Routing protocol preference on S series switches


Preference of routing protocols
Different routing protocols, including the static routing protocol may discover different routes to the same destination, but not all these routes are optimal. At a certain moment, only one routing protocol determines the preferred route to a certain destination. To select the optimal route, routes of these routing protocols including the static route are configured with preferences. When multiple sources of routing information exist, the route learned by the routing protocol with the highest preference (a smaller value indicates a higher preference) becomes the optimal route. The optimal route is then saved to the local routing table.

Routers define the external preference and internal preference. External preferences are manually configured for routing protocols. The following lists the default external preferences of routing protocols:
- Direct: 0
- OSPF: 10
- IS-IS: 15
- Static: 60
- RIP: 100
- OSPF ASE: 150
- OSPF NSSA: 150
- IBGP: 255
- EBGP: 255
Value 0 indicates direct routes and value 255 indicates routes learned from unreliable sources. A smaller value indicates a higher preference.
Except for direct routes, the priorities of routing protocols can be manually configured. In addition, the preference of each static route varies.

Internal preferences of routing protocols cannot be manually configured. The following lists the internal preferences of routing protocols:
- Direct: 0
- OSPF: 10
- IS-IS Level-1: 15
- IS-IS Level-2: 18
- Static: 60
- RIP: 100
- OSPF ASE: 150
- OSPF NSSA: 150
- IBGP: 200
- EBGP: 20
During route selection, a router first compares the external preferences of routes. When the same external preference is set for different routing protocols, the router selects the optimal route based on the internal preference. For example, two routes (an OSPF route and a static route) are available. Both routes can reach the destination, and the preferences of the two routes are set to 5. In this case, the router determines the optimal route according to the internal preferences. The internal preference (the value is 10) of OSPF is higher than that (the value is 60) of the static route. Therefore, the router selects the route discovered by OSPF as the optimal route.

Other related questions:
S series switches routing protocol priority problem
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How does the S series switch receive the routing protocol message?
When the S-series and E-series switches receive the protocol packets, the main control board (CPU) and the main control board (CPU) complete the related calculation and send the control message to update the routing information of the service board

Why do some routing policies not take effect for a routing protocol on S series switches
When a routing policy contains the routing attribute modification action, the routing policy does not take effect because the routes advertised by the routing protocol cannot carry the attribute. For example, a routing policy is configured to modify the next-hop attribute of OSPF external routes, the routing policy does not take effect because DD packets used to advertise routes do not carry the next-hop attribute. Some routing policies only take effect for certain protocols. For example, the policy where community attributes are matched can only be applied to BGP. In addition, the BGP protocol runs based on point-to-point neighbor relationships; therefore, some policies can be used only on a peer device.

How long is the default GR period of the OSPF routing protocol used by S series switches
For S series switches supporting OSPF, the default graceful restart (GR) period of the OSPF routing protocol is 120 seconds. If an OSPF neighbor relationship does not enter the Full state within 120 seconds, the GR fails. For example, when there are a large number of OSPF link-state advertisements (LSAs), LSA synchronization may not be complete within the default GR period, causing the GR to fail. In this case, run the graceful-restart period period command in the OSPF view to change the GR period on the Restarter.

Routes imported into IS-IS are invalid on S series switches
Q: Why are routes imported into IS-IS invalid? A: For S series switches supporting IS-IS, if the IS-IS level is not set when routes are imported into IS-IS, level-2 IS-IS is used by default. That is, routes are imported into level-2 IS-IS. To import routes to areas of other levels, specify the area level.

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