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How To Use Path Lookup

Unicast Path Lookup

Unicast form

First Hop Algorithm And VRF

The First Hop Algorithm adds the possibility to start the path lookup simulation from a different device than the closest one.

If the Automatic option is selected, IP Fabric initiates the path lookup simulation from the closest possible device. VRF is also automatically selected by default but can be changed manually.

First hop algorithm

The User defined First Hop Algorithm can be useful when simulating a flow where the source network is not known by IP Fabric.

To simulate such a flow, it is necessary to specify the starting point by entering the name of the device and the interface which will be used to start the path lookup.

Packets will use the indicated source IP address.

User-Defined First Hop Algorithm

In the example below, the source network is not known by IP Fabric. To show the path between a client in this network and a server in a network known by IP Fabric:

User Defined First hop algorithm example

  1. Select the User defined First Hop Algorithm.
  2. Search for the device where you want to start the path: L43EXR1.
  3. Select the source interface: Et0/1.
  4. Enter the source IP from the network outside the scope of IP Fabric:
  5. Finally, enter the destination IP:

User Defined First hop algorithm

This is the result you will get:

User Defined First hop algorithm Result

Source/Destination IP Address and Port

A plain IP address or a CIDR (Classless Inter-Domain Routing) can be used as a source/destination IP address, for example, when simulating path lookup from a host to a network.

By default, the ICMP protocol and the Echo request option are chosen for path lookup.

Source and destination

When switched to Web HTTP/HTTPS, TCP destination ports 80 and 443 with the (web|http|https) application are set by default.

HTTP default

When extending details, the transport protocol and range of ports can be specified for a source and for a destination. When more destination ports are specified, IP Fabric will analyze all of them individually during the path lookup.

Source and destination ports

The port can be changed to an arbitrary one for TCP/UDP protocols.

The following flags can also be set for TCP traffic: None / ACK / FIN / SYN / RST / PSH / URG.

TTL and Fragment Offset

In More details, TTL (Time to live) and Fragment offset can be set – thus affecting the path lookup output. The default TTL is 128 and Fragment offset is set to 0.

TTL and Fragmentation


When evaluating security rules and security appliances on the path check traffic on L7, an application can be checked on the path lookup.

It’s almost impossible to standardize application names across all vendors. You can define your own application name with regular expressions.


The application name input is simply a string, so it must be defined exactly as it appears in a security rule!


Source/Destination IP Regions

When testing access to or from the internet, source or destination IP regions can be set.

Example: Europe, China, etc.

By default, IP regions are not evaluated.


IP regions are represented as strings, so they must be defined exactly as they are appear in a security rule!


Path Lookup Mode

If you’ve used a network CIDR instead of a single IP address, you will have the option between:

  • Network Mode – Simulation starts and ends with whole networks; individual hosts are not considered.
  • Host Mode – Simulation starts and ends with each host. It is limited to 255 hosts, source and destination combined.

Then click Submit.

This is how the path lookup might look:

Path lookup example

Security Rules

Path lookup drop

If Drop is selected, the path lookup will stop when a security rule denies traffic.

Path lookup continue

If Continue is selected, the path lookup continues and does not apply the policy’s deny; in the detail pane, it is labeled as (not applied).

Multicast Tree Lookup

If you want to understand how a certain multicast flow is used, you can use the Multicast Tree Lookup tab. Just select the correct option and enter the relevant details.

Multicast form

You will then see the Multicast Tree:

Multicast example

And you will have access to a lot of information regarding the Multicast forwarding decision:

Multicast path inspector

Host To Gateway

To find out more details between a host and its network gateway, you can use the Host To Gateway tab. You only need to provide the host, and you will the details:

Host To Gateway form

Inspecting and Adjusting Path Lookup

Path Controls

With the right mouse click, more options are enabled:

Path controls

After opening the details with Explore, you can select the destination link to proceed with packet analysis:

Path detail

Understand the Path Selection

To understand the decision taken by a device, right-click the device and click Explore. You will then be presented with the details. If you have more than one interface where the flow can come from, you will need to select the interface you want to look at. Similarly, if you have several interfaces that can be used to forward the traffic, you will have to choose one. Then, in the middle of the table, you will see the forwarding decision:

Forwarding decision

In this example, we are looking at the device L21C11, which has 2 incoming interfaces and one forwarding for this flow:

Forwarding decision animation

Visualization Setup

You can set up what you want to prioritize in the view. Simply move the bars up or down.

Visualization setup