Services
7750 SR OS Services Guide Page 43
Default SAP on a Dot1q Port
This feature introduces default SAP functionality on Dot1q-encapsulated ports. This is similar to
the functionality provided by Q1* SAP on QinQ encapsulated ports, meaning that on On dot1q-
encapsulated ports where a default SAP is configured, all packets with q-tags not matching any
explicitly defined SAPs will be assigned to this SAP. SAPs with default QinQ encapsulation are
supported in VPLS, Epipe, IES and VPRN services. Both DHCP snooping and IGMP snooping
are supported for QinQ SAPs. In this context, the character “*” indicates default which means
allow through. A 0 value means that it should not be there which allows the Qtag to be missing.
One of the applications where this feature can be applicable is an access connection of a customer
who uses the whole port to access Layer 2 services. The internal VLAN tags are transparent to the
service provider. This can be provided by a null encapsulated port. A dedicated VLAN (not used
by the user) can be used to provide CPE management.
In this type of environment, logically two SAPs exist, a management SAP and a service SAP. The
management SAP can be created by specifying a VLAN tag which is reserved to manage the CPE.
The service SAP covers all other VLANs and behaves as a SAP on a null-encapsulated port.
There a few constraints related for the use of default SAP on a Dot1q-encapsulated port:
• This type of SAP is supported only on VPLS and Epipe services and cannot be created in
IES and VPRN services as it cannot preserve VLAN tag markings.
• For VPLS SAPs with STP enabled, STP listens to untagged and null-tagged BPDUs only.
All other tagged BPDUs are forwarded like other customer packets. This is the same
behavior as null-encapsulated ports.
• IGMP snooping is not supported on a default SAP. This would require remembering
VLAN tags per hosts. By not allowing IGMP snooping of this SAP, all IGMP packets will
be transparently forwarded.
• This type of SAP is mutually exclusive with a SAP defined by explicit null encapsulation
(for example, 1/1/1:0). This avoids conflict as to which SAP untagged frames should be
associated.
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