Advertising selected fabric paths for service routes in virtual nodes

1. A method comprising:

selecting, by a first virtual routing node of a single-chassis network device having a plurality of forwarding components and a plurality of fabric links coupling respective pairs of the plurality of forwarding components at respective fabric interfaces of the plurality of forwarding components, a fabric interface of a forwarding component of the plurality of forwarding components that has an egress interface toward a network destination and that is associated with the first virtual routing node;

advertising, by the first virtual routing node to a second virtual routing node of the single-chassis network device, the fabric interface as a next hop for the network destination;

in response to receiving the fabric interface, storing, by the second virtual routing node to a context data structure of the second virtual node, the fabric interface as a next hop for the network destination;

selecting, by the second virtual routing node and based on the context data structure, the fabric interface from among a plurality of fabric interfaces as a next hop for the network destination; and

forwarding, by the second virtual routing node, network traffic destined for the network destination to the selected fabric interface.

2. The method of claim 1 , wherein the network destination comprises a network address prefix advertised by a customer edge network device, wherein advertising the fabric interface further comprises advertising the fabric interface as service-layer reachability information in association with a local address of the first virtual routing node.

3. The method of claim 2 , wherein selecting comprises selecting the fabric interface from among a plurality of fabric interfaces associated with the customer edge network device.

4. The method of claim 2 , wherein the customer edge network device allows access to a Virtual Private Network (VPN) service, and wherein advertising comprises advertising a VPN label as network layer reachability information for the fabric interface.

5. The method of claim 1 , wherein advertising comprises advertising the fabric interface over a single-hop Border Gateway Protocol (BGP) session established between the first virtual routing node and the second virtual routing node of the single-chassis network device.

6. The method of claim 5 , wherein advertising comprises advertising the fabric interface in a Border Gateway Protocol (BGP) update message over the single-hop BGP session.

7. The method of claim 1 , further comprising:

receiving, by the first virtual routing node, a packet of the network traffic at the selected fabric interface; and

without sending the packet across the plurality of fabric links, outputting, by the first virtual routing node, the packet from the single-chassis network device at the egress interface of the forwarding component of the plurality of forwarding components.

8. The method of claim 1 , wherein the plurality of forwarding components comprises a first subset of the plurality of forwarding components used by the first virtual routing node and a second subset of the plurality of forwarding components used by the second virtual routing node, the second subset distinct from the first subset.

9. The method of claim 1 , further comprising:

converting, by the first virtual routing node, a forwarding next hop (FNH)-level load balance distribution into packet-forwarding engine (PFE)-level load balance distribution,

wherein selecting the fabric interface comprises selecting a plurality of fabric interfaces based on the PFE-level load balance distribution, and

wherein advertising comprises advertising network addresses of each of a plurality of fabric interfaces as respective next hops for the network destination based on the PFE-level load balance distribution, and information specifying a load balance distribution across the plurality of fabric interfaces; and

storing, by the second virtual routing node, the information specifying the load balance distribution to the context data structure.

10. A single-chassis network device comprising:

a plurality of forwarding components;

a plurality of fabric links coupling respective pairs of the plurality of forwarding components at respective fabric interfaces of the plurality of forwarding components;

a first virtual routing node configured to forward packets using a first subset of the plurality of forwarding components; and

a second virtual routing node configured to forward packets using a second subset of the plurality of forwarding components, the second subset distinct from the first subset,

wherein the first virtual routing node is configured to:

select a fabric interface of a forwarding component of the plurality of forwarding components that has an egress interface toward a network destination and that is associated with the first virtual routing node;

advertise to the second virtual routing node, the fabric interface as a next hop for the network destination, and

wherein the second virtual routing node is configured to, in response to receiving the fabric interface, store, to a context data structure of the second virtual node, the fabric interface as a next hop for the network destination, select, based on the context data structure, the fabric interface from among a plurality of fabric interfaces as a next hop for the network destination, and forward network traffic destined for the network destination to the selected fabric interface.

11. The single-chassis network device of claim 10 , wherein the first virtual routing node is configured to, in response to receiving network traffic from the second virtual routing node on the fabric interface of the forwarding component, output the network traffic from the egress interface of the forwarding component onto a link external to the single-chassis network device.

12. The single-chassis network device of claim 10 , wherein the first virtual routing node is configured to advertise the fabric interface over a single-hop Border Gateway Protocol (BGP) session established between the first virtual routing node and the second virtual routing node of the single-chassis network device, wherein the fabric interface is advertised in a Border Gateway Protocol (BGP) update message sent over the single-hop BGP session.

13. The single-chassis network device of claim 10 , wherein the first virtual routing node is configured to convert a forwarding next hop (FNH)-level load balance distribution into packet-forwarding engine (PFE)-level load balance distribution, select a plurality of fabric interfaces based on the PFE-level load balance distribution, advertise network addresses of each of a plurality of fabric interfaces as respective next hops for the network destination based on the PFE-level load balance distribution, and information specifying a load balance distribution across the plurality of fabric interfaces; and

and wherein the second virtual routing node is configured to store the information specifying the load balance distribution to the context data structure.

14. The single-chassis network device of claim 10 , wherein the network destination comprises a network address prefix advertised by a customer edge network device, wherein the first virtual routing node is configured to advertise the fabric interface as service-layer reachability information in association with a local address of the first virtual routing node.

15. The single-chassis network device of claim 14 , wherein the customer edge network device allows access to a Virtual Private Network (VPN) service, and wherein the first virtual routing node is configured to advertise a VPN label as network layer reachability information for the fabric interface.

16. The single-chassis network device of claim 14 , wherein the first virtual routing node is configured to select the fabric interface from among a plurality of fabric interfaces associated with the customer edge network device.

17. The single-chassis network device of claim 10 , wherein the first virtual routing node is configured to, in response to receiving a packet of the network traffic at the selected fabric interface, output the packet from the single-chassis network device at the egress interface of the forwarding component of the plurality of forwarding components without sending the packet across the plurality of fabric links.

18. A computer-readable storage medium comprising instructions for causing one or more programmable processors of a single-chassis network device to:

select, by a first virtual routing node of the single-chassis network device having a plurality of forwarding components and a plurality of fabric links coupling respective pairs of the plurality of forwarding components at respective fabric interfaces of the plurality of forwarding components, a fabric interface of a forwarding component of the plurality of forwarding components that has an egress interface toward a network destination and that is associated with the first virtual routing node;

advertise, by the first virtual routing node to a second virtual routing node of the single-chassis network device, the fabric interface as a next hop for the network destination;

in response to receiving the fabric interface, store, to a context data structure of the second virtual node, the fabric interface as a next hop for the network destination;

select, by the second virtual routing node and based on the context data structure, the fabric interface from among a plurality of fabric interfaces as a next hop for the network destination; and

forward, by the second virtual routing node, network traffic destined for the network destination to the selected fabric interface.