Importance of SID Depth and Routing Domains in Segment Routing

In Segment Routing the concept of source routing is present:

In computer networking, source routing, also called path addressing, allows a sender of a packet to partially or completely specify the route the packet takes through the network. In contrast, in conventional routing, routers in the network determine the path incrementally based on the packet’s destination


In prevalent IP networks per-hop lookup is performed based on the single primary destination address in the packet. Consider a situation where a stack of IP addresses is present per packet and needs to be processed by the intermediate routers. There would be a requirement from the hardware in the line cards. In this hypothetical situation how deep of a stack of addresses can be processed by the router chipsets and hardware ?

Similarly the SID Depth or the Maximum SID Depth is a parameter in segment routing enabled network devices. To route from a ingress to an egress the path selected by the source should be entirely capable of handling the number of SIDs ( MPLS Labels in SR MPLS) that are pushed onto the packet. Because the path selected by the source is in effect translated into a stack of labels (in SR MPLS) therefore the number of labels that the each device in the path can handle is an important design consideration.

Also, in Segment Routing MPLS the SID i.e. the labels are distributed via the IGP. So an end to end path label stack is supposed to be either in a single IGP area or if multiple routing areas or domains are required then some tricks are required to push and handle a label that is not distributed by the IGP. Lets see now: An external entity will need to program the ingress source node to push a stack of labels which includes a label not distributed by the IGP. This being the source there will be a resultant intermediate destination where at some point on some hop a label will be popped and the next label will be not have been learnt via the IGP.

In some way the burden of end-to-end connectivity over multiple hops is being shifted from the distributed IP routing control plane into a central label stack distribution authority.

I wonder if where we had IP Planning and IP configurations we will have label planning and label configurations.

Shifting a portion of the intelligence present on distributed nodes to a central authority.


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