[Day #44 Pyats Series] Cross-check BGP route attributes (AS path, next hop) using pyATS for Cisco [Python for Network Engineer]

• Trainer Sagar Dhawan
• Posted on September 6, 2025
• No Comments

[Day #44 Pyats Series] Cross-check BGP route attributes (AS path, next hop) using pyATS for Cisco [Python for Network Engineer]

Introduction on the Key Points

Welcome back to Day 44 of our 101 Days of pyATS Series, where we dive deep into vendor-agnostic network automation and validation techniques. Today, we’re focusing on one of the most critical aspects of inter-domain routing: Cross-checking BGP route attributes like AS Path and Next Hop.

Understanding and validating BGP attributes is essential when working with large-scale, multi-vendor enterprise or service provider networks. Whether you’re a beginner or an experienced professional looking to scale operations, using pyATS and Genie simplifies the validation of dynamic routing protocols like BGP.

If you’re learning Python for Network Engineer, this article is a great use case for applying your skills in real-world BGP route validation—without manual CLI fatigue.

We’ll walk you through:

• Why validating BGP attributes is important
• A Cisco-based topology
• Automated script using pyATS
• Output validation with CLI and parsed data
• How you can start applying this to large-scale infra
• A complete sales-funnel style call-to-action to join our live training led by Trainer Sagar Dhawan

Let’s get started

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Topology Overview

In today’s lab, we are using a simple Cisco 3-router topology running iBGP and eBGP:

• R1 ↔ R2: eBGP Peering
• R2 ↔ R3 ↔ R4: iBGP Peering
• Prefixes are advertised and re-advertised across domains

We’ll validate BGP attributes like:

• AS-Path: To check proper AS traversal
• Next-Hop: To ensure reachability and path correctness

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Topology & Communications

• R1 is originating prefixes 1.1.1.0/24 into eBGP
• R2 receives the route with AS-Path [65001] from R1
• R2 redistributes into iBGP with R3 and R4
• We want to validate consistency of AS-Path and Next-Hop across all routers

This is extremely useful in large environments where misconfigurations can break redundancy or loop-free routing.

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Workflow Script

Here’s your pyATS job script using Genie parser:

# File: bgp_attribute_validation.py

from genie.testbed import load
import re

testbed = load('testbed.yml')

devices = ['R1', 'R2', 'R3', 'R4']
prefix_to_check = '1.1.1.0/24'

for device_name in devices:
    device = testbed.devices[device_name]
    device.connect(log_stdout=False)
    
    output = device.parse('show ip bgp')
    
    if prefix_to_check in output['vrf']['default']['address_family']['ipv4 unicast']['routes']:
        route_data = output['vrf']['default']['address_family']['ipv4 unicast']['routes'][prefix_to_check]
        as_path = route_data['index'][1].get('as_path', 'Not Found')
        next_hop = route_data['index'][1].get('next_hop', 'Not Found')

        print(f"{device_name}: Prefix {prefix_to_check}")
        print(f"  AS-Path  : {as_path}")
        print(f"  Next-Hop : {next_hop}")
    else:
        print(f"{device_name}: Prefix {prefix_to_check} not found in BGP table.")

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Explanation by Line

Let’s break down what’s happening:

• load('testbed.yml') – Loads device connectivity parameters
• Loop over all devices – Connect to each router
• device.parse('show ip bgp') – Genie automatically parses structured output
• Extract AS-Path & Next-Hop – From specific index (best path)
• Print for human-readable comparison

This script will help you quickly pinpoint anomalies across routers.

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testbed.yml Example

testbed:
  name: BGP-Testbed
  credentials:
    default:
      username: cisco
      password: cisco

devices:
  R1:
    os: iosxe
    type: router
    connections:
      cli:
        protocol: ssh
        ip: 192.168.100.1

  R2:
    os: iosxe
    type: router
    connections:
      cli:
        protocol: ssh
        ip: 192.168.100.2

  R3:
    os: iosxe
    type: router
    connections:
      cli:
        protocol: ssh
        ip: 192.168.100.3

  R4:
    os: iosxe
    type: router
    connections:
      cli:
        protocol: ssh
        ip: 192.168.100.4

Make sure SSH access is configured properly on your routers.

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Post-validation CLI Screenshots (Real expected output)

Here’s what you should see when cross-verifying manually:

On R2:

R2# show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  65001
    10.0.0.1 from 10.0.0.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best

On R3:

R3# show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24
Paths: (1 available, best #1, table default)
  65001
    10.0.0.2 from 10.0.0.2 (2.2.2.2)
      Origin IGP, localpref 100, valid, internal, best

This CLI helps you correlate Genie output with native command results.

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FAQs

1. What are the key BGP route attributes we need to validate using pyATS?

When working with BGP, the two most critical attributes that determine path selection and forwarding are:

• AS Path: Helps in loop prevention and path selection (shortest AS path wins).
• Next Hop: Indicates the next router to reach the destination prefix.

With pyATS, we parse the routing table or BGP table and validate:

• Whether the AS_PATH follows expected patterns (e.g., no unauthorized AS in the path).
• Whether the NEXT_HOP is reachable and belongs to the expected peer.

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2. Why is verifying the AS Path using pyATS automation important in large-scale networks?

In enterprise or ISP-grade BGP deployments, AS path manipulation is common for traffic engineering. Verifying AS path ensures:

• You’re not receiving routes via unauthorized AS paths (security filtering).
• Traffic engineering policies (like AS path prepending) are correctly implemented.
• pyATS helps automate these verifications across hundreds of routers, removing human error from the equation.

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3. How do I extract AS path and next hop from a Cisco BGP router using pyATS?

You can use the Genie parser for the command:

show ip bgp

The parsed output gives structured access like:

parsed_output['vrf']['default']['address_family']['ipv4 unicast']['routes']['<prefix>']['index'][<index>]['next_hop']
parsed_output['vrf']['default']['address_family']['ipv4 unicast']['routes']['<prefix>']['index'][<index>]['as_path']

This makes it easy to write logic in pyATS scripts to:

• Loop through routes
• Match specific AS paths
• Verify the expected next-hop IP

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4. Can pyATS help compare AS paths across different routers for consistency?

Yes, absolutely!
You can define multiple testbed devices in a single pyATS test and collect BGP route data from all of them. Then:

• Compare AS paths for a given prefix across routers.
• Detect inconsistent propagation or routing anomalies (e.g., route leaking, misconfigurations).

Use this method to validate edge router consistency, especially in iBGP scenarios or when dealing with multiple upstream providers.

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5. How do I write a pyATS test to ensure the BGP next hop is within a valid peer subnet?

Use logic like:

if next_hop not in expected_peer_subnet:
    self.failed("Next hop is outside the expected range.")

You can even enhance it by checking next-hop reachability using:

ping <next_hop>

via pyats.aetest or unicon libraries to validate connectivity. This ensures the next-hop is not blackholing traffic due to IGP/BGP inconsistencies.

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6. What are common errors that can be caught with pyATS BGP attribute checks?

Some real-world issues you can proactively detect:

• Incorrect AS prepending (extra/prepended ASes not matching policy).
• Hijacked routes (unexpected AS in the path).
• Invalid or unreachable next hops.
• Missing routes (not received from certain peers).
• AS path loops or excessive path lengths.

By building these checks into your CI/CD pipelines, you prevent outages before they happen.

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7. Is it possible to validate both IPv4 and IPv6 BGP attributes using pyATS?

Yes. The Genie parsers support:

• show ip bgp → IPv4 BGP routes
• show bgp ipv6 unicast → IPv6 BGP routes

You can extract:

['address_family']['ipv6 unicast']['routes'][…]

The structure remains similar, so you can build dual-stack validation workflows where both AS_PATH and NEXT_HOP are validated across address families, ensuring IPv6 parity with IPv4 routing.

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YouTube Link

Watch the Complete Python for Network Engineer: Cross-check BGP route attributes (AS path, next hop) using pyATS for Cisco [Python for Network Engineer] Lab Demo & Explanation on our channel:

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