The Segment Routing-Traffic Engineering (SR-TE) Per-Flow policy (PFP) On-Demand Next-Hop (ODN) with auto-steering (Per-Flow ODN/AS) is a mechanism that allows the steering of traffic on a segment routing policy based on the attributes of the packets. SR-TE PFP ODN with auto steering (Per flow ODN/AS) is a mechanism that allows the steering of traffic on an SR policy based on the attributes of the packets. Packets are classified using Cisco’s Modular QoS CLI (MQC) framework and then marked using internal tags known as Forward Classes (FCs). A PFP is then used to route the marked packets based on the mappings between an FC and its corresponding path. This means that the traffic is steered based on its QoS markings and switched to the appropriate path based on the FC of the packet.

A PFP is identified by <color, endpoint>. It is configured with a per-flow forwarding class table with up to eight entries, with each entry indexed by an FC and pointing to a Per Destination Policy (PDP).

Note	The following features are supported: 250 PFP+PDP (Combination) 6 PE and 6 VPE 10k VPNV4 prefix limit L3VPN Inter AS Option B for SR PFP IPv6 over PFP

In the Segment Routing Traffic Engineering mechanism, the prefix that needs an SR-TE routing path is associated with a color-extended community (an attribute that assigns color to the prefixes). Currently, BGP has the capability to attach the color-extended community based only on the neighbor command routemap outbound configuration. To color the prefixes based on attributes such as Source-VRF, Destination-VRF, CE-neighbor, and Source protocol, the following ways of attaching color are introduced:

• VRF Export Coloring

• VRF Import Coloring

• Route Redistribution Coloring into BGP

• Neighbor In-bound Coloring

Additionally, in Cisco IOS XE releases prior to 17.7.1a, any new color-extended community attached to the prefix replaces the existing color-extended community available in the prefix. To be able to add the new color-extended community to the existing list of color-extended communities instead of replacing, the keyword additive is added to the route-map command as part of Cisco IOS XE 17.7.1a:

route-map SRTE-color-map permit
set extcommunity color < 1-4294967295> [additive]

Note

When a BGP update is received with multiple color-extended communities, the highest color value in the list is used for SR policy creation, and the binding SID corresponding to the SR policy is used as the routing path for the received BGP path. If the SR policy corresponding to the highest color is not available, BGP uses the interface as the routing path for the update.

PFP consists of a bundle output chain element (OCE), and each hash of the bundle OCE consists of a PDP policy (PDP tunnel). In this scenario, a PDP policy is created for the default IGP or RIB learned path. This means that a separate PDP policy is created for every default IGP or RIB learned path. Therefore, this implementation will eventually increase the number of policies and will not scale.

From Cisco IOS XE 17.9.1a, you can configure forwarding class in a PFP using the RIB path option. Instead of configuring a PDP, the RIB option uses the IGP shortest path to the policy destination.

PFP has a binding SID, similar to the PDP. The traffic-steering mechanism is also the same as PDP, either through BSID or through RIB.

A PFP is in the operational UP state based on the following conditions:

• The default FC is configured with a PDP, and it is in the operational UP state.

• The default FC is configured with the RIB path and is resolved.

Note	The state of the nondefault FC does not affect the PFP state.

After a packet is steered on the PFP, according to the FC marked by Modular QoS CLI (MQC) at ingress, the following scenarios show the path of the packet:

• If PFP is in the Down state, the packet is dropped.

• If no FC is attached to a packet, the packet is forwarded with a default FC in PFP.

• If an FC is attached to a packet that points to a resolved RIB path or an operational PDP, the packet is forwarded to it.

• If an FC attached on a packet points to a nonexisting unresolved RIB path or a nonoperational PDP, the packet is forwarded to the default FC.

Consider the following topology:

Perform the following steps to configure ODN for PFP:

1. Configure QOS on PE1:

   
   class-map DSCP
           match DSCP AF41

   • Set the forward class on the class map:

     policy-map per-flow
     	class DSCP
     	set forward-class 1
     

   • Attach the policy map on the corresponding interface:

     interface GigabitEthernet0/0/3
     	service-policy type epbr input PFP
     

2. Configure SR-TE PFP on PE1:

   • Set the forward class on PFP:

   on-demand color 4500
    authorized
   candidate-paths
     preference 2
       per-flow
           forward-class 0 color 100
           forward-class 0 rib
           forward-class 2 color 102
   

   • Attach the segment list to PDP:

     policy  perflow_pdp
     color 100 end-point 10.5.5.5
     candidate-paths
      preference 2
       explicit segment-list srte1 weight 10
       !
       constraints
         segments 
           dataplane mpls 
     

   • Set the segment list to SR-TE:

     segment-routing traffic-eng
       segment-list name srte1
            index 1 mpls label 16002 
            index 2 mpls label 16005
     

3. Configure SR-TE PFP on PE2:

   ip prefix-list pfp seq 5 permit 10.35.0.0/16 le 32 

   • Attach the route map to PFP:

     route-map pfp permit 10
      match ip address prefix-list pfp
      set extcommunity color 4500
     

   • Activate the BGP routes:

     router bgp 100
     !
      address-family vpnv4
       neighbor 10.1.1.1 activate
       neighbor 10.1.1.1 send-community extended
       neighbor 10.1.1.1 route-map pfp out
     

4. View the PFP output:

   show segment-routing traffic-eng policy name *6.6.6.6|4090 detail
   
   Name: *6.6.6.6|4090 (Color: 4090 End-point: 6.6.6.6)
   Owners : BGP
   Status:
   Admin: up, Operational: up for 01:29:41 (since 06-21 14:09:05.510)
   Candidate-paths:
   Preference 1 (BGP):
   Per-flow Information (active):
   Forward PDP PDP BSID RW
   Class Color Status Status
   -------- ---------- ------- --------
   0 rib n/a n/a
   1 129 up Done
   2 130 up Done
   3 131 up Done
   4 132 up Done
   Default Forward Class: 0
   Attributes:
   Binding SID: 39
   Allocation mode: dynamic
   State: Programmed
   IPv6 caps enabled
   Tunnel ID: 65568 (Interface Handle: 0x26)
   Per owner configs:
   BGP
   Binding SID: dynamic
   Stats:
   5 minute output rate 0 bits/sec, 0 packets/sec
   Packets: 500524 Bytes: 88056352
   
   Event history:
   Timestamp Client Event type Context: Value
   --------- ------ ---------- -------: -----
   06-21 14:09:05.489 BGP Policy created Name: BGP
   06-21 14:09:05.490 BGP Set colour Colour: 4090
   06-21 14:09:05.490 BGP Set end point End-point: 6.6.6.6
   06-21 14:09:05.490 BGP Set dynamic pce Path option: per flow
   06-21 14:09:05.510 BGP BSID allocated FWD: label 39
   06-21 14:09:05.510 FH Resolution Policy state UP Status: PFP RESOLVED CP: 1
   06-21 14:09:05.551 FH Resolution REOPT triggered Status: REOPTIMIZED CP: 1
   06-21 14:09:05.576 FH Resolution REOPT triggered Status: REOPTIMIZED CP: 1
   06-21 14:09:05.602 FH Resolution REOPT triggered Status: REOPTIMIZED CP: 1
   06-21 14:09:05.626 FH Resolution REOPT triggered Status: REOPTIMIZED CP: 1
   

Use the following command to verify SR-TE Per-Flow Class (ODN) and Automated Steering (PCE Delegated):

show segment-routing traffic-eng policy name *10.5.5.5|4500

Name: *10.5.5.5|4500 (Color: 4500 End-point: 10.5.5.5)
Owners : BGP
Status:
Admin: up, Operational: up for 00:03:50 (since 09-07 16:07:02.938)
Candidate-paths:
Preference 2 (BGP):
Per-flow Information (active):
Forward PDP PDP BSID RW
Class Color Status Status
-------- ---------- ------- --------
0 100 up Done
1 101 up unknown Pending
2 102 up unknown Pending
Default Forward Class: 0
Attributes:
Binding SID: 72
Allocation mode: dynamic
State: Programmed
IPv6 caps enabled
Tunnel ID: 65675 (Interface Handle: 0x2D)
Per owner configs:
BGP
Binding SID: dynamic
Stats:
5 minute output rate 0 bits/sec, 0 packets/sec
Packets: 9 Bytes: 584