Question 48 - JN0-636 discussion

The exhibit shows the configuration of filter-based forwarding on an SRX Series device. Filter-based forwarding is a feature that allows the device to use firewall filters to direct traffic to different routing instances based on the match criteria. In this scenario, the device has two routing instances - ISP-1 and ISP-2 - and two firewall filters - FBF and FBF-ISP-1. The FBF filter is applied to the ge-0/0/1 interface as an input filter. The FBF filter has one term that matches the traffic from the 172.25.0.0/24 network and directs it to the ISP-1 routing instance. The ISP-1 routing instance has a static route to the next hop 172.20.0.2. The FBF-ISP-1 filter is applied to the ge-0/0/0 interface as an output filter. The FBF-ISP-1 filter has one term that matches the traffic to the 172.20.0.2 next hop and sets the forwarding class to expedited-forwarding.

The problem in this scenario is that the traffic from the other network (172.25.1.0/24) is not being forwarded to the upstream 172.21.0.2 neighbor. This is because the FBF filter does not have a term that accepts the traffic from the 172.25.1.0/24 network. The FBF filter only has one term that matches the traffic from the 172.25.0.0/24 network and directs it to the ISP-1 routing instance. The traffic from the 172.25.1.0/24 network does not match this term and is therefore discarded by the implicit deny action at the end of the filter. The traffic from the 172.25.1.0/24 network should be forwarded to the ISP-2 routing instance, which has a static default route to the next hop 172.21.0.2.

To solve this problem, you must add another term to the FBF filter to accept the traffic from the 172.25.1.0/24 network. This term should have the action accept, which means that the traffic will be forwarded according to the routing table of the master routing instance. The master routing instance has a static default route to the ISP-2 routing instance, which in turn has a static default route to the next hop 172.21.0.2. By adding this term, the traffic from the 172.25.1.0/24 network will be forwarded to the upstream 172.21.0.2 neighbor as expected.

The configuration of the new term in the FBF filter could look something like this:

[edit firewall family inet filter FBF] term 2 { from { source-address { 172.25.1.0/24; } } then { accept; } }

Reference: Juniper Security, Professional (JNCIP-SEC) Reference Materials source and documents:

https://www.juniper.net/documentation/en_US/junos/topics/concept/firewall-filter-option-filterbased-forwarding-overview.html

https://www.juniper.net/documentation/en_US/junos/topics/example/filter-based-forwardingexample.html