SR LSP
SR LSPs are established using the segment routing technique, and use a prefix or node segment to guide data packet forwarding. Segment Routing Best Effort (SR-MPLS BE) uses an IGP to run the shortest path algorithm to compute an optimal SR LSP.
The establishment and data forwarding of SR LSPs are similar to those of LDP LSPs. SR LSPs have no tunnel interfaces.
SR LSP Creation
Creating an SR LSP involves the following operations:
Devices report topology information to a controller (if the controller is used to create an LSP) and are assigned labels.
The devices compute paths.
Table 5-2 describes the process of using prefix labels to create an LSP shown in Figure 5-5.
Step |
Device |
Operation |
|---|---|---|
1 |
D |
An SRGB is configured on D and a prefix SID is configured on the loopback interface for generation and delivery of forwarding entries. After that, D encapsulates the SRGB and prefix SID into LSP packets and floods the LSP packets to the entire network through an IGP. After receiving the LSP packets, the other devices parse the packets, obtain the prefix SID advertised by D, compute label values based on local SRGBs and outgoing labels (OuterLabels) based on next-hop devices. They run the IGP to compute a label forwarding path and generate forwarding entries. |
2 |
C |
C parses the prefix SID released by D and computes a label value based on the local SRGB (36000 to 40960). The value is calculated using the following formula: Label = SRGB start value + Prefix SID value = 36000 + 100 = 36100 IS-IS calculates an OuterLabel based on the following formula: OuterLabel = SRGB start value advertised by the next-hop device + Prefix SID value = 16000 + 100 = 16100 Here, the next-hop device is D, and D releases the SRGB (16000 to 40960). |
3 |
B |
The calculation process is similar to C: Label = 26000 + 100 = 26100 OuterLabel = 36000 + 100 = 36100 |
4 |
A |
The calculation process is similar to C: Label = 18000 + 100 = 18100 OuterLabel = 26000 + 100 = 26100 |
Data Forwarding
Similar to MPLS, SR label operations include pushing a label into a label stack, swapping for a label stack, and popping out a label.
Push: After a packet enters an SR LSP, the ingress node adds a label between the Layer 2 and IP header. Alternatively, the ingress node adds a label stack above the existing label stack.
Swap: When packets are forwarded in an SR domain, a node searches the label forwarding table for a label assigned by a next hop and swaps the label on the top of the label stack with the matching label in each SR packet.
Pop: After the packets leave an SR domain, a node finds an outbound interface mapped to the label on the top of the label stack and removes the top label.
Table 5-3 describes the data forwarding process on the network shown in Figure 5-6.
Step |
Device |
Operation |
|---|---|---|
1 |
A |
Receives a data packet, adds label 26100 to the packet, and forwards the packet. |
2 |
B |
Receives the labeled packet, swaps label 26100 for label 36100, and forwards the packet. |
3 |
C |
Receives the labeled packet, swaps label 36100 for label 16100, and forwards the packet. |
4 |
D |
Removes label 16100 and forwards the packet along a matching route. |