D-Link xStack DGS-3620 Series Reference Manual page 242

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xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
Type-7 LSA (Link State Advertisement) packets are
used to import external routes into the NSSA.
These packets can originate from NSSA ASBRs or
NSSA ABRs and are defined by setting the P-Bit in
the LSA type-7 packet header. Each destination
network learned from external routes is converted
into Type-7 LSA packets. These packets are
specific for NSSA switches and the route
information contained in these packets cannot
leave the area unless translated into Type-5 LSA
packets by Area Border Routers. See the following
table for a better description of the LSA type-7
packet seen here.
Parameter
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
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.
Figure 6-42 LSA Type-7 Packet
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