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MPLS explicit-null

‎10-12-2017 07:10 AM

Hi Team,

 

What is the benifit of configuring the explicit-null under protocols/mpls at the EDGE level (LER) ?

In other term, what is the use case of configuring it at LER level ?

 

BR/

Zakour

JNCIS-SP
11 REPLIES 11
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Re: MPLS explicit-null

‎10-12-2017 07:45 AM

Hi Zakor,

 

When you configure MPLS explicit-null labels, label 0 is advertised to the egress router of an LSP. When label 0 is advertised, the egress router (instead of the penultimate router) removes the label. Ultimate-hop popping ensures that any packets traversing an MPLS network include a label

 

We can say the implicit-null label is used by default and works well if the network has not particular QoS requirement, or if the QoS is end to end between the CPEs. On the other hand, the explicit-null label is used when the MPLS core network needs to know the QoS values.

 

Hope this helps

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Re: MPLS explicit-null

‎10-12-2017 12:03 PM

Hello,

 


@mzakour wrote:

Hi Team,

 

What is the benifit of configuring the explicit-null under protocols/mpls at the EDGE level (LER) ?

 


This is "explicit-null"/EN for RSVP, LDP explicit-null/EN is configured at different stanza.

And AFAIK, since JUNOS supports "ultimate-hop-popping"/UHP in recent releases https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/mpls-ultimate-hop-poppin..., there isn't any EN benefit remaining, in my view.

You may see that EN is internally processed differently than any other label on certain platforms like PTX with FPC1/2 cards although I agree that PTX as LER is not a common use case.

HTH

Thx

Alex

 

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Re: MPLS explicit-null

‎03-09-2018 02:09 AM

Hello,

 

I am a bit confused regarding the above topic. Do we need to configure explicit null 'only' on the egress router or on all routers along the LSP? 

 

Thanks in advance.

 

Suresh

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Re: MPLS explicit-null

[ Edited ]
‎03-09-2018 02:22 AM

Hi Suresh,

 

explicit-null only to be configured on Egress rotuer

 

Explicit Null

 

When Explicit Null is configured a label value of 0 is signalled to the penultimate router which instructs the router to push an MPLS Shim Label onto packets that are destined for the final hop. In certain cases the requirement for Explicit Null may be desirable, for example to maintain CoS values attributed to the LSP, as the CoS value would normally be discarded at the penultimate router if PHP were enabled.

 

Configuration Examples

 

When enabled Explicit Null will pass a label value of 0 downstream from the tail end router so should be applied at this point in the network.

For RSVP the feature is configured under the MPLS protocol heirarchy and will apply to all egress LSPs signalled by RSVP:

set protocols mpls explicit-null

 

To apply the feature to LDP the statement is configured directly under the LDP protocol heirarchy and will apply to all LDP signlled LSPs:

 

set protocols ldp explicit-null

 

 

Hope this helps

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Re: MPLS explicit-null

‎03-09-2018 02:27 AM

Hi Vadivel,

 

Thanks for your prompt response. It makes sense. 

 

Alternatively, can we only configure UHP (Ultimate Hop Popping) on egress router? So the egress router will perform the pop action and the cos values will be preserved end to end?

 

Thanks

 

Suresh

 

 

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Re: MPLS explicit-null

[ Edited ]
‎03-09-2018 02:44 AM

Hi Suresh,

 

Both explicit-null and  ultimate-hop-popping  does the same work. but there is one difference with respect to 

tehconfig

 

> set protocols mpls ultimate-hop-popping  

> set protocols mpls explicit-null

 

First statement needs to be configured in the ingress such that in the path message it requests the egress to use UHP
Second statement needs to be configured on the egress such that it sends a label 0 upstream as part of RESV message

 

Please refer below link for UHP

 

https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/mpls-ultimate-hop-poppin...

 

 

Hope this helps

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Re: MPLS explicit-null

‎03-09-2018 03:07 AM

Just one last question.

 Hi Vadeivel,

 

Thanks for the brief explanation. 

 

Just one last question Smiley Happy

 

 

CE1--PE1--P1--P2--PE2--CE2

 

PE01 - Ingress router

PE02 - Egress router

 

By default, P2 will perform a pop action (PHP) and forward the frame to PE2. So PE2 doesn't need to do two lookups. 

 

After P2 pops the label, would the frame be forwarded out to PE2 by using IP route table or the label just popped using mpls.0 table?

 

Hope I explained the question clearly. 

 

Suresh

 

 

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Re: MPLS explicit-null

‎03-09-2018 03:46 AM

Hi Suresh,

 

P2 just POP the label using mpls.0 table

 

You can refer the below link for VPN label forwarding in detail:

 

https://www.juniper.net/documentation/en_US/junos/topics/concept/vpn-forwarding-across-providers-cor...

 

Hope this helps

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Re: MPLS explicit-null

‎03-09-2018 03:47 AM
P2 will pop top label and send the packet out to PE2 after doing label lookup using mpls.0 table
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Re: MPLS explicit-null

‎03-27-2018 11:07 PM

It will pop the label and forward using mpls.0 table not IP routing table.

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Thanks
Amit
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Re: MPLS explicit-null

‎09-29-2019 11:45 AM

Basically is useful to disable penultimate-hop when we are using MPLS3VPN too.

"set protocols ldp explicit-null"

Best regards,