cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
704
Views
10
Helpful
4
Replies
Highlighted
Beginner

Calling Segment Routing experts

Good afternoon everyone. I have a very simple question regarding SR TE using dynamic dynamic candidate path. I am reading C.Filsfils' "Segment Routing Part II" book, in which there is this example: 

 

Screenshot 2019-06-01 at 13.54.46.png

The example given in the book basically has constrain configured on R1 to avoid the GREEN path and that's okay. 

 

Now I have a use case, where my topology is slightly more complicated, but let's say for example, that I have a specific requirement to use the link between routers 23 and 24. Let's say we give this link a color "purple".

 

Is it possible, following the example topology above, to simply define an alternative logic (e.g. not using exclude-any, but rather using include-any) doing something like this (I don't care if I will go via GREEN or BLUE path in this particular case. All I want to do is the policy to establish a path using the link between 23 and 24):

 

policy 1-to-3
color 1 end-point ipv4 1.1.1.3
!
candidate-paths preference 10 dynamic metric type igp ! ! constraints affinity include-any purple

is this supposed to work or not? My thinking is that the SR should simply follow best-path to Router 23 then using either prefix sid for 24 or an adjacency sid to get across that particular link. Is “include-any” an implicit method where the path is built based on any of the specified colours only and therefore that won’t work as there isn’t a purple link available from 1 to 23? I am aware of other methods I can use but I am wondering if I can simply give a link in the network a color and then simply define a policy saying "use this link"? 

 

Hope that makes sense and thanks in advance for your time.

1 ACCEPTED SOLUTION

Accepted Solutions
Highlighted
Cisco Employee

Re: Calling Segment Routing experts

Hi Daniel,

 

is this supposed to work or not?

 

No, it won't. As you mentioned, the remaining links are not colored with purple.

 

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

View solution in original post

4 REPLIES 4
Highlighted
Cisco Employee

Re: Calling Segment Routing experts

Hi Daniel,

 

is this supposed to work or not?

 

No, it won't. As you mentioned, the remaining links are not colored with purple.

 

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

View solution in original post

Highlighted
Beginner

Re: Calling Segment Routing experts

Thanks very much Harold, that makes sense.

 

The whole reason I am asking this question is because I have a use case where I need to consider TE using SR, but I have a limitation of using more than 3 labels throughout the network.. This isn’t ideal especially considering the fact we must also provision VPWS as well as L3 VPNs using SR TE to steer the traffic... So I was wondering what’s the best approach (if possible at all) to do this across 6-7 devices, where the best path to the destination doesn’t necessarily follow the IGP. Adding TI LFA in the equation also doesn’t help, but this is something I am willing to accept as a short period of time during the IGP reconvergence. 

 

Any hints or ideas as of tips & tricks I can try to reduce the label stack will be much appreciated.


Thanks,

Danny

Highlighted
Participant

Re: Calling Segment Routing experts

Just for completeness. What you want to accomplish (include a specific link
or set of links/nodes) can be done easiest by using loose hop or hops with
your LSP.

The use of colours is tricky. There are basically only two options:

1) if you include a color it needs to be present on all links along the path

2) if you use exclude that's where you can single out a specific link to be
excluded
adam
Highlighted
Participant

Re: Calling Segment Routing experts

Just for completeness. What you want to accomplish (include a specific link or set of links/nodes) can be done easiest by using loose hop or hops with your LSP.

The use of colours is tricky. There are basically only two options:

1) if you include a color it needs to be present on all links along the path

2) if you use exclude that's where you can single out a specific link to be excluded

adam
CreatePlease to create content
Content for Community-Ad

Cisco COVID-19 Survey