Orhan Ergun 2 Comments

MPLS Traffic Engineering Fast Reroute Link Protection – I will explain it in plain English and the most important point of MPLS TE FRR Link Protection among other Fast Reroute mechanisms.

 

Sometimes I am surprised that we have a very long technology names in networking 🙂

 

Okay, let me explain briefly what is fast reroute , then why we use fast reroute in Layer 3 with MPLS , and finally explain how MPLS TE FRR works.

 

As you can understand , I will use FRR for short, instead of fast reroute. And MPLS TE for short, instead of MPLS Traffic Engineering.

 

What is Fast Reroute ?

 

The detection of link failure, the propagation of information to every device in the flooding domain, and calculating and installing the new paths into the routing and forwarding tables of the devices will require some time. Interior gateway protocol parameters for propagation and detection can be changed, and convergence time might be reduced to even less than one second. But for some applications like voice, this may not be enough.

 

We may need latency to be less than 100 or 200 ms in order to reroute traffic without experiencing adverse effects. MPLS traffic engineering can often provide a backup path within 50 ms, because the alternate path is calculated and installed into the routing and forwarding information bases before failure happens.

 

The critical point is to calculate and install backup/alternate path in routing and forwarding tables. That’s why fast reroute is a proactive mechanism but convergence is reactive (After a failure the alternate path is calculated)

 

Why Fast Reroute with MPLS ?

 

Fast Reroute can be done in DWDM , Sonet/SDH layers as well. So FRR can be done in physical layer of OSI.

 

But generally with physical layer protection, extra hardware is required to divert the traffic to an alternate/backup path in case of failure. With SONET/SDH, APS (Automatic Protection Switching).

 

MPLS doesn’t require an hardware at the physical layer.

 

MPLS FRR is Layer 3 protection mechanism.

 

RSVP-TE is required to have MPLS TE FRR.

 

Before the failure, while network is running in steady state, backup/alternate path is calculated, signaled and the path is installed in every routers routing and forwarding tables with the help of RSVP-TE.

 

 

What is MPLS Traffic Engineering Fast Reroute Link Protection ?

 

MPLS TE FRR Link Protection is a local protection mechanism. In case of a link failure, traffic is rerouted to the backup path.

 

mpls te frr link protection

MPLS TE FRR Link Protection

 

Why I said, MPLS Traffic Engineering Fast Reroute Link Protection is a local protection mechanism ?

 

On the above picture, there is a primary LSP which is red one.

 

Headend of the LSP is Router A , Tailend of the LSP is Router D.

 

Which mean , source of MPLS TE tunnel is Router A , Destination of the MPLS TE tunnel is Router D.

 

Router E and F in the between.

 

Router B and C , can be used a backup path, in case link or node fails of the Router E and F.

 

As in the above picture, in case the link failure between Router E and F, and traffic is going from Router A towards Router D, who should react to that failure ?

 

Router E right ?

 

Router E can send the traffic to Router B or Router C.

 

Since the router which reacts to a failure is attached to failed link, MPLS TE FRR Link Protection is local protection mechanism.

 

If the link between Router F and Router D fails, it wouldn’t be a local protection anymore. In that case, Router E could react if we are doing link protection.

 

Link protection cannot protect the traffic if the link between Router A and the Router E fails, because there is no other path from Router A to any other router in the network. No backup path, no protection !

 

Last but not least, with MPLS Traffic Engineering Fast Reroute Link Protection, we only protect the traffic against link failure.

 

On the above topology, if Router E or Router F fails, then link protection couldn’t find an alternate path.

 

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