Routing Table

Routing Table

 

Quiz: https://networkdirection.net/labsandquizzes/quizzes/juniper-jncia/routing-tables

Lab: https://networkdirection.net/labsandquizzes/labs/jncia-labs/routing-table

 

Notes

Junos uses various different routing table for different purposes:

  • inet.0 – IPv4
  • inet6.0 – IPv6
  • inet.1 – Multicast forwarding cache
  • inet.2 – Multicast RPF
  • inet.3 – IPv4 MPLS

 

Routes in the routing table that are marked as active are the ones that are installed into the forwarding table.

If there are two or more equal-cost routes, Junos will select one to install in the forwarding table. There is no ECMP by default, but this can be changed with a policy.

The forwarding table contains both layer-3 information and layer-2 information (MAC addresses).

 

Static routes can be configured with next-hop resolution. This means that the next-hop does not need to be directly connected to the device.

When this is used, the routing table is consulted to find the real next-hop that’s needed to reach this next-hop.

If we want to create a black-hole route, we can create a static route with the ‘discard’ or ‘reject’ next-hop. This is similar to using the ‘null0’ destination on a Cisco router.

 

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uRPF (Unicast Reverse Path Forwarding) checks if a packet is valid before it will accept it. It does this by comparing the source IP to the routing table.

In loose mode, the source IP must have a valid route in the routing table. If it doesn’t, it is dropped.

In strict mode, the route must exist, but in addition the packet must have been received on the correct next-hop interface (according to the routing table). If not, it’s dropped.

Loose mode is used when asymmetric routing is a concern, such as at the edge of the network. Strict mode is used in the core, where there are deterministic symmetric routes.

 

Command Summary

Command Mode Description
show route Operational Show the routing table
show route terse Operational Show a summary of the routing table
show route forwarding-table Operational Show the forwarding table
set routing-options static route DEST next-hop < ip | interface> Configuration Create a static route
show route hidden Operational Show hidden routes
deactivate routing-options static route Configuration Deactivate a static route
activate routing-options static route Configuration Activate a static route
set routing-options static … resolve Configuration Configure next-hop resolution

 

Additional References

Understanding Junos OS Routing Tables

https://www.juniper.net/documentation/en_US/junos/topics/concept/routing-tables-understanding.html

 

Understanding Unicast RPF (Routers)

https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/interfaces-configuring-unicast-rpf.html

 

 

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