MPLS Protection and Restoration – Part 4 (Local Protection, Fast Reroute) - www.ipcisco.com : www.ipcisco.com
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MPLS Local Protection (Fast Reroute)

The goal of the protection is minimizing the down time. To do this, protection must be done near to the failure. Local protection does this. By doing this types of protection, it avoids using entire path protection. Local protection reroutes the traffic rapidly around the failure, so this mechanism is called “Fast Reroute (FRR)”.

In local protection the aim is not using another path after one path fails. Here, the aim is guiding the traffic to a different way excluding the failure point only. In other words, this is a temporary solution before the exact traffic route. Fast Reroute (FRR) keeps the traffic until a new LSP is given to that traffic by Head-End. It is not aim to keep traffic until the primary comes back.

As we say above, the protection is as close to the point of failure as possible for Fast Reroute (FRR). And the failover time is less than 50ms.

Fast Reroute (FRR) is applicable for LSPs established using RSVP-TE. And Fast Reroute (FRR) can only protect the Primary LSP-Path, Secondary Paths can not be protected.

In Secondary Paths (End-to-End Protection), the configuration overhead was too much. Fast Reroute (FRR) reduce this configuration overhead with automatic path calculation.

Fast Reroute (FRR) can not be used in IP networks. Why? Because in IP networks, the forwarding decisicon is done on every node. But in MPLS, source routing is using. Source routing, is the routing which the decision is done by Head-End and never change again till end of the link. So, Fast Reroute (FRR) is used in MPLS networks.

We can clasify the local protection mechanisms. If we clasify it by resurce that is protected, there are two types:

– Link Protection
– Node Protection

We will talk about this protection types in the following article.

With Fast Reroute (FRR), multiple Primary LSP-paths can be protected as long as they have shared paths. This methods clasified according to the number of LSPs protected. Local Protection (Fast Reroute) includes two methods. These methods are:

– One-to-One Backup (1:1)
– Many-to-One (Facility) Backup (1:N)

Implementations has their own advantages, so both One-to-One Backup and Facility Backup is used with these names usually.

The possible configuration options are:
– One-to-One Node Protection
– One-to-One Link Protection
– Facility Node Protection
– Facility Link Protection


Fast Reroute (FRR) Router Roles

Some of the routers has a special name according to the point they are located for MPLS Fast Reroute. These special routers are:

Head-End : The point that primary LSP-Path(protected) is configured and originates.
Tail-End : The point that primary LSP-Path(protected) is terminated.
PLR(Point of Local Repair) : Where the protection tunnel originates.
MP (Merge Point) : Where the protection tunnel terminates and merges into the original protected LSP-Path.

There are two important points in MPLS protection and restoration mentioned above. These points are the Point of Local Repair (PLR) and Merge Point (MP). Here, the PLR is the node, which notices the failure. And after this it is the point, that the traffic forwarding begins to an alternate path. PLR is also the router, that informs the ingress router (Head-End) about the main LSP problem by PATH Error Messages. After bypass or detour the failure, the traffic need to connect the main link again. And the point that the traffic connect to the main path again, is the MP.

There is also a Detour Merge Point (DMP). This is the point where multiple detours join together along the protected path. If they protect the same LSP and if the exerss point is same, then multiple detours can merge. Detour Merging is enabled by default on Alcatel-Lucent 7750 Service Routers and there is no option to disable it.

mpls point of local repair (PLR) and merge point(MP),MPLS Recovery

PLR and MP,MPLS Recovery


Except Tail-End every router tries to establish Fast Reroute (FRR) tunnel on itself and tries to be PLR.

For Fast Reroute (FRR), the Head-End must know the exact path of the LSP-path before signalling. This can be accomplished in several ways:

– A path with full strict hops (no need for CSPF)
– A path with loose hops (CSPF must be enabled)
– A path with mixture of strcit and loose hops (CSPF must be enabled)

One-to-One Protection


In this One-to-One Protection type, a seperate tunnel protection is required for each LSP. This One-to-One Protection tunnel is called “detour”.

One-to-One Backup Detour Tunnels,MPLS Recovery

One-to-One Backup Detour Tunnels,MPLS Recovery


If a failure occurs in the LSP that is protected by One-to-One Backup, the labels are swapped and sent into the alternate path.

In one-to-one backup, a single LSP is protected. In this type, the traffic come to the MP with the different label and top label is not used. This provide tighter control over backup tunnel. In facility backup this is diffciult, because the are multiple LSPs that share the same backup.

mpls one-to-one fast reroute backup traffic forwarding

One-to-One Backup Traffic Forwarding, Detour Tunnel

Let’s check the One-to_one Protection configuration on Alcatel-Lucent 7750 Service Routers for loose hops.

A:R2>config>router>mpls# 
------------------------------
	path "fully_loose"
		no shutdown
	lsp "to_RouterB"
		to 4.4.4.4
		cspf
		fast-reroute one-to-one
			node-protect
		primary "fully-loose"
		exit
	no shutdown
	exit

In above example, loose hops and Fast Reroute (FRR) is used. Because of this usage, CSPF must be enabled to make the calculations.

Facility Protection



In Facility Protection type, a single Protection tunnel is used for multpile LSPs. Facility Protection tunnel is called “bypass”. This medhod allows multiple LSPs to be bound the same bypass tunnel. This brings more efficient resource usage. This is the less resource intensive version of Fast Reroute (FRR).

Facility Backup Bypass Tunnel,MPLS Recovery
Facility Backup Bypass Tunnel,MPLS Recovery


If a failure occurs in the LSP that is protected by Facility Backup, labels are swapped and a second label pushed before sending in the alternate path.

In Facility Backup, there are number of LSPs to protect. And here, on the backup LSP, a tunnel backup label is used at the top of the protected LSP label. At the backup link, the labels swap and at the last router before the MP, the top label removed. This is PHP (Penultimate Hop Poping), also defined before. And at MP, the protected LSP label remains the same.

mpls many-to-one facility backup traffic forwarding,MPLS Recovery

Facility Backup Traffic Forwarding, Bypass Tunnel

In Facility Protection, the selection of Merge Point (MP) is different and more topology efficient.

A:R2>config>router>mpls# 
------------------------------
	path "fully_loose"
		no shutdown
	lsp "to_RouterB"
		to 4.4.4.4
		cspf
		fast-reroute facility
			node-protect
		primary "fully-loose"
		exit
	no shutdown
	exit

In Facility Protection, the main idea is, protecting as many LSP as possible by an established Bypass tunnel. To do this Bypass tunnel merges with the original LSP-Path at the closest downstream router. For Link Protection, MP is the PLR’s next hop router (1 hop away). For Node-protection, MP is the PLR’s next next hop router (2 hop away).

Think about Node Protection. Firstly the Next-Next Hop is located and then check if any other LSP ends there too. If there is another LSP thatends at the same node, then send it on Bypass tunnel. The Protected LSP count increased 1.

For Link Protection, the above steps are also true. But this sime instead of Next-Next Hop, Next Router is located.

At the Merge Point (MP) the traffics need to be seperated. Think about a Bypass tunnel built for two LSPs. Here, at the Merge Point of both of this LSPs, a mechanism needed to make this classification.

For Link Protection, the label that is waited by the next router, is encapsulated in the Bypass tunnel. When it comes to the Merge Point (MP), the label that is waited by next router, is ready.

For Node Protection, the label that the Next-Next router waits, must be known. This information is in Record Route Object (RRO).
Here, the LSP traffic also encapsulated in Bypass tunnel.

Bypass tunnels are separate LSPs. They do not belong to any primary LSP-Path. They have different Tunnel IDs, LSP-IDs and session names (unlike one-to-one detours).

one-to-one -> separate detours tunnels for each LSP
facility -> same Bypass tunnel for many LSP, one LSP do not own the Bypass tunnel

You can check the other parts of these MPLS Protection and Restoration Article series below.

MPLS Protection and Restoration – Part 1
MPLS Protection and Restoration – Part 2 (End-to-End Protection, Secondary Paths)
MPLS Protection and Restoration – Part 3 (Path Diversity in Secondary Paths)
MPLS Protection and Restoration – Part 4 (Local Protection, Fast Reroute)
MPLS Protection and Restoration – Part 5 (Fast Reroute Protection Types)
MPLS Protection and Restoration – Part 6 (RSVP Objects Used for MPLS Fast Reroute)
MPLS Protection and Restoration – Part 7 (Verification Commands on Alcatel-Lucent Routers)
MPLS Protection and Restoration – Part 8 (Actions After the Failure)
MPLS Protection and Restoration – Part 9 (Convergence Factors)
MPLS Protection and Restoration – Part 10 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 11 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 12 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 13 (Juniper Configuration Samples)


…YOU CAN REACH ALL THE “MPLS” ARTICLES AND CONFIGURATIONS BELOW…


MPLS Basics
What is MPLS?
Enabling MPLS on Cisco Router

MPLS Label Distribution Protocol, LDP
MPLS Label Distribution Protocol, LDP – Part 1
MPLS Label Distribution Protocol, LDP – Part 2
MPLS Label Distribution Protocol, LDP – Part 3 (Configuration on ALU)
MPLS Label Distribution Protocol, LDP – Part 4 (ECMP)
MPLS Label Distribution Protocol, LDP – Part 5 (Export and Import Policy, Prefix Aggregation)
MPLS Label Distribution Protocol, LDP – Part 6 (T-LDP)
MPLS Label Distribution Protocol, LDP – Part 7 (CR-LDP)

MPLS, RSVP-TE
MPLS, RSVP-TE – Part 1
MPLS, RSVP-TE – Part 2(Alcatel-Lucent Configuration)

MPLS Protection And Restoration
MPLS Protection and Restoration – Part 1
MPLS Protection and Restoration – Part 2 (End-to-End Protection, Secondary Paths)
MPLS Protection and Restoration – Part 3 (Path Diversity in Secondary Paths)
MPLS Protection and Restoration – Part 4 (Local Protection, Fast Reroute)
MPLS Protection and Restoration – Part 5 (Fast Reroute Protection Types)
MPLS Protection and Restoration – Part 6 (RSVP Objects Used for MPLS Fast Reroute)
MPLS Protection and Restoration – Part 7 (Verification Commands on Alcatel-Lucent Routers)
MPLS Protection and Restoration – Part 8 (Actions After the Failure)
MPLS Protection and Restoration – Part 9 (Convergence Factors)
MPLS Protection and Restoration – Part 10 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 11 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 12 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 13 (Juniper Configuration Samples)


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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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