Flex Algo not working with OSPF | IOS-XR

[steven.crutchley]

Hi all,

I have the following setup, trying to get Flex Algo to work with OSPFv2 on an IOS-XR device. I've distribute link-states and even added mpls traffic-eng router-ids (which I'm not sure if I technically need to do) but still the policy will just not come up. The output below shows only one device in the topology but all other have a similar setup (with router-ids and distribute link state and a reference to flex-algo 240 and an appropriate node-sid under their OSPF instance). 

I think I'm missing something obvious but cannot see what. Funnily enough this works with IS-IS.

Can anyone see what is wrong?

RP/0/RP0/CPU0:PE3#sh segment-routing traffic-eng policy color 110
Sun Jun 16 15:03:10.016 UTC

SR-TE policy database
---------------------

Color: 110, End-point: 10.1.1.101
  Name: srte_c_110_ep_10.1.1.101
  Status:
    Admin: up  Operational: down for 00:17:04 (since Jun 16 14:46:06.023)
  Candidate-paths:
    Preference: 100 (configuration) (inactive)
      Name: FLEX-ALGO-TEST
      Requested BSID: dynamic
      Constraints:
        Prefix-SID Algorithm: 240
        Protection Type: protected-preferred
        Maximum SID Depth: 10 
  Attributes:
    Forward Class: 0
    Steering labeled-services disabled: no
    Steering BGP disabled: no
    IPv6 caps enable: no
    Invalidation drop enabled: no
    Max Install Standby Candidate Paths: 0
          
RP/0/RP0/CPU0:PE3#sh run router ospf                             
Sun Jun 16 15:03:18.960 UTC
router ospf CORE
 apply-group CORE-OSPF
 distribute link-state
 router-id 10.1.1.103
 segment-routing mpls
 segment-routing sr-prefer
 address-family ipv4 unicast
 area 0
  mpls traffic-eng
  interface Loopback0
   prefix-sid index 103
   prefix-sid algorithm 240 index 303
  !
  interface GigabitEthernet0/0/0/3
  !
  interface GigabitEthernet0/0/0/4
   cost 100
  !
 !
 mpls traffic-eng router-id Loopback0
 flex-algo 240
  priority 200
  advertise-definition
  affinity exclude-any
   BLUE
  !
 !
!

RP/0/RP0/CPU0:PE3#sh run segment-routing 
Sun Jun 16 15:05:06.629 UTC
segment-routing
 global-block 17000 23999
 traffic-eng
  policy FLEX-ALGO-TEST
   color 110 end-point ipv4 10.1.1.101
   candidate-paths
    preference 100
     constraints
      segments
       sid-algorithm 240
      !
     !
    !
   !
  !
  affinity-map
   name BLUE bit-position 6
  !
  pcc
   source-address ipv4 10.1.1.103
   pce address ipv4 10.1.1.98
    precedence 100
    password encrypted 00141253514C5B140B61
   !      
   pce address ipv4 10.1.1.99
    precedence 50
    password encrypted 00141253514C5B140B61
   !
  !
 !
!

RP/0/RP0/CPU0:PE3# 

[Harold Ritter]

Hi @steven.crutchley ,

There are four things missing from the configuration you provided:

1. You should configure the following under your SR policy.

segment-routing
traffic-eng
policy FLEX-ALGO-TEST
candidate-paths
preference 100
dynamic
metric
type igp

2. Make sure your interfaces are configure as point to point.

router ospf CORE

area 0

interface GigabitEthernet0/0/0/3
network point-to-point
!

interface GigabitEthernet0/0/0/4
network point-to-point
!

3. Add the affinity-map under router ospf.

router ospf CORE
affinity-map
BLUE bit-position 6

4. Colour the link you want to exclude under router ospf.

router ospf CORE

area 0

interface GigabitEthernet0/0/0/3
affinity flex-algo
BLUE

Regards,

[steven.crutchley]

Thanks. I have added igp as the metric and it seems to come up. But I don't think it is avoiding the BLUE links. 

This leads me to have two questions about flex-algo:

1. So SR policies always need to have a metric defined (in this case igp)? - I thought they would just default to igp... 

2. For coloring links... do I color them under the IGP like this?

router ospf CORE
 area 0
  interface GigabitEthernet0/0/0/0
   affinity flex-algo
    BLUE
   !
  !
 !
!
end

Or do I colour them under SR... like this:

segment-routing
 traffic-eng
  interface GigabitEthernet0/0/0/0
   affinity
    name BLUE
   !
  !
  affinity-map
   name BLUE bit-position 6
  !

And when do I use which?

Is the color under the IGP for Flex-Algo and then under SR for the normal (non-flex-algo) SR policies?

[Harold Ritter]

Hi @steven.crutchley ,

> 1. So SR policies always need to have a metric defined (in this case igp)? - I thought they would just          > default to igp... 

There is no default. It needs to be configured

> 2. For coloring links... do I color them under the IGP like this?

Yes, this is how it needs to be done for Flex Algo. You would need to configure it under SR if you wanted to configure a SRTE policy using colors.

> Is the color under the IGP for Flex-Algo and then under SR for the normal (non-flex-algo) SR policies?

That is correct.

Regards,

[Adrian Brooks]

Thanks, I found this exchange useful as I had got muddled when trying to test a SR TE policy linked to a flex-algo topology configured with link affinities.

I notice that the metric type defined in the flex-algo overrules the metric type in the SR TE policy (when the SR TE policy references the flex-algo via the constraints section). I suppose this makes sense, but it caused a bit of initial head-scratching to work out what was going on.

Adrian