SR-TE Flex-Algo w/ Affinity

Load sr-te.flex-algo.init.cfg

configure
load bootflash:sr-te.flex-algo.init.cfg
commit replace
y

Configure flex algo 128 to use the IGP path but exclude RED links. Configure the R3-R5 link as color RED. Traffic from CE101 to CE107 should then take the path R1-R3-R9-R5-R7.

Answer

#All routers
router isis 1
 affinity-map RED bit-position 1
 !
 flex-algo 128
  affinity exclude-any RED
  advertise-definition

#R1
segment-routing
 traffic-eng
  on-demand color 10
   dynamic
    sid-algorithm 128

#R3
router isis 1
 interface GigabitEthernet0/0/0/5
  affinity flex-algo RED

#R5 (technically optional)
router isis 1
 interface GigabitEthernet0/0/0/3
  affinity flex-algo RED

#R7
router isis 1
 interface Loopback1
  address-family ipv4 unicast
   prefix-sid algorithm 128 index 107

Explanation

When using link affinity in a flex-algo definition, you must define the affinity and assign affinities to interfaces under the IGP. This is true even if you configured the same affinity under segment-routing traffic-eng.

To start, all routers will define the flex algo and its constraint (avoid RED links). All routers must define the bit mapping for affinity name “RED.”

#All routers
router isis 1
 affinity-map RED bit-position 1
 !
 flex-algo 128
  affinity exclude-any RED
  advertise-definition

All routers will advertise the flex algo definition. The bit mapping is 0 index-based, so bit 1 maps to 0x2. The flex algo definition advertisement says to exclude this admin group.

Next, we must define the affinity on R3 and R5 under the IGP.

#R3
router isis 1
 interface GigabitEthernet0/0/0/5
  affinity flex-algo RED

We must define this under the IGP because the affinity must be advertised as part of an application specific link attribute. The affinity must be linked to the flex-algo application. It is different from the general MPLS-TE link affinity.

As before, we define the ODN policy on R1, and advertise the additional prefix SID for this flex algo on R7.

#R1
segment-routing
 traffic-eng
  on-demand color 10
   dynamic
    sid-algorithm 128

#R7
router isis 1
 interface Loopback1
  address-family ipv4 unicast
   prefix-sid algorithm 128 index 107

We’ll now color routes from R7:

#R7
extcommunity-set opaque LOW_LATENCY
 10
end-set
!
route-policy LOW_LATENCY_ODN
 set extcommunity color LOW_LATENCY
end-policy
!
router bgp 100
vrf BLUE
  neighbor 192.168.107.107
   address-family ipv4 unicast
    route-policy LOW_LATENCY_ODN in

R1 instantiates the ODN policy, which uses a single label:

A traceroute from CE101 to CE107 confirms the path is correct: