Chained Composite Next Hops for VPNs and Layer 2 Circuits

The Juniper Networks PTX Series Packet Transport Routers, MX Series 5G Universal Routing Platforms with MIC and MPC interfaces, and T4000 Core Routers are principally designed to handle large volumes of traffic in the core of large networks. Chained CNHs help to facilitate this capability by allowing the router to process much larger volumes of routes. A chained CNH allows the router to direct sets of routes sharing the same destination to a common forwarding next hop, rather than having each route also include the destination. In the event that a network destination is changed, rather than having to update all of the routes sharing that destination with the new information, only the shared forwarding next hop is updated with the new information. The chained CNHs continue to point to this forwarding next hop, which now contains the new destination.

When the next hops for MPLS LSPs are created on the routers, the tag information corresponding to the innermost MPLS label is extracted into a chained CNH. The chained CNH is stored in the ingress Packet Forwarding Engine. The chained CNH points to a next hop called the forwarding next hop that resides on the egress Packet Forwarding Engine. The forwarding next hop contains all the other information (all of the labels except for the inner-most labels as well as the IFA/IP information corresponding to the actual next-hop node). Many chained composite next hops can share the same forwarding next hop. Additionally, separating the inner-most label (that is the VPN label) from the forwarding next hop and storing it on the ingress PFE (within the chained composite next hop) helps to conserve egress Packet Forwarding Engine memory by reducing the number of rewrite strings stored on the egress Packet Forwarding Engine.

Table 1 shows support for chained CNHs for ingress or transit routers on the MPLS network.

Table 1: Support for Chained Composite Next Hops
Platform	L2 VPN	L3 VPN	L2 CKT
PTX Series	Ingress and transit	Ingress and transit	Ingress only
MX Series	Ingress only	Ingress only	Ingress only

To enable chained CNHs on a T4000 router, the chassis must be configured to use the enhanced-mode option in network services mode.

Benefits of chained composite next hops

Chained CNH optimizes the memory and performance of the router by reducing the size of the forwarding table. The router can use the same next-hop entry in the forwarding table for routes with different destinations when the next-hop is the same. This reduces the number of entries in the forwarding table and reduces the number of changes when the next hop entry has to be modified.