D-Link DGS-3600 Series User Manual page 211

Field
Link State
Packet Header
Network Mask
E - bit
Forwarding
Address
TOS
Metric
External Route
Tag
The N-Bit
Contained in the options field of the Link State Packet header, the N-Bit is used to ensure that all members of an NSSA agree on
the area configurations. Used in conjunction with the E-Bit, these two bits represent the flooding capability of an external LSA.
Because type-5 LSAs cannot be flooded into the NSSA, the N-Bit will contain information for sending and receiving LSA type-7
packets, while the E-bit is to be cleared. An additional check must be created for the function that accepts these packets to verify
these two bits (N and E-Bit). Bits matching the checking feature will be accepted, while other bit combinations will be dropped.
The P-Bit
Also included in the Options field of the LSA type-7 packet, the P-Bit (propagate) is used to define whether or not to translate the
LSA type-7 packet into an LSA type-5 packet for distribution outside the NSSA.
LSA Type-7 Packet Features

LSA Type-7 address ranges for OSPF areas are defined as a pair, consisting of an IP address and a mask. The packet will
also state whether or not to advertise and it will also contain an external route tag.

The NSSA ASBR will translate external routes into type-7 LSAs to be distributed on the NSSA. NSSA ABRs will
optionally translate these type-7 packets into type-5 packets to be distributed among other OSPF areas. These type-5
packets are indiscernible from other type-5 packets. The NSSA does not support type-5 LSAs.

Once border routers of the NSSA have finished translating or grouping type-7 LSAs into type-5 LSAs, type-5 LSAs
should be flushed or reset as a translation or an aggregation of other type-7 LSAs.

The forwarding addresses contained in translated type-5 LSAs must be set, with the exception of an LSA address range
match.
®
xStack DGS-3600 Series Layer 3 Gigabit Ethernet Managed Switch
Description
This field will hold information concerning information regarding the LS Checksum,
length, LS sequence number, Advertising Router, Link State ID, LS age, the packet
type (Type-7), and the options field. The Options byte contains information regarding
the N-Bit and the P-Bit, which will be described later in this section.
The IP address mask for the advertised destination.
The type of external metric. If the E-bit is set, the metric specified is a Type 2 external
metric. This means the metric is considered larger than any link state path. If the E-bit
is zero, the specified metric is a Type 1 external metric. This means that is
comparable directly to the link state metric.
Data traffic for the advertised destination will be forwarded to this address. If the
Forwarding Address is set to 0.0.0.0, data traffic will be forwarded instead to the
advertisement's originator.
Yet, if the network between the NSSA ASBR and the adjacent AS is advertised in the
area as an internal OSFP route, this address will be the next hop address.
Conversely, if the network is not advertised as internal, this field should be any of the
router's active OSPF interfaces.
The Type of Service that the following cost is relevant to.
The cost of this route. The interpretation of this metric depends on the external type
indication (the E-bit above).
A 32-bit field attached to each external route. This is not used by the OSPF protocol
itself.
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