•
Segment Routing Mapping Client (SRMC) —Receives prefix-SID mappings advertised by the
SRMS and creates SR-MPLS label forwarding entries.
As shown in Figure 11 , Device A and Device B are running SR-MPLS, and Device B, Device C, and
Device D are running LDP. After Device D assigns a label for destination address 3.3.3.3/32, Device
B and Device C each install an LDP label forwarding entry.
The following steps establish a mapping between the SRLSP and the LDP LSP:
1. Device B acts as an SRMS. It assigns SID index value 201 to the IP address of loopback interface 0 on Device D (3.3.3.3/32), and then advertises an IP prefix-SID mapping TLV to
Device A.
2. Device A acts as an SRMC. After receiving the advertised mapping TLV, it creates an
SR-MPLS label forwarding entry.
A mapping is established between the SRLSP and the LDP LSP.
Figure 11 SR to LDP packet forwarding
IP data
16201
IP data
16201
IP data
SR
20
IP data
LDP
IP data
Type
SR
Device A
SRMC
In Label
16201
Out Label
16201
Device B
SRMS
Type
LDP
Device C
In Label
20
Out Label
-
Device D
Type
SR
LDP
In Label
16201
-
Out Label
-
20
If Device A uses an SRLSP to send a packet to Device D, the packet is forwarded as follows:
3. Ingress node Device A adds label 16201 to the packet and then sends the packet to transit node Device B.
4. When transit node Device B receives the packet, it searches for a label forwarding entry that matches incoming label 16201. Because the entry does not have an outgoing label, Device B searches for a valid LDP outgoing label for destination address 3.3.3.3/32. Then, Device B adds the LDP label (20) to the packet as the SR outgoing label and forwards the packet to the next hop (Device C).
5. When transit node Device C receives the packet, it searches for an LDP label forwarding entry for incoming label 20. Then, it removes the label from the packet and forwards the packet to egress node Device D.
6. The egress node forwards the packet by IP address.
LDP to SR
As shown in Figure 12 , Device C and Device D are running SR-MPLS, and Device A, Device B, and
Device C are running LDP. Each device running LDP assigns a label to destination address
3.3.3.3/32 and then installs an LDP label forwarding entry.
The following steps establish an association between the SR-MPLS label and the LDP label:
11
