What are the rules for an S series switches to select and refresh router IDs

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On an S series switches, the rules for selecting and refreshing router IDs are as follows:

If the router id command is executed, the configured router ID is used.
If the router id command is not executed and the loopback interfaces with IP addresses configured already exist, the switch selects the largest IP address from the loopback interface addresses as the router ID.
If the router id command is not executed and the loopback interfaces with IP addresses configured do not exist, the switch selects the largest IP address from the IP addresses of other interfaces as the router ID. In this case, the Up or Down state of interfaces is not considered.
The router ID is reelected only when the interface IP address that is elected as the router ID is deleted or modified.
Interface status changes do not result in reelection of the router ID.
If the IP address of a non-loopback interface is originally selected as the router ID and currently the IP address of a loopback interface is configured, the router ID will not be reelected.
If a larger interface IP address is set, the router ID will not reelected.
If the master and slave MPUs are installed on the switch, the system backs up the configured router ID and the router ID selected among interface IP addresses.
During system startup, when the protocol attempts to obtain the router ID, the RM may not obtain the information about all interface IP addresses. In this case, a smaller interface IP address may be selected as the router ID to return to the protocol. This is not considered as a problem.
When the router ID is changed, you need to manually reset the protocol to obtain a new router ID.
The router ID cannot be 0.0.0.0 or 255.255.255.255.

Other related questions:
Rules based on which OSPF selects router IDs of S series switches
A router ID is used to uniquely identify a device on an OSPF network. Therefore, each router ID in an OSPF process must be unique. Otherwise, an OSPF neighbor relationship cannot be established and routing information is incorrect. You are advised to configure a unique router ID for each OSPF process on an OSPF device. You can configure the router ID manually. If no router ID is specified through a command, the system selects an IP address of an interface as the router ID. The largest loopback interface IP address is used as the router ID. If no loopback interface is configured, the largest physical interface IP address is used as the router ID. The router ID is reselected in the following scenarios: The ospf xx router-id x.x.x.x command is used to reconfigure a router ID of an OSPF device. The router ID of a system is reconfigured and the OSPF process is restarted. The IP address that was selected as the router ID of a system is deleted and the OSPF process is restarted.

What are rules for route selection
  • Rule 1: The route with higher priority is preferred. A small value indicates a high priority. If a route cannot be selected using this rule, go to rule 2.
  • Rule 2: If two routes have the same priority, the route with the higher internal protocol preference is preferred. A small value indicates a high priority. If a route cannot be selected using this rule, go to rule 3.
  • Rule 3: If two routes have the same priority and belong to the same protocol, the route with the lower-cost value is preferred. If the cost values of two routes are the same, load balancing can be formed. If the number of routes for forming load balancing exceeds the maximum value (such as m) set by the system, the total number of only the first m routes act as the active routes.

  • What are address allocation rules for S series switches
    When functioning as a DHCP server, an S series switch (excluding the S1700) adds IP addresses in an address pool to different IP address lists based on the IP address status. Unallocated IP addresses belong to the allocatable IP address list. Allocated IP addresses belong to the in-use IP address list. Conflicting IP addresses belong to the conflicting IP address list. IP addresses that cannot be allocated belong to the unallocatable IP address list. The DHCP server selects an IP address for a client from the address pool in the following sequence: 1. IP address statically bound to the MAC address of the client on the DHCP server 2. IP address that has been allocated to the client 3. IP address specified in the Option50 field (requested IP address) in the DHCP Discover message from the client 4. Largest allocatable IP address 5. If the DHCP server does not find any allocatable IP address, it searches for the expired IP addresses and conflicting IP addresses in turn, and then allocates a valid IP address to the client. If all the IP addresses are in use, the DHCP server replies with a DHCP NAK message to notify the client that no IP address is available. After receiving the DHCP NAK message, the DHCP client sends a DHCP Discover message to apply for a new IP address. The device can exclude some IP addresses that cannot be allocated through DHCP from address pools. For example, if IP segment 192.168.1.100/24 has been manually configured for the DNS server, the DHCP server needs to exclude this IP address from the address pool on network segment 192.168.1.0/24. In this way, IP address 192.168.1.100 will be not allocated through DHCP, preventing IP address conflicts.

    Address allocation rules on S series switch
    When an S series switch (except S1700 switches) functions as a DHCP server, the switch adds IP addresses in an address pool to different IP address lists based on the IP address status. Unallocated IP addresses belong to the allocatable IP address list. Allocated IP addresses belong to the in-use IP address list. Conflicting IP addresses belong to the conflicting IP address list. IP addresses that cannot be allocated belong to the unallocatable IP address list. The DHCP server selects an IP address for a client from the address pool in the following sequence: 1. IP address statically bound to the MAC address of the client on the DHCP server 2. IP address that has once been allocated to the client 3. IP address specified in the Option 50 field (requested IP address) in the DHCP Discover message sent by the client 4. Largest allocatable IP address 5. If the DHCP server does not find any allocatable IP address, it searches for the expired IP addresses and conflicting IP addresses in turn, and then allocates a valid IP address to the client. If all the IP addresses are in use, the DHCP server replies with a DHCP NAK message to notify the client that no IP address is available. After receiving the DHCP NAK message, the DHCP client sends a DHCP Discover message to apply for a new IP address. The switch can exclude some IP addresses that cannot be allocated through DHCP from the address pool. For example, if 192.168.1.100/24 has been manually configured for the DNS server, the DHCP server needs to exclude IP address 192.168.1.100 from the address pool on network segment 192.168.1.0/24. In this way, IP address 192.168.1.100 will not be allocated through DHCP, preventing IP address conflicts.

    What route selection rules should be followed when external routes with the same prefix are imported into OSPF on an S series switch
    Question: What route selection rules should be followed when external routes with the same prefix are imported into an S series switch supporting OSPF? Answer: Imported external OSPF routes include Type 1 and Type 2 routes. By default, Type 2 routes are imported. When Type 1 external routes are imported, in route calculation, the cost of the route to be advertised is the sum of the LSA cost and the cost of an external route. When Type 2 external routes are imported, OSPF considers that the cost of an external route is much higher than that of an intra-area route. Therefore, only the LSA cost value is used in route calculation. When Type 1 external routes with the same prefix are imported, the route selection rules are as follows: The route with the lowest cost is selected. The cost is the sum of the cost of the route to the ASBR and the LSA cost. If the costs of the routes are the same, the route in a Type 5 LSA is selected. When Type 2 external routes with the same prefix are imported, the route selection rules are as follows: The imported route with the smallest LSA cost value is selected. If the LSA cost values are the same, the route with the lowest cost of the route to the ASBR is selected. If the costs of the routes are the same, the route in a Type 5 LSA is selected.

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