Segment routing BGP SR SID

[sabyasachi.kar85]

I am not able to understand any significance between BGP LU and BGP SR SID in ISIS Multidomain environment. 

Without BGP SID, i am able to reach end to end. With BGP SR also same thing. Can any one explain what is the usecase of BGP SR replacing BGP LU. 

Thanks in advance. 

[pigallo · Accepted solution]

Hello @sabyasachi.kar85 ,

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@sabyasachi.kar85 wrote:

Without BGP SID, i am able to reach end to end. With BGP SR also same thing. Can any one explain what is the usecase of BGP SR replacing BGP LU. 

Thanks in advance. 

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The point is you should not think about BGP SR replacing BGP LU.
You should think instead about BGP SR as technology that can leverage BGP-LU to signal prefix-SIDs rather than LDP labels.
They do not replace each other.
When you work interdomain by choosing BGP-LU you gain multidomain reachability either if you signal LDP labels or BGP prefix-SIDs.
It's up to you to figure out which transport method works better for your network.

Should you need further clarification, feel free to ask.

best regards,

[sabyasachi.kar85]

Thank you for the response. 

Could you please let me know use cases where only BGP SR SID will work instead of BGP LU. Any use cases where BGP LU itself will not work at all. 

I just want to know where we explicitly work with BGP SR SID. No BGP LU is required. 

[pigallo]

Hello,

BGP LU is just an address-family. The same as ipv4 unicast or ipv6 unicast for example.
Anyway, consult RFC for further details: https://tools.ietf.org/html/rfc3107

BGP LU cannot work if you do not signal anything, ok ?  So, BGP LU is needed to signal labels or prefix-SID.

Nobody stops you to advertise a prefix-SID via BGP IPV4 unicast AFI as well. The problem is that it's not usable advertisement or, even worse, you may risk to harm your peers.

Kind regards,