Hello,
The domain ID is used in VPN scenarios to identify and distinguish domains, but not to prevent loops. 

As shown in the following figure, OSPF is used in different sites. On the PEs, the BGP routes imported to OSPF( VPN instances) are advertised to CEs as external LSAs (type-5 LSAs) by default. However, for the destinations of different nodes in the same OSPF domain, such routes should be advertised as Type 3 LSAs. Therefore, the domain ID is required to identify the same OSPF domain. In this case, the PEs consider that the MPLS domain is a super area 0. After receiving LSAs 1, 2, and 3 from the peer, the local PE summarizes them into LSA3 and advertise to the corresponding CE. If the PE receives LSA5, it still sends LSA5 to the CE. If the domain IDs are different, the PE imports all received routes as Type 5 LSAs.

I hope this helps.

Hi user!

OSPF Domain ID
If inter-area routes are advertised between local and remote OSPF areas, these areas are considered to be in the same OSPF domain.

Important characteristics of OSPF domain IDs include:
- Domain IDs identify and differentiate different domains.
- Each OSPF domain has one or more domain IDs. One domain ID must be a primary ID, while the others are secondary IDs.
- If an OSPF instance does not have a specific domain ID, its ID is considered as null.

Before a PE advertises the BGP routes learned from a remote PE to CEs, the PE checks the domain IDs carried in the BGP routes to determine the type of OSPF routes (Type 3, Type 5, or Type 7) to be advertised.

If local domain IDs are the same as or compatible with the remote domain IDs carried in the BGP routes, the PE advertises Type 3 routes. Otherwise, the PE advertises Type 5 or Type 7 routes.

Routing Loop Prevention

Routing loops may occur between PEs and CEs when OSPF and BGP learn routes from each other.

The preference of an OSPF route is higher than that of a BGP route. Therefore, BGP routes on PE1 and PE2 to 10.1.1.1/32 are replaced by the OSPF route. The OSPF route with the destination address 10.1.1.1/32 and next hop of CE1 is active in the routing tables of PE1 and PE2.

The BGP route then becomes inactive, causing LSAs generated when this route is imported by OSPF to be deleted. As a result, the OSPF route is withdrawn. There is no OSPF route in the routing table, and the BGP route becomes active again. This cycle causes route flapping.

OSPF VPN provides a solution to this problem.

https://support.huawei.com/enterprise/en/doc/EDOC1000141935/2e3eb6b4/enhanced-ospf-functions

Thanks!

Hi User,

domain-id (OSPF)

Function

The domain-id command sets an ID for an OSPF domain.

The undo domain-id command restores the default setting.

By default, the domain ID is null.

Format

domain-id { null | domain-id [ type type value value | secondary ] ^* }

undo domain-id [ domain-id [ type type value value ] ]

Parameters

Parameter	Description	Value
domain-id	Specifies the ID of an OSPF domain.	The value can be an integer or in dotted decimal notation. If it is an integer, the value ranges from 0 to 4294967295, and it is converted to dotted decimal notation when the ID is displayed. If it is in dotted decimal notation, it is displayed as entered.
null	Indicates that the OSPF domain ID is null.	-
type type	Specifies the type of the OSPF domain ID.	It can be 0005, 0105, 0205, or 8005. By default, it is 0005.
value value	Specifies the value of the type of the OSPF domain ID.	The value is a hexadecimal number that ranges from 0x0 to 0xffff, and the default value is 0x0.
secondary	Indicates the ID of a secondary domain.	The maximum number of domain-id secondary in each OSPF process is 1000.

Views

OSPF view

Default Level

2: Configuration level

Usage Guidelines

Usage Scenario

Domain IDs are used to identify domains.

If the local OSPF area and an OSPF area of a remote VPN attempts to exchange Type 3 LSAs, the two areas must be in the same OSPF domain. You can run the domain-id command to configure the same domain ID for the two OSPF areas.

The routes that are imported from a PE switch are advertised using External-LSAs. The routes destined for different nodes in the same OSPF domain are advertised based on Type 3 LSAs. This requires that the nodes in the same OSPF domain be configured with the same domain ID.

If the undo domain-id command without any parameter is executed, the primary domain ID will be deleted.

OSPF direct routes to the PE do not carry the domain ID, while BGP direct routes to the PE carry the domain ID.

Configuration Impact

Before sending routes to a remote CE switch, a PE switch sends Type 3 LSAs or Type 5 LSAs to the CE based on domain ID. If local domain IDs are the same as or compatible with remote domain IDs in BGP routes, the PE advertises Type 3 routes. If local domain IDs are different from or incompatible with remote domain IDs in BGP routes, the PE advertises Type 5 routes.

Precautions

• Each OSPF domain has one or multiple domain IDs. One of them is a primary ID and the others are secondary IDs.

• If an OSPF instance does not have a specific domain ID, its ID is considered as null.

• If the value of the domain ID is 0, secondary cannot be configured.

• The maximum number of domain-id secondary items configured in an OSPF process is 1000.

• The domain-id command is forbidden in public networks.

Example

# Set an OSPF domain ID.

<Quidway> system-view

[Quidway] ospf 1

[Quidway-ospf-1] domain-id 234

https://support.huawei.com/enterprise/en/doc/EDOC1100116542/cafb264d/domain-id-ospf

Thanks i hope this will help you.