Indirect Next Hop

Created Jul 26, 2018 23:55:00 191 0 0 0
Indirect next hop is a technique that speeds up route convergence. It can change the direct association between route prefixes and next hops into an indirect association. Then next-hop information can be refreshed independently, and the prefixes of the same next hop do not need to be refreshed one by one. This speeds up route convergence.

Purpose
In the scenario requiring route iteration, when IGP routes or tunnels are switched, FIB entries are quickly refreshed. This implements fast traffic convergence and reduces the impact on services.

Mapping Between the Route Prefix and Next Hop
The mapping between the route prefix and next hop is the basis of indirect next hop. To meet the requirements of route iteration and tunnel iteration in different scenarios, next-hop information involves the address family, original next-hop address, and tunnel policy. The system assigns an index to information about each next hop, performs route iteration, notifies the iteration result to the routing protocols, and distributes FIB entries.

On-Demand Route Iteration
In on-demand route iteration, when a dependent route is changed, only the next hop related to the dependent route is re-iterated. If the destination address of a route is the original next-hop address or network segment address of next-hop information, route changes affect the iteration result of next-hop information. Otherwise, route changes do not affect next hop-information. Therefore, when a route changes, you can re-iterate only the related next hop according to the destination address of the route.

In tunnel iteration, when a tunnel alternates between Up and Down, you just need to re-iterate the next-hop information whose original next-hop address is the same as the destination address of the tunnel.

Iteration Policy
An iteration policy controls the next-hop iteration result to meet the requirements of different application scenarios. In route iteration, iteration behaviors do not need to be controlled by the iteration policy. Instead, iteration behaviors only need to comply with the longest matching rule. The iteration policy needs to be used only when VPN routes are iterated to tunnels.

By default, the system selects LSPs for a VPN without performing load balancing. If load balancing or other types of tunnels are required, you need to configure a tunnel policy and bind the tunnel policy to a tunnel. After a tunnel policy is applied, the system uses the tunnel bound in the tunnel policy or selects a tunnel according to the priorities of different types of tunnels.

Refreshment of Indirect Next Hop
On the forwarding plane, public network routes are forwarded based on the next hop and outbound interface while VPN routes are forwarded based on the public network tunnel in addition to the next hop and outbound interface. Before indirect next hop is used, forwarding information, including the next hop, outbound interface, and tunnel token, needs to be added into the FIB entry using the route prefix. In this case, the route convergence speed is relevant to the number of route prefixes. After indirect next hop is used, many route prefixes correspond to a shared next hop. Forwarding information is added into the FIB entry using the next hop, and the traffic with the relevant route prefixes can be switched simultaneously. Therefore, the route convergence speed becomes faster.
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