Overview

In the first blog of this series, we covered the Segment Routing Problem definition, Drivers, use cases,  and the benefits it brings to the table. (for more details refer to this article). This post we will explain Segment Routing Building Blocks, without further ado let’s start with SR SID.

Lab Setup

The Network Setup that we will use in this series is shown below:

• It includes Cisco and Juniper virtual routers.
• Below devices forms an MPLS network with (3 Core + 4 Provider Edge routers).
• Segment Routing is used and preferred over LDP.

One of the key properties of SR is that MPLS labels are distributed via the Interior Gateway Protocol (IGP), ISIS or OSPF, rather than a dedicated label distribution protocol. Another benefit is the concept of Source Routing that implies encoding the path a packet traverse by means of a stack of headers applied to the packet when launching it into the network. These headers/labels what we call SID “Segment ID” in the SR world.

Node Segments

Allow us to ask you this question;  What is the range of label blocks reserved for SR in Cisco IOS-XR, and Juniper JunOS ?

• IOS-XR –> 16000 till 23900
• JunOS –> By default no reserved block

Node SID is a special type of Prefix-SID. Each node in the network has a node segment associated with its loopback address. (however, it can have multiple prefix segments).

Any other node in the network can send packets to it along the shortest IGP path by using that node segment.

The routers in the network use the following two parameters in order for each node in the network to build the Node segment for each node in the network

1. Node index: Each router must have a unique node index. This is also known as a Node-SID
2. Label Stack: This is defined in terms of a start-label and a label-range. The label range must be wide enough to accommodate all of the routers in the domain (including anticipated future growth). This label range is known as the segment routing global block (SRGB).

Below is the basic configuration to enable SR on IOS-XR devices and specify the Node-SID using the index

router isis 100

net 49.0000.0000.0001.00

address-family ipv4 unicast

metric-style wide

segment-routing mpls

!

interface Loopback0

passive

address-family ipv4 unicast

prefix-sid index 1

!

!

Below is the basic configuration to enable SR on JunOS devices and specify the Node-SID using the index

isis {

source-packet-routing {

node-segment ipv4-index 402;

}

level 2 wide-metrics-only;

level 1 disable;

}

With the above configuration, we can observe the Node-SID advertised in the ISIS database as shown below

LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd  ATT/P/OL

XR-P01.00-00        * 0x00000009   0xfab6        697  /*            0/0/0

Area Address:   49

NLPID:          0xcc

IP Address:     10.100.1.1

Metric: 0          IP-Extended 10.100.1.1/32

Prefix-SID Index: 1, Algorithm:0, R:0 N:1 P:0 E:0 V:0 L:0

Prefix Attribute Flags: X:0 R:0 N:1

Hostname:       XR-P01

Router Cap:     10.100.1.1, D:0, S:0

Segment Routing: I:1 V:0, SRGB Base: 16000 Range: 8000

In IOS-XR by default, the OS reserves a label block from 16000 with a range of 8000 to be allocated for SR global block for Node segments.

For Juniper devices, we can see the Node-SID and the SRGB block as shown below

LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd  ATT/P/OL

MX-P02.00-00          0x00000026   0xc871        899  /1198         0/0/0

Area Address:   49

Hostname:       MX-P02

Router Cap:     10.100.1.2, D:0, S:0

Segment Routing: I:1 V:1, SRGB Base: 12560 Range: 4096

SR Algorithm:

Algorithm: 0

Prefix-SID Index: 402, Algorithm:0, R:0 N:1 P:0 E:0 V:0 L:0

In Juniper by default, there is no default label block allocated for SR and the system computes a block for SR in case an SRGB block is not statically configured.

With the above basic configuration, each router calculates the label to reach a specific node using the following formula:

Node Segment to reach PeerX = SRGB advertised by downstream peer + Node-index advertised for PeerX

Node Segment Calculated for PeerX = SRGB for local router + Node-index advertised for PeerX

So we can see that the following is the MPLS forwarding table for XR-P01 to reach MX-PE01

RP/0/RP0/CPU0:XR-P01#sh mpls  forwarding

Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes

Label  Label       or ID              Interface                    Switched

------ ----------- ------------------ ------------ --------------- ------------

16004  Pop         SR Pfx (idx 4)     Gi0/0/0/2    172.20.1.6      2156

16402  Pop         SR Pfx (idx 402)   Gi0/0/0/0    172.20.1.1      3224

16403  Pop         SR Pfx (idx 403)   Gi0/0/0/1    172.20.1.3      731

16406  12966       SR Pfx (idx 406)   Gi0/0/0/0    172.20.1.1      198

4774        SR Pfx (idx 406)   Gi0/0/0/1    172.20.1.3      733

The below diagram outlines the calculation

We can see that the label assigned for each node across the network is not consistent and it is very similar to LDP. Thus the recommended approach is to unify the SRGB on all devices in order to have a consistent calculation for the Node Segment across the network

Below is the configuration required on IOS-XR devices

segment-routing

global-block 400000 400999

Below is the configuration required on JunOS devices

isis {

source-packet-routing {

srgb start-label 400000 index-range 1000;

node-segment ipv4-index 403;

}

}

We can verify that the Node-Segment assigned to all the devices is consistent as shown below from XR-P01 output

RP/0/RP0/CPU0:XR-P01#sh mpls  forwarding

Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes

Label  Label       or ID              Interface                    Switched

------ ----------- ------------------ ------------ --------------- ------------

400402 Pop         SR Pfx (idx 402)   Gi0/0/0/0    172.20.1.1      474

400403 Pop         SR Pfx (idx 403)   Gi0/0/0/1    172.20.1.3      474

400406 400406      SR Pfx (idx 406)   Gi0/0/0/0    172.20.1.1      0

400406      SR Pfx (idx 406)   Gi0/0/0/1    172.20.1.3      297

400407 400407      SR Pfx (idx 407)   Gi0/0/0/0    172.20.1.1      217

400407      SR Pfx (idx 407)   Gi0/0/0/1    172.20.1.3      0

The below is the output from Juniper MX-PE03 node that outlines the Node-SID

MX-PE03# run show route table inet.3

inet.3: 5 destinations, 5 routes (5 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

10.100.1.1/32      *[L-ISIS/14] 00:03:43, metric 2000

to 172.20.1.15 via ge-0/0/0.0, Push 400001

> to 172.20.1.17 via ge-0/0/1.0, Push 400001

10.100.1.2/32      *[L-ISIS/14] 02:13:37, metric 1000

> to 172.20.1.15 via ge-0/0/0.0

10.100.1.3/32      *[L-ISIS/14] 00:52:20, metric 1000

> to 172.20.1.17 via ge-0/0/1.0

10.100.1.4/32      *[L-ISIS/14] 00:27:56, metric 2000

> to 172.20.1.17 via ge-0/0/1.0, Push 400004

10.100.1.7/32      *[L-ISIS/14] 00:11:38, metric 2000

to 172.20.1.15 via ge-0/0/0.0, Push 400407

> to 172.20.1.17 via ge-0/0/1.0, Push 400407

The below diagram outlines the Node-SID in the network

Let’s conclude this article by listing the advantages of Using Node Segments Instead of LDP:

1. You have already seen the first advantage – the fact that when configuring the same SRGB on each router, the label required to reach a given router is the same throughout the network, rather than changing hop-by-hop as in the LDP case.
2. the second advantage is having fewer protocols to configure and monitor. LDP can be removed from the network.
3. The path can be treated as direction instead of hop-by-hop, when you point to 40406 all routers in the middle will freely independently decide how to reach it.