Segment Routing with traffic engineering steers the traffic without maintaining the state in the network. The state is being maintained in the packet itself which provides lot of advantages in terms of savings of network resources.
In previous post, we have seen how to steer traffic by using “Segment Routing Explicit Path”, “Segment Routing On Demand Next Hop for L3VPN (ODN)” and “Deploy Network Slice By Using Flex-Algo”. “Deploy Network Slice By Using Flex-Algo” also leverages the affinity links so that Flex-Algo(k) must not take the path which is having affinity-map configured.
In today’s post, we will see how to steer traffic with constraints by using dynamic path. In other words, we can also say that how we can leverage adjacency-SID while we steer the traffic on headend router. Below is topology used for demonstration purpose only. Configurations will remain be the same as per the earlier post of segment routing.
As per above topology, L4 shows the best path to reach PE7 (7.7.7.7) via GigabitEthernet0/0/0/1 -> Interface towards node S2. On L4, will color GigabitEthernet0/0/0/1 with blue affnity and GigabitEthernet0/0/0/0 with red infinity. SR-TE policy is created with contraint that don’t use affinity blue link which is GigabitEthernet0/0/0/1 for color 128 and destination node 7.7.7.7. By default GigabitEthernet0/0/0/1 is selected by IGP but using adjacency-sid (affinity) we can steer the traffic to GigabitEthernet0/0/0/0.
Below output is showing the best path to reach 7.7.7.7/32 (PE7)
RP/0/0/CPU0:L4#show route isis
i L2 1.1.1.1/32 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 1.3.0.0/24 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 1.5.0.0/24 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 2.2.2.2/32 [115/10] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 2.3.0.0/24 [115/20] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 2.5.0.0/24 [115/20] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 3.3.3.3/32 [115/20] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 3.10.0.0/24 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 3.11.0.0/24 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 5.5.5.5/32 [115/20] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 5.6.0.0/24 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 5.8.0.0/24 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 6.6.6.6/32 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 6.7.0.0/24 [115/40] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 6.8.0.0/24 [115/40] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 7.7.7.7/32 [115/40] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 7.12.0.0/24 [115/50] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 7.13.0.0/24 [115/50] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
i L2 8.8.8.8/32 [115/30] via 2.4.0.2, 21:45:34, GigabitEthernet0/0/0/1
RP/0/0/CPU0:L4#
Tracerute before configuring link affinity
RP/0/0/CPU0:L4#traceroute vrf test 100.0.0.7
1 2.4.0.2 [MPLS: Labels 16007/24009 Exp 0] 19 msec 9 msec 29 msec
2 2.5.0.5 [MPLS: Labels 16007/24009 Exp 0] 9 msec 9 msec 9 msec
3 5.6.0.6 [MPLS: Labels 16007/24009 Exp 0] 9 msec 9 msec 9 msec
4 6.7.0.7 9 msec * 9 msec
RP/0/0/CPU0:L4#
Apply below configuration for link affinity
RP/0/0/CPU0:L4#sh run segment-routing
segment-routing
traffic-eng
interface GigabitEthernet0/0/0/0
affinity
name red
!
!
interface GigabitEthernet0/0/0/1
affinity
name blue
!
!
policy L4toL7
color 128 end-point ipv4 7.7.7.7 -> candidate path instantiation whenever color is 128 and destination will 7.7.7.7
candidate-paths
preference 100
dynamic
metric
type igp
!
!
constraints
affinity
exclude-any -> exclude blue link which GigabitEthernet0/0/0/1
name blue
!
!
!
!
!
!
affinity-map
name red bit-position 1
name blue bit-position 2
!
!
!
RP/0/0/CPU0:L4#