We than use a protocol for label distribuition. We can use label distribuition protocol (LDP) or Tag distruibtion Protocol (TDP).

Router(Config)ip cef – Enables the CEF on all routers interfaces

Router(Config-if)mpls ip – enables the mpls label switching on the interface

Router(config-if)mpls label protocol ldp – specifies the label distribution protocol to be used by the interface.

Router(config-if)mpls mtu size – configure the maximum MTU size for the labels
as we are adding labels there will be additional data we do not want the packet to be greater than 1500, the default size. If this happens fragmentation can occur.

Config t
ip cef
interface fa 0/0
mpls ip
mpls label protocol ldp

config t
ip cef
interface fa 0/0
mpls ip
mpls label protocol ldp

LDP neighborship should form between the routers

config t
ip cef
interface fa 0/0
mpls ip
mpls label protocol ldp

Verify MPLS

show mpls interfaces
show mpls ldp neighbor
show mpls ldp bindings


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