OSPF - Options Interview

Default Route in NSSA

There are two ways to have a default route in an NSSA. When you configure an area as NSSA, by default the NSSA ABR does not generate a default summary route. In the case of a stub area or an NSSA totally stub area, the NSSA ABR does generate a default summary route.

Default Summary Route

By defining an area as a NSSA totally stub area, the NSSA ABR generates a default summary route. As mentioned, if the NSSA area were not defined as totally stub, then a default summary route is not generated by NSSA ABR. This configuration generates a default summary route for a NSSA totally stub area.

router ospf 1
 Area 1 nssa no-summary

Default Type 7

This configuration generates a type 7 default route. You can configure this command on any NSSA ASBR or NSSA ABR with these rules:

• NSSA ASBR can generate a default only when it has a default route in its routing table.
• The default route must be known through non-OSPF protocol
• NSSA ABR can generate a default route with or without a default route in its own routing table.

This command is used in order to generate an NSSA default route:

router ospf 1
 Area 1 nssa default-information-originate

Below lists the fields in the OSPF Type-1 Router-LSA:

Field	Description
V bit	Indicates whether the advertising router is an endpoint of a virtual link.
E bit	Indicates whether the advertising router is an Autonomous System Border Router (ASBR).
B bit	Indicates whether the advertising router is an Area Border Router (ABR).
Number of Links	Indicates the number of links of the advertising router. A Router-LSA would contain the information of all router links of the advertising router.
Link Type	Indicates the 4 types of router links. Note: There are 2 types of point-to-point links – numbered and unnumbered.
Link ID andLink Data	The Link ID and Link Data values vary according to the Link Type. The Link ID identifies the object to which a link connects to; while the Link Data provides extra information for a link. Note: The LSA header contains the Link-State ID while an LSA entry contains the Link ID allowing for easier differentiation.
Metric	Indicates the OSPF cost of a router link.
ToS, ToS Metric	Represent the Type of Service and are normally set to 0.

Below describes the bits in the Options field:

DN	Used in MPLS-based Layer 3 Virtual Private Networks as defined in RFC 2547 – BGP/MPLS VPNs. When a route learnt from a customer network via OSPF is advertised across a BGP/MPLS VPN using Multiprotocol BGP, and advertised back to a customer network via OSPF, a loop can occur in which the OSPF route is redistributed back to the VPN service provider network via BGP. The DN bit prevents this type of routing loop. When an OSPF router received a Type 3, 5, or 7 LSA with the DN bit set, it cannot use that LSA in the OSPF route calculations.
O	The O bit is set when the originating router supports Type 9, 10, and 11 Opaque LSAs.
DC	The DC bit is set when the originating router supports OSPF over Demand Circuits.
L	Indicates whether the OSPF packet contains a Link-Local Signaling (LLS) data block. This bit is set only in Hello and DBD packets. If the OSPF packet is cryptographically authenticated, the LLS data block must also be cryptographically authenticated. Reference: RFC 4813 – OSPF Link-Local Signaling.
N	The N bit is used only in Hello packets. The N bit is set when the originating router supports Type-7 NSSA-External-LSAs. Neighboring routers with mismatched N bit value will not form neighbor relationship. This restriction ensures that all OSPF routers within an area support NSSA capabilities. When the N bit is set to 1, the E bit must be 0.
P	The P bit is used only in Type-7 NSSA-External-LSA headers. Due to this reason, the N and P bits can share the same position in the Options field. The P (Propagate) bit is set to inform the NSSA ABR to translate Type-7 LSAs into Type-5 LSAs.
MC	The MC bit is set when the originating router supports Multicast extensions to OSPF (MOSPF). Note: Cisco does not support MOSPF, mainly due to the reasons that it uses a dense-mode multicast forwarding scheme and is protocol dependent. A Cisco router would generate a %OSPF-4-BADLSATYPE error message upon receiving a Type-6 LSA. The ignore lsa mospf router subcommand configures an OSPF router to ignore Type-6 LSAs and therefore prevents the router from generating the error message.
E	The E (ExternalRoutingCapability) bit is set when the originating router is capable of accepting AS External LSAs. It will be set to 1 in all AS External LSAs and in all LSAs originated in the backbone and non-stub areas; and will be set to 0 in all Hellos and LSAs originated within a stub area. Additionally, this bit is used in to Hello packets to indicate the capability of a router interface to send and receive Type-5 AS-External-LSAs. Neighboring routers with mismatched E bit value will not form neighbor relationship. This restriction ensures that all OSPF routers within an area support the stub capabilities.
MT	The MT bit is set when the originating router supports Multitopology OSPF, MT-OSPF. However, MT-OSPF is still under the proposal stage and is not generally adopted yet. Older OSPF specifications specified this bit position as the T bit. The T bit was set when the originating router support TOS-based routing. However, OSPF TOS-based routing has never been deployed; therefore the T bit was also never been used.

The Link ID, Link Data, Link Type, Number of TOS, Metric, TOS, and TOS metric fields appear one or more times in a Router-LSA corresponding to the Number of Links field.
Note: Cisco supports only TOS = 0. The ToS and ToS Metric fields appear corresponding to the Number of TOS field. Ex: If Number of TOS is 2, there will be 2 32-bit words containing 2 instances of these fields. If Number of TOS is 0, there will be no instances of these fields.

Below lists the Link ID and Link Data values for the various Type-1 Router-LSA link types:

Type	Description	Link ID	Link Data
1	Point-to-point numbered connection to another router	The Router ID of neighboring router	Physical interface IP address of the router to the network
1	Point-to-point unnumbered connection to another router	The Router ID of neighboring router	The MIB-II ifIndex value of the unnumbered interface [1]
2	Connection to a transit network	Physical interface IP address of the DR	Physical interface IP address of the router to the network
3	Connection to a stub network	IP network number or subnet number	Subnet mask
4	Virtual link	The Router ID of virtual link neighbor	Physical interface IP address of the router to reach the virtual link neighbor

[1] – This value usually starts with 0.