Flexible SR-TE label stack allocation for services

SR OS supports a dynamic egress label limit configuration mode that extends the number of allowed MPLS labels in the egress label stack by not counting specific labels in the BGP next-hop resolution check when those labels are not used. The configuration mode exists in EVPN services configured on Epipe, VPLS, and VPRN (EVPN-IFL), and in IP-VPN services.

Classic CLI


+-- dynamic-egress-label-limit
    no dynamic-egress-label-limit

MD CLI


+-- dynamic-egress-label-limit <boolean>

When the dynamic-egress-label-limit command is configured, the always-computed labels are no longer considered when resolving the next hop of the route. As a result, the following rules apply to the specified services:

For VPRN services, the OAM label is never computed. This is true whether the BGP next hop is being resolved over an auto-bind tunnel or an SDP in the vprn>spoke-sdp context. The dynamic mode is supported for EVPN-IFL and IP-VPN families.
For EVPN (Epipe or VPLS) services with dynamic-egress-label-limit configured, the control word (CW) and ESI label are only computed if they are used.
- In the case of the CW, the system reduces the egress label limit by one label when the CW is configured in the service. The CW is always accounted when the dynamic-egress-label-limit command is not configured.
- When the dynamic-egress-label-limit command is configured, the ESI label is not accounted for in Epipes or VPLS services without an ES; however, the ESI label is always accounted if dynamic-egress-label-limit is not configured.

When no dynamic-egress-label-limit is configured, the behavior follows the procedures described in Control plane handling of egress label stack limitations.

In summary, when the dynamic-egress-label-limit is configured, the total amount of labels (X) configured in X= (max-sr-labels Y + additional-frr-labels Z) can go higher for EVPN and IP-VPN services.

The following table summarizes the required behavior.

Table: Egress label stack limits for BGP services based on dynamic-egress-label-limit
Features that reduce the Label Stack		no dynamic-egress-label-limit			dynamic-egress-label-limit
Features that reduce the Label Stack		IP-VPN (VPRN)	EVPN-IFL (VPRN)	EVPN VPLS EVPN Epipe	IP-VPN (VPRN)	EVPN-IFL (VPRN)	EVPN VPLS EVPN Epipe
Always computed	Service label	1	1	1	1	1	1
	OAM label ¹	1	1	0	0	0	0
	CW	0	0	1	0	0	0
	ESI label	0	0	1	0	0	0
Computed if configured	Hash label (mutex with EL)	1	1	0	1	0	0
	Entropy EL+ELI	2	2	2	2	2	2
	CW	0	0	0	0	0	1
	ESI label ²	0	0	0	0	0	1
Required labels		2	2	3	1	1	1
Required labels + All Options		4	4	5	3	3	5
Maximum available labels		12	12	10	12	12	10
Maximum available transport labels without options		10	10	7	11	11	9
Maximum available transport labels with options		8	8	5	9	9	5

R-VPLS and B-VPLS services, with EVPN-MPLS enabled, also support the dynamic-egress-label-limit command when dynamic-egress-label-limit is configured, the CW is accounted for only if the control-word command is added.

In R-VPLS services, the ESI label is not accounted because the routed encapsulation is always larger (and either ESI label for bridged traffic, or routed traffic without ESI label is transmitted by the R-VPLS).
In B-VPLS services, only the CW applies; there is no ESI label.