Packet Forwarding In Mpls Sr - H3C SR6600 Command Reference Manual

To configure an SRLSP, you must specify the label stack for packets to be forwarded along the
SRLSP on the ingress node. Each label in the stack corresponds to a prefix SID or adjacency
SID. The system can look for the outgoing label and next hop based on the prefix SID or
adjacency SID.
•
Configure SR nodes to use an IGP to dynamically establish an SRLSP.
SR nodes follow these steps to establish SRLSPs:
a. Use an IGP to collect prefix SID information from the MPLS SR network.
b. Calculate the shortest paths to other SR nodes based on the collected prefix SID
information and the IGP network topology.
c. Establish SRLSPs based on the shortest paths.
•
Configure a controller to deploy SRLSP configuration to the device so the device creates
SRLSPs.
For more information about controller configuration, see the user guide for the controller.

Packet forwarding in MPLS SR

Based on the SID allocation method, MPLS SR uses one of the following packet forwarding
methods:
•
Prefix SID-based packet forwarding—The ingress node encapsulates the prefix SID for the
egress node to a packet. The transit nodes forward the packet based on label forwarding
entries.
•
Adjacency SID-based packet forwarding—The ingress node encapsulates the label stack
that contains the adjacency SIDs of all links along the forwarding path to a packet. Each transit
node uses the top label in the label stack to determine the next hop and pops the top label
before forwarding the packet to the next hop.
•
Prefix and adjacency SID-based packet forwarding—The nodes use prefix SID-based
packet forwarding in combination with adjacency-based packet forwarding.
Prefix SID-based packet forwarding
Figure 3 shows how a packet is forwarded along the SRLSP from Device A to Device E based on
prefix SIDs. In this example, the outgoing label for the packet is 21201 on Device A.
1. Ingress node Device A searches for a forwarding entry for label 21201, adds outgoing label
20201 to the packet and sends the packet to the next hop (Device B).
2. When transit node Device B receives the packet, it searches for a label forwarding entry that
matches the label in the packet. Then, Device B uses the outgoing label of the matched entry
(19201) to replace the label in the packet and forwards the packet to the next hop (Device C).
3. Transit nodes Device C and Device D process the packet in the same way Device B does.
4. When egress node Device E receives the packet, it removes the label and forwards the packet
by IP address.
Figure 3 Prefix SID-based packet forwarding
Device A
In Label Nexthop
21201 Device B
20201
IP data IP data
Device B
Out Label In Label
20201
In Label Nexthop Out Label
20201 Device C 19201
19201 18201
IP data
Device C
Nexthop Out Label
19201 Device D 18201
In Label
18201
4
17201
IP data
Device D Device E
In Label
17201
Nexthop Out Label
Device E 17201