Tag Archives: Traffic-Engineering

Segment Routed L2VPN TE – Cisco IOS-XR

Hi All

Let’s see Segment routing in action in this blog particularly on IOS-XR. Segment routing is quite new concept which is picking pace these days. In my earlier blog I listed the differences between Segment routing and RSVP-TE and SR can replace it and there are certain areas where it may not be able to help however L3VPN and L2VPN Traffic Engineering is surely one area where it can be used and in this blog we will use SR as TE while configuring the L2VPN.

For this we will take NCS5508 as our router platform in below topology where we will configure the L2VPN SR-TE between NCS5508-1 to NCS5508-3 via NCS5508-8.

Segment Routing in IOS-XR


Let’s see the SR config first.

SR beauty is that there is no special protocol needed to run it. SR Labels will be advertised in OSPF/ISIS and these protocols have been uplifted to carry them. SR Labels are carried in Type 10 Opaque area LSA as TLV.

If you are familiar with OSPF config in IOS-XR, most of the config below looks similar to you as we have just enabled OSPF under area0 and added interfaces under it.

However there are 3 configs highlighted in RED which we have enabled for Segment routing.

RP/0/RP0/CPU0:ncs5508-1#show running-config router ospf
router ospf 1
 distribute link-state
 segment-routing mpls
 nsf ietf
 segment-routing sr-prefer
 area 0
 mpls traffic-eng
 interface Loopback0
 passive enable
 prefix-sid index 1 explicit-null
 interface HundredGigE0/1/0/0
 cost 1
 network point-to-point
 interface FortyGigE0/2/0/8
 cost 4
 network point-to-point
 interface FortyGigE0/2/0/10
 cost 4
 network point-to-point
 interface FortyGigE0/2/0/18
 cost 4
 network point-to-point
 mpls traffic-eng router-id Loopback0

segment-routing mpls , this command causes OSPF to originate RI LSA, Extended Prefix and Extended Link LSAs. It enables MPLS on all interfaces in area(s) enabled for SR and programs SR MPLS labels for forwarding.

segment-routing sr-prefer is used to set the preference of segment routing (SR) labels over label distribution protocol (LDP) labels in case both are available towards destination in your network.

prefix-sid index 1 explicit-null — A prefix SID is associated with an IP prefix. The prefix SID is manually configured from the segment routing global block (SRGB) range of labels. The prefix segment steers the traffic along the shortest path to its destination. A node SID is a special type of prefix SID that identifies a specific node. It is configured under the loopback interface with the loopback address of the node as the prefix. The prefix SID is globally unique within the segment routing domain.

Let’s verify it

RP/0/RP0/CPU0:ncs5508-1#show ospf sid-database
SID Database for ospf 1 with ID

SID Prefix/Mask
-------- ------------------
1 (L)

In the same way we have configured the Node-SID as same index as last octet on lo0 interface.

RP/0/RP0/CPU0:ncs5508-1#show ospf database opaque-area
 OSPF Router with ID ( (Process ID 1)
Type-10 Opaque Link Area Link States (Area 0)
LS age: 782
 Options: (No TOS-capability, DC)
 LS Type: Opaque Area Link
 Link State ID:
 Opaque Type: 7
 Opaque ID: 1
 Advertising Router:
 LS Seq Number: 800006fa
 Checksum: 0xed8b
 Length: 44
Extended Prefix TLV: Length: 20
 Route-type: 1
 AF : 0
 Flags : 0x40
 Prefix :
SID sub-TLV: Length: 8
 Flags : 0x50
 MTID : 0
 Algo : 0
 SID Index : 1
RP/0/RP0/CPU0:ncs5508-1#show mpls forwarding
Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes
Label  Label       or ID              Interface                    Switched

—— ———– —————— ———— ————— ————

16002  Exp-Null-v4 SR Pfx (idx 2)     Hu0/1/0/0     0
16003  16003       SR Pfx (idx 3)     Hu0/1/0/0     0
16004  Exp-Null-v4 SR Pfx (idx 4)     Fo0/2/0/8     0
16005  16005       SR Pfx (idx 5)     Fo0/2/0/8     6421133
16006  16006       SR Pfx (idx 6)     Hu0/1/0/0     0
       16006       SR Pfx (idx 6)     Fo0/2/0/8     0
16007  16007       SR Pfx (idx 7)     Hu0/1/0/0     0
16008  Exp-Null-v4 SR Pfx (idx 8)     Fo0/2/0/18     0

Now let’s create a Segment routed TE EVPN based P2P L2 Circuit. 🙂

Ideally we know that Controller is needed to play with Segment routed labels and Controller can insert the appropriate labels required for TE however if you don’t have Controller, you can configure the path by explicitly giving the path through which traffic will be going.

So we will start with l2vpn xconnect taking edge interface on NCS5508-1 and assigning a EVPN EVI 1100 with source and target ac-id (attachment circuit id) and associate it with pw-class which we will define in next step.


RP/0/RP0/CPU0:ncs5508-1#show running-config l2vpn xconnect group evpn-vpws p2p vpws1
 xconnect group evpn-vpws
 p2p vpws1
 interface HundredGigE0/2/0/2.1100
 neighbor evpn evi 1100 target 11003 source 11001
 pw-class vpws1-class

Pw-class is associated with sr-te policy to steer traffic through the network. An SR-TE policy path is expressed as a list of segments that specifies the path, called a segment ID (SID) list. Each segment is an end-to-end path from the source to the destination, and instructs the routers in the network to follow the specified path instead of the shortest path calculated by the IGP

RP/0/RP0/CPU0:ncs5508-1#show running-config l2vpn pw-class vpws1-class
 pw-class vpws1-class
 encapsulation mpls
 preferred-path sr-te policy vpws1-policy
RP/0/RP0/CPU0:ncs5508-1#show running-config segment-routing traffic-eng policy vpws1-policy
 policy vpws1-policy
 color 10 end-point ipv4
 preference 200
 type te
 preference 300
 explicit segment-list vpws1-path

So in our policy, we have defined one preferred path which is dynamic and if that fails it should failover to explicitly configured segment list defined via path vpws1-path.

RP/0/RP0/CPU0:ncs5508-1#show running-config segment-routing traffic-eng segment-list vpws1-path
 segment-list vpws1-path
 index 10 address ipv4
 index 20 address ipv4

So if we see currently the route towards NCS5508-3, it’s going via IGP Route and not taking our defined list which is expected.

RP/0/RP0/CPU0:ncs5508-1#show route
Wed Jun 27 14:49:59.487 UTC
Routing entry for
 Known via "ospf 1", distance 110, metric 3, labeled SR, type intra area
 Installed Jun 27 14:47:18.930 for 00:02:40
 Routing Descriptor Blocks, from, via HundredGigE0/1/0/0
 Route metric is 3
 No advertising protos.

So let’s see our L2VPN status.

RP/0/RP0/CPU0:ncs5508-1#show l2vpn xconnect group evpn-vpws detail
Group evpn-vpws, XC vpws1, state is up; Interworking none
 AC: HundredGigE0/2/0/2.1100, state is up
 Type VLAN; Num Ranges: 1
 Rewrite Tags: []
 VLAN ranges: [1100, 1100]
 MTU 9016; XC ID 0x1000001; interworking none
 packets: received 157064234, sent 157063216
 bytes: received 234968088320, sent 234966565392
 drops: illegal VLAN 0, illegal length 0
 EVPN: neighbor, PW ID: evi 1100, ac-id 11003, state is up ( established )
 XC ID 0xc0000001
 Encapsulation MPLS
 Source address
 Encap type Ethernet, control word disabled
 Sequencing not set
 Preferred path Active : SR TE vpws1-policy, Statically configured, fallback enabled
 Tunnel : Up

 EVPN  Local Remote
 ------------ ------------------------------ -----------------------------
 Label 64007 64006
 MTU   9016  9016
 Control word disabled disabled
 AC ID 11001 11003
 EVPN type Ethernet Ethernet

So if we go n shut the primary dynamic path we can see the forwarding table moves over to our segment-list defined for label 16003 which is for NCS5508-3.

RP/0/RP0/CPU0:ncs5508-1#config t
Wed Jun 27 14:58:04.096 UTC
RP/0/RP0/CPU0:ncs5508-1(config)#int HundredGigE0/1/0/0
RP/0/RP0/CPU0:ncs5508-1#show mpls forwarding
Local Outgoing Prefix Outgoing Next Hop Bytes
Label Label or ID Interface Switched
------ ----------- ------------------ ------------ --------------- ------------
16002 16002 SR Pfx (idx 2) Fo0/2/0/18 0
16003 16003 SR Pfx (idx 3) Fo0/2/0/18 0


So thats all, i hope you like the blog and let me know your feedback.