D-Link DGS-3612G Product Manual - Page 231

xStack® DGS-3600 Series Layer 3 Managed Gigabit Ethernet Switch Field Description Link State Packet Header 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. Network Mask The IP address mask for the advertised destination. E - bit 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. Forwarding Address 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. TOS The Type of Service that the following cost is relevant to. Metric The cost of this route. The interpretation of this metric depends on the external type indication (the E-bit above). External Route A 32-bit field attached to each external route. This is not used by the OSPF protocol Tag itself. 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. 215