Forwarding policy is a tool that allows you to control and affect the path traffic takes through the network. CoS can be integrated with policy by allowing you to select forwarding paths based on a packet’s forwarding class.
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CoS-Based Forwarding
CBF provides the ability to select a packet’s routing path based on its forwarding class. For example, you might want to specify a particular path to carry high priority traffic, while best-effort traffic takes a less preferred path. CBF is available for IPv4, IPv6 and MPLS packets.
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CoS-Based Forwarding
The slide lists the steps to configure CBF. These steps are expanded on the following slides.
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CoS-Based Forwarding
The first configuration s tep involves identifying the traffic that should use CBF. Using routing policy, define the desired routes within route filters. The resulting action for the policy must include the cos-next-hop-map statement along with the name of the next-hop map.
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CoS-Based Forwarding
The next configuration step involves specifying the next hops for each forwarding class. Create a next-hop map that specifies one or more next-hops for each forwarding class. You can optionally specify more than one next-hop for a forwarding class, creating load balancing across the multiple paths. You can also mix IP next-hops with MPLS next-hops under a single forwarding class.
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CoS-Based Forwarding
Much like firewall filters, routing policy on its own does nothing, until you apply it. The final configuration step involves applying the routing policy to routes exported to the forwarding table.
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CoS-Based Forwarding
When you configure CBF within a network that uses OSPF as the interior gateway protocol (IGP), you must specify the next-hop as an interface name (or next-hop alias), not as an IP address. The reason for this is OSPF adds routes with the interface as the next hop for point-to-point interfaces; the next hop does not contain the IP address.
When a forwarding class is identified within a next-hop map and configured with both LSP next hops and IP next hops, the LSP next hops are preferred.
If a next-hop map does not specify all possible forwarding classes, the Junos OS assigns traffic for the unspecified forwarding classes to a default forwarding class. The default forwarding class is the class associated with queue 0. If the default forwarding class is not specified in the next-hop map, Junos randomly designates the default class.
