Segment Routing Control Plane – ISIS

Segment Routing Deployment – Control and Data Plane gives fair understanding of how to deploy segment routing by using IS-IS on Cisco XR based platforms. In this post, I will be focusing how the segment routing control plane gets established by using IS-IS protocol.

Remember below mentioned points before moving ahead:

1. Every router must know what type of Data Plane capability to use like capable of processing SR-MPLS-encapsulated IPv4 packets on all interfaces or SR-MPLS-encapsulated IPv6 packets on all interfaces [SR-Capability TLV]
2. Every router must know what the SID-Index of the destination route is [Extended IP Reachability TLV]
3. Every router must know about the SRGB block to be used [SR-Capability TLV]
4. Every router must know what the SRLB (Segment Routing Local Block – Mainly Used For Adj-Sids, This is local to router and can be sent to PCE or can be used statically on headed router for Segment Routing Traffic Engineering Use Case) [SR-Capability TLV]

All the above four functions/tasks are taken care by ISIS Router-Capability TLV(242) and Extended IP Reachability TLV – (135,235)

1. Router-Capability TLV(242)
As per RFC 8667, section 3; Router Capability TLV (242) carries below mentioned different Sub-TLV. Router Capability TLV, helps to exchange the SRGB information along with type of data plane used.
a. SR-Capabilities Sub-TLV – This is type 2 sub-TLV. It contains the information of the SRGB Range, SID/Label value and capability of processing SR-MPLS-encapsulated IPv4 packets or IPv6 (Flags).

The SR-Capabilities sub-TLV has the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| Type | Length | Flags |

| Range |
// SID/Label Sub-TLV (variable) //

Example of the advertisement of SRGB ranges by SR capable nodes(routers).
SR-Capable Node Advertises: Range: 100, SID value: 100
Receiving routers build the SRGB as follows: [100,199]. Index 0 means label 100, Index 1 means label 101, Index 99 means label 100.


Share on facebook
Share on twitter
Share on linkedin

Leave a Reply

Your email address will not be published. Required fields are marked *

Become a member

Full Access to 739 Lessons. New Lessons Added Every Week!

Awesome Deal! Get 2 Months for FREE!

No Obligations. Cancel At Any Time!