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.
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#
Check the output of SR-TE database L4:
RP/0/0/CPU0:L4#show segment-routing traffic-eng policy
SR-TE policy database
---------------------
Name: L4toL7 (Color: 128, End-point: 7.7.7.7)
Status:
Admin: up Operational: up for 00:01:52 (since May 2 07:46:40.533)
Candidate-paths:
Preference 100:
Constraints:
Affinity:
exclude-any:
blue
Path Metrics:
Margin Absolute: 0
Margin Relative: 0%
Maximum SID Depth: 10
Dynamic (active)
Metric Type: IGP, Path Accumulated Metric: 130
24002 [Adjacency-SID, 1.4.0.4 - 1.4.0.1] -> Adjacency Sid
16007 [Prefix-SID, 7.7.7.7] -> Prefix-Sid
Attributes:
Binding SID: 24014 -> Binding Sid
Allocation mode: dynamic
State: Programmed
Policy selected: yes
Forward Class: 0
Steering BGP disabled: no
IPv6 caps enable: no
RP/0/0/CPU0:L4#
Check the below output to see adjacency-sid is allocated to Gi0/0/0/0 which is red link affinity.
RP/0/0/CPU0:L4#show mpls forwarding
Local Outgoing Prefix Outgoing Next Hop Bytes
Label Label or ID Interface Switched
------ ----------- ------------------ ------------ --------------- ------------
16001 16001 SR Pfx (idx 1) Gi0/0/0/1 2.4.0.2 0
16002 Pop SR Pfx (idx 2) Gi0/0/0/1 2.4.0.2 0
16003 16003 SR Pfx (idx 3) Gi0/0/0/1 2.4.0.2 0
16005 16005 SR Pfx (idx 5) Gi0/0/0/1 2.4.0.2 0
16006 16006 SR Pfx (idx 6) Gi0/0/0/1 2.4.0.2 0
16007 16007 SR Pfx (idx 7) Gi0/0/0/1 2.4.0.2 0
17001 17001 SR Pfx (idx 1001) Gi0/0/0/1 2.4.0.2 0
17002 Pop SR Pfx (idx 1002) Gi0/0/0/1 2.4.0.2 0
17003 17003 SR Pfx (idx 1003) Gi0/0/0/1 2.4.0.2 0
17005 17005 SR Pfx (idx 1005) Gi0/0/0/1 2.4.0.2 0
17006 17006 SR Pfx (idx 1006) Gi0/0/0/1 2.4.0.2 0
17007 17007 SR Pfx (idx 1007) Gi0/0/0/1 2.4.0.2 0
24000 Aggregate test: Per-VRF Aggr[V] \
test 14272
24001 Pop SR Adj (idx 1) Gi0/0/0/0 1.4.0.1 0
24002 Pop SR Adj (idx 3) Gi0/0/0/0 1.4.0.1 0 -> Adjacency-Sid
24003 Pop SR Adj (idx 1) Gi0/0/0/1 2.4.0.2 0
24004 Pop SR Adj (idx 3) Gi0/0/0/1 2.4.0.2 0
24009 Aggregate test_without_odn: Per-VRF Aggr[V] \
test_without_odn 1784
RP/0/0/CPU0:L4#
Below output is showing whenever the color 128 prefix is received from PE7, BSID (24014) is attached to it.
RP/0/0/CPU0:L4#show bgp vpnv4 unicast vrf test 100.0.0.7
BGP routing table entry for 100.0.0.7/32, Route Distinguisher: 100:100
Not advertised to any peer
Local
7.7.7.7 C:128 (bsid:24014) (metric 40) from 8.8.8.8 (7.7.7.7)
Received Label 24009
Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best, import-candidate, imported
Received Path ID 0, Local Path ID 1, version 78
Extended community: Color:128 RT:100:100
Originator: 7.7.7.7, Cluster list: 8.8.8.8
SR policy color 128, up, not-registered, bsid 24014
Source AFI: VPNv4 Unicast, Source VRF: test, Source Route Distinguisher: 100:100
RP/0/0/CPU0:L4#
Run traceroute to 100.0.0.7 which is vrf test and being advertised by 7.7.7.7. Below output shows that the path is changed to Gi0/0/0/0 which is red link affinity. Earlier it was going via Gi0/0/0/1. Check the above traceroute output and match it with the below one. Difference can be seen easily.
RP/0/0/CPU0:L4#traceroute vrf test 100.0.0.7
1 1.4.0.1 [MPLS: Labels 16007/24009 Exp 0] 19 msec 19 msec 9 msec
2 1.5.0.5 [MPLS: Labels 16007/24009 Exp 0] 0 msec 9 msec 9 msec
3 5.6.0.6 [MPLS: Labels 16007/24009 Exp 0] 9 msec 9 msec 19 msec
4 6.7.0.7 9 msec * 9 msec
RP/0/0/CPU0:L4#
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