As described in Data path support, the egress IOM can push a maximum of 12 labels; however, this number may be reduced if other fields are pushed into the packets. For example, for a VPRN service, the ingress LER can send an IP VPN packet with 12 labels in the stack, including one service label, one label for OAM, and 10 transport labels. However, if entropy is configured, the number of transport labels is reduced by two (Entropy Label (EL) and Entropy Label Indicator (ELI)). Similarly, for EVPN services, the egress IOM may push specific fields that reduce the total number of supported transport labels.
To avoid silent packet drops in cases where the egress IOM cannot push the required number of labels, SR OS implements a set of procedures that prevent the system from sending packets if it is determined that the SR-TE label stack to be pushed exceeds the number of bytes that the egress IOM can put on the wire.
Table: Label stack egress IOM restrictions on FP-based hardware for IPVPN and EVPN services describes the label stack egress IOM restrictions on FP-based hardware for IPVPN and EVPN services.
| Features that reduce the label stack | Source service type | |||||
|---|---|---|---|---|---|---|
| IP-VPN (VPRN) | EVPN-IFL (VPRN) | EVPN VPLS or EVPN Epipe | EVPN B-VPLS (PBB-EVPN) | EVPN-IFF (R-VPLS) | ||
Always Computed1 |
Service Label |
1 |
1 |
1 |
1 |
1 |
OAM Label |
1 |
1 |
0 |
0 |
0 |
|
Control Word |
0 |
0 |
1 |
1 |
1 |
|
ESI Label |
0 |
0 |
1 |
0 |
0 |
|
Computed if configured2 |
Hash Label (mutex with EL) |
1 |
1 |
0 |
0 |
0 |
Entropy EL+ELI |
2 |
2 |
2 |
2 |
2 |
|
|
Required Labels3 |
2 |
2 |
3 |
2 |
2 |
|
Required Labels + Options3 |
4 |
4 |
5 |
4 |
4 |
|
Maximum available labels4 |
12 |
12 |
10 |
6 |
9 |
|
Maximum available transport labels without options5 |
10 |
10 |
7 |
4 |
7 |
|
Maximum available transport labels with options5 |
8 |
8 |
5 |
2 |
5 |
|
The total number of labels configured in the command max-sr-labels label-stack-size [additional-frr-labels labels] must not exceed the labels indicated in the "Maximum available transport labels with/without options" rows in Table: Label stack egress IOM restrictions on FP-based hardware for IPVPN and EVPN services . If the configured LSP labels exceed the available labels in the table, the BGP route next hop for the LSP is not resolved and the system does not even try to send packets to that LSP.
For example, for a VPRN service with EVPN-IFL where the user configures entropy-label, the maximum available transport labels is eight. If an IP Prefix route for next-hop X is received for the service and the SR-TE LSP to-X is the best tunnel to reach X, the system checks that (max-sr-labels + additional-frr-labels) is less than or equal to eight. Otherwise, the IP Prefix route is not resolved.
The same control plane check is performed for other service types, including IP shortcuts, spoke SDPs on IP interfaces, spoke SDPs on Epipes, VPLS, B-VPLS, R-VPLS, and R-VPLS in I-VPLS or PW-SAP. In all cases, the spoke SDP is brought down if the configured (max-sr-labels + additional-frr-labels) is greater than the maximum available transport labels. Table: Maximum available transport labels for IP shortcuts and spoke SDP services indicates the maximum available transport labels for IP shortcuts and spoke SDP services.
| Features that reduce the label stack | Source service type | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| IP shortcuts | Spoke-sdp interface | Spoke-sdp Epipe | Spoke-sdp VPLS | Spoke-sdp B-VPLS | Spoke-sdp R-VPLS | Spoke-sdp R-VPLS I-VPLS | PW-SAP Epipe/interface | ||
Always Computed6 |
Service Label |
0 |
1 |
1 |
1 |
1 |
1 |
1 |
1/1 |
OAM Label |
0 |
1 |
1 |
1 |
1 |
0 |
0 |
0/0 |
|
IPv6 label |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
0/0 |
|
Computed if configured7 |
Hash Label (mutex with EL) |
0 |
1 |
1 |
1 |
1 |
1 |
0 |
0/0 |
Entropy EL+ELI |
2 |
2 |
2 |
2 |
2 |
2 |
2 |
0/0 |
|
Control Word |
0 |
1 |
1 |
1 |
1 |
1 |
1 |
0/0 |
|
Required Labels8 |
1 |
2 |
2 |
2 |
2 |
1 |
1 |
1/1 |
|
Required Labels + Options8 |
3 |
5 |
5 |
5 |
5 |
4 |
4 |
1/1 |
|
Maximum available labels9 |
12 |
9 |
10 |
10 |
6 |
8 |
4 |
10/7 |
|
Maximum available transport labels without options 10 |
11 |
7 |
8 |
8 |
4 |
7 |
3 |
9/7 |
|
Maximum available transport labels with options10 |
9 |
4 |
5 |
5 |
1 |
4 |
0 |
8/6 |
|
In general, the labels shown in Table: Label stack egress IOM restrictions on FP-based hardware for IPVPN and EVPN services and Table: Maximum available transport labels for IP shortcuts and spoke SDP services are valid for network ports that are null or dot1q encapsulated. For QinQ network ports, the available labels are deducted by one.