How does an S series switch select the RPF route

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If the longest match policy is not configured for route selection, a multicast router selects an optimal route as the RPF route based on route precedences.
If the longest match policy is configured for route selection, a multicast router selects a route with the longest matched mask in RPF selection. If the masks of routes have the same length, the router selects an optimal route based on route precedences.

A multicast router selects an optimal route respectively from the static multicast routing table, MBGP routing table, MIGP routing table, and unicast routing table, and then selects one route from the four routes as the RPF route. Route selection rules are as follows:
- If the longest match policy is configured for route selection:
1. A route with the longest matched mask is selected from the four routes.
2. If the masks of four routes have the same length, the route with the highest preference is selected.
3. If the four routes have the same preferences, a route is selected in the order of static multicast route, MBGP route, MIGP route, and unicast route.

- If the longest match policy is not configured for route selection:
1. The route with the highest preference is selected.
2. If the four routes have the same preferences, a route is selected in the order of static multicast route, MBGP route, MIGP route, and unicast route.

Other related questions:
Why does the switch not select a direct route as the optimal route
By default, the switch selects routes in the following order: Longest mask > route preference > route cost When a direct route is not selected as the optimal route, check whether the switch has imported an external route with a longer mask than the direct route. If such a route exists, check whether the switch imports the more exact external route. This may be caused by incorrect routing configuration.

How S series switches select an NTP server
S series switches (except S1700 switches) select an NTP server as follows: Question: After an NTP server is configured for S1, S2 configures S1 and the NTP server as active/standby NTP servers and sets the priority of S1 to be higher than that of the NTP server. Which NTP server takes effect for S2? Answer: S1, as an NTP client, belongs to an NTP server. When both S1 and the NTP server are configured as the NTP servers for S2, the priority of S1 is lower than the priority of the NTP server regardless of the priority settings on S2. Therefore, the NTP server of S1 takes effect for S2. The NTP server priority settings only take effect for the NTP servers irrelevant to each other.

How does OSPF on S series switches create ECMP routes
Question: Two routes learned from two areas with the same cost are not ECMP routes. Why? Answer: According to RFC 2328, OSPF can generate ECMP routes only when the following conditions are met: The routes are learned from the same area. The routes have the same priority and cost. RFC 2328 does not define selection rules of intra-area and inter-area routes for switches. When conditions of multiple routes are the same, a switch may not select the last advertised route. If routes are Type 5 external routes, the route with the largest area ID is preferentially selected, as defined in RFC 2328.

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 OSPF on an S series switch does not preferentially select a route with a small cost value and through the backbone area
OSPF defined by RFC 2328 must be compatible with RFC 1583. As defined in RFC 2328, if "RFC1583 Compatibility" is disabled, the routes learned through common areas are preferred regardless of their costs when routes can be learned through common areas and the backbone area. This helps reduce the burden of the backbone area.

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