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Path Diversity in Secondary Paths



As we say in the following articles, while using Secondary LSP-Paths, sharing links between Primary and Secondary LSP-Paths must be avoided. We can say this term “Path Diversity”.

There are some methods to achieve Path Diversity. These methods are:

– Using Full Strict hop LSP-Paths
– Admin Groups
– Shared Risk Groups


Using Full Strict hop LSP-Paths

Primary and secondary LSP-Paths are configured with fully strict hops in this method. This is difficult to configure and cause a huge operational overhead in large scale networks. Troubleshooting is also difficult in this method. But this method can be a good choice for small scale networks, because there are limited number of redundancy options.

While using full strict hop methods, redundant paths can not be used. There is only one option to do this, using two secondary paths. But this is more configuration overhead. MPLS FRR (Fast Reroute) is also the other solution for using redundant paths in the topology. We will talk about MPLS FRR (Fast Reroute) in the following posts.

Let’s see how to configure strict hops in Alcatel-Lucent 7750 Service Router according to below topology.

Strict Path, primary path, secondary path,MPLS Recovery

Paths with Strict Hops

primary "primary_path". 
	hop 1 10.1.0.1 strict
	hop 2 10.3.0.1 strict
	hop 3 10.5.0.1 strict
	no shutdown
exit

primary "secondary_path"
	hop 1 10.2.0.1 strict
	hop 2 10.4.0.1 strict
	hop 3 10.6.0.1 strict
	no shutdown
exit

lsp "toRouterB"
	to 2.2.2.2
	primary "primary_path"
	exit
	secondary "secondary_path"
	exit
	no shutdown
exit

Admin Groups


With this Admin Groups method, redundant links are assigned to different Admin Groups. Primary and Secondary LSP-Paths are configured loose hops that exclude either one of the groups. Admin groups can be asymetrics.

In the below example, the Primary LSP-Path can use any of the links in the upper plane. And the Secondary LSP-Path can use any of the links in the lower plane.

Admin Groups, primary path, secondary path,MPLS Recovery

Admin Groups

A:RouterX>config>router>mpls# 
------------------------------
		admin-group "UP" 1
		interface "toRouter1"
			admin-group "UP"
		exit
		interface "toRouter2"
			admin-group "UP"
		exit

A:RouterY>config>router>mpls# 
------------------------------
		admin-group "LOW" 2
		interface "toRouter3"
			admin-group "LOW"
		exit
		interface "toRouter4"
			admin-group "LOW"
		exit

A:RouterX>config>router>mpls# 
------------------------------
		admin-group "UP" 1
		admin-group "UP" 2
		path "fully_loose-1"
			no shutdown
		exit
		path "fully_loose-2"
			no shutdown
		exit

A:RouterX>config>router>mpls# 
------------------------------
lsp "toRouterB"
	to 2.2.2.2
	cspf
	primary "fully_loose-1"
		exclude "LOW"
	exit
	secondary "fully_loose-2"
		exclude "UP"
	exit
	no shutdown
exit

SRLG (Shared Risk Link Group)



SRLG is the another mechanism for path diversity. This method gives more freedom to the Primary path during path decision. In the previous method, in Admin Group method, this selection can be done only with the limited group of links.

Here, the main point is this, do not use the links under the same SRLG with Primary LSP-Path.

SRLG allows the operator to create secondary LSPs that are automatically disjointed from the primary LSP-Path.

As we talked about before, while selecting a active Secondary LSP-Path, path-preference is important. But if path-preference is equal, then the Secondary LSP-Path with SRLG enabled is selected as active.

Let’s see the SRLG configuration on Alcatel-Lucent 7750 Service Router according to the below topology.

SRLG (Shared Risk Resource Group), primary path, secondary path,MPLS Recovery

SRLG (Shared Risk Resource Group)

A:RouterX>config>router>mpls# 
------------------------------
		srlg-group "SRLG-U" value 1
		interface "toRouter1"
			srlg-group "SRLG-U"
		exit
		interface "toRouter2"
			srlg-group "SRLG-U"
		exit

A:RouterY>config>router>mpls# 
------------------------------
		srlg-group "SRLG-L" value 2
		interface "toRouter3"
			srlg-group "SRLG-L"
		exit
		interface "toRouter4"
			srlg-group "SRLG-L"
		exit

A:RouterA>config>router>mpls# 
------------------------------
		srlg-group "SRLG-U" value 1
		srlg-group "SRLG-L" value 2
		path "fully_loose-1"
			no shutdown
		exit
		path "fully_loose-2"
			no shutdown
		exit

A:Router1>config>router>mpls# 
------------------------------
lsp "toRouterB"
	to 2.2.2.2
	cspf
	primary "fully_loose-1"
	exit
	secondary "fully_loose-2"
		stanby
		srlg
	exit
	no shutdown
exit

Above, the Primary LSP-Path is established over the router at upper plane. And CSPF prunes all the links that are in the same SRLG with the first LSP during calculating path for Secondary LSP-Path.

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)


…You Can Join Our “IPCisco” Groups Below!!!…




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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