MPLS VPN FOR ABSOLUTE BEGINNERS

ROUTING TABLE ANIMATION

MPLS VPN   MPLS stands for MultiProtocol Label Switching VPN stands for Virtual Private Network   In conventional packet forwarding every router opens the IP packet and looks at IP header to find out destination IP address and then with the help of its routing table takes independent decision to forward the packet. Handling the bulky IP header and reconstructing the ip packet again reduces the speed.   But MPLS works at layer 2. It classifies each packet based on FEC (Forwarding Equivalence Class). Subsequent routers simply look at the label and route the packet after swapping the label with a new one.

This is the trace route output of 61.1.196.1 using the website whatismyipaddress.com.

 

The visual tracert tool displays the path Internet packets traverse to reach a specified destination. The tool works by identifying the IP addresses of each hop along the way to the destination network address. The estimated geophysical location of each hop is identified using MaxMind's GeoIP database. After all of the hops locations' are identified, the path to the destination is plotted on a Google Map. Concerning trace routes The IP packet is having a field called "time to live(TTL)".Normally its intial value is 30. In case of windows, it may even be 128.. Every time a host forwards a packet, it decrements the TTL value of the packet by one. If the TTL value of a packet is zero and it has yet to reach its destination, the packet is discarded and the host will send a response back to the sender notifying them that the destination was not reached. The main purpose of TTL is to prevent packets from endlessly looping around the Internet, never finding their destination. A “trace route” works by increasing the TTL value of each successive packet sent. The first packet is sent with a TTL value of one (implying that it will make a single hop). The next packet has a TTL value of two, and so on. By doing so, a destination unreachable response packet is elicited from each hop. These returning packets are used to produce a list of hosts that the packets have traversed en route to the destination.

What is FEC(Forward Equivalence Class)?

In a non-mpls routing, each router looks into the packet header, picks up the destination address and compare this address with the routing table entries and selects the best match (longest match). All packets forwarded to this particular match are treated as same FEC.

In cases MPLS, all packets that are forwareded in the same manner by an LSR belongs to one FEC.

MPLS assigns each packet to a FEC only at the ingress LSR.

 

What is LIB?

• An FEC is represented by Labels along the Label Switch Path(LSP) from the ingress router to egress router.
• Eash label is valid only between two LSRs.
• The table which contains the information of next hop label is called as LIB.
• The ingress and egress LSRs are also called as Edge Routers

 

Which protocols are used in MPLS?

BGP, IS-IS, or OSPF as used as Layer 3 protocol.

BGP, LDP, or RSVP-TE is used as Label Distribution Protocol.

 

What is Penultimate Hop Pop(PHP?

Normally, the Egress Router has to do two look-ups. First it has to look into the Connection Table (also known as Forwarding Table) to decide whether to SWAP or POP. Once it has decided to Pop, then again it has to look for information based on the IP header of the packet.

In some MPLS implementations, PHP is used to avoid the first look-up. The penulitmate router (router prior to the egress router) itself will pop the label and sends just the unlabeled ip packet. Hence the first lookup is eliminated.

 

Some excellent Tutorials on MPLS VPN

converge digest


Juniper

What is LDP(Label Distribution Protocol)?

A set of Labels from the Ingress router to Egress Router forms a LSP (Label Switched Path). LDP helps in forming this LSP by mapping network layer routing to the data link layer switch path. LDP distribute labels to its LDP peers. RSVP-TE or CR-LDP are LDPs. LDP establishs LSP and reserve necessary resources for this data path.

What is a Label Switch Path?
An LSP is a specific path traffic path through an MPLS network. An LSP is provisioned using Label Distribution Protocols (LDPs) such as RSVP-TE or CR-LDP. Either of these protocols will establish a path through an MPLS network and will reserve necessary resources to meet pre-defined service requirements for the data path.

a. What is a Label?
It is 32 bit and placed between Layer 2 header and IP header. It contains the label value (20bits) and other information.

5. Compare MPLS with ATM.

MPLS has more advantages over ATM. Hence many service providers migrate their ATM networks to MPLS networks.