MPLS Traffic Engineering:: Part – 1 :: Theory

Whenever data is moved and we want the data not to follow the routing protocol dedicated path but to redirect to the path which we select for the traffic to follow. When we determined the path for traffic or data to follow so there are some concept on which Multiprotocol Label Switching - Traffic Engineering (MPLS-TE) is based:

When Routing Protocol is configured on a node it results about the interface parameters like Bandwidth information:

1. MPLS-TE also gives us some information like Bandwidth of the link, how much bandwidth is left (remaining) or Bandwidth is available. Basically it gives the link properties (all these are link constraints).

2. Protocol is needed which carry information and these information are carried by Link State Protocol either OSPF or IS-IS.

3. Constraint based SPF or Path Calculation algorithm is needed for the determination of best path to various routes. This CBSPF use many constraints for path selection other than bandwidth.

4. We have to make sure that the connectivity is available on that path which is selected for traffic. Resource Reservation Protocol (RSVP) is used to reserve bandwidth on the path. For example, we need 10 Mbps on a link, so RSVP ensures this much of bandwidth & RSVP also do the label, changing end-to-end changing (information).

Path Message: Which ensure Bandwidth?
Reserve Message: Confirmation of Bandwidth



In TE, the tunnel headed router decided which path the traffic should follow

a: The LSP Path/Tunnel is one way

b: TE Tunnel configuration is done on Head router, that’s why one way

c: We can use RSVP and LDP parallel like some traffic will pass through tunnel which will use Tunnel Label while some traffic will pass out of Tunnel which will use LDP Label.

d: We can have multiple Tunnels so we can have more than one label.



e: In MPLS TE we basically decided: How to allow the traffic in tunnel means How to route the traffic to the tunnel? & How to feed the traffic in Tunnel?

f: In TE we basically engineer, Like we select another path other than the path which is selected by routing protocols. Routing protocols select best path using only metric which TE use many other parameters for it.

I hope this will be informative for you !

Cheers :)

BFD Session status Changes to Down & Up

While working on DataCom Network (composed of hundreds of NE routers, configured with MPLS, IS-IS, QoS, MPLS-VPN, MPLS-TE, BGP and list goes on and on……..!, I saw this alarm “BFD Session status Changes to down” and “BFD Session status changes to up”. After doing some research, studying and help, I came to know about the root cause and main reason for this alarm.

There can be different reason for alarm to appear:

1. The status of the interface goes down on which the BFD session is established between the two peers.

2. The BFD peer session is deleted or shutdown explicitly.

3. The link is not able to forward packets because of congestion or may be the link status is down.

In my case it was NSA (Non-Service Affecting) and I just need to confirm the status so for that we need to remember few commands which are:


display interface [interface name]

display bfd session all

There are bundle of more commands, if you need any help regarding that please feel free to reach me.

Note: If you are not familiar about BFD? Wait for my next blog on BFD

I Hope this will be informative for you :)

Multiprotocol Label Switching Fast Reroute (MPLS FRR)

In actual MPLS FRR is a feature of RSVP-TE, it is also called MPLS local restoration or MPLS local protection. MPLS FRR gives protection to the LSP path in a network where each LSP is protected by backup path. The node which redirects traffic after path failure to the backup path is known as Point of Local Failure (PLR) and the node where backup LSP merger with primary LSP is called Merge Point (MP). This protection is purely local as compare to protection which is enable at IP Layer (Layer-3) which take a bit more time then this which is not acceptable in real time application (VoIP, Video Conferencing etc). This local protection takes even less than 50 ms.

There are two types of protection approaches:

1. One-to-One Local Protection

In one-to-one approach, PLR maintain a separate path for each LSP across the path or network. This method creates a detour LSP for each protected path at the point of each local failure. For more detail, study RFC 4090

2. Many-to-One Local Protection

In this method, PLR create a single path that can be used to protect multiple LSP. In this method a same tunnel can be used for multiple LSP which acts as a protected path for all links which face failure. See RFC 4090 for more details.




Refer to the figure above, we have Node-A and Node-E as the start and end point for which the primary path (LSP) is from Node-A to Node-E through Node-B and Node-D. While the secondary path for Node-A to reach Node-E is through Node-C. We assume that for primary path FRR is enabled & once it is enable all the remaining nodes on a network came to know about this feature. Assume the link between Node-D and Node-E is down by any means, so the first node who realize this breakdown is Node-D which immediately inform Node-B and Node-A. For Node-A to get the failure message it will take some time and Node-D already know about the link failure & FRR is enable for the LSP so Node-D will use the detour path for the communication (Node-D-C-E) to avoid any loss and get rid of the link failure which will carry the traffic to final destination. This whole process will take less than 50 ms. On the other hand when the backup path (secondary LSP) comes up, traffic will be switched to the secondary LSP and detour path will turn down.

I Hope this will be informative for you :)

Ref: RFC4090