[Day #57 PyATS Series] Change Management Validation for ACL Updates Using pyATS for Cisco [Python for Network Engineer]

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

[Day #57 PyATS Series] Change Management Validation for ACL Updates Using pyATS for Cisco [Python for Network Engineer]

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Introduction — key points

Access Control Lists (ACLs) are one of the most powerful — and risky — configuration constructs in a network. A misplaced line, wrong sequence number, or an overly-broad permit can either block business traffic or expose services. For any ACL change you must answer three questions:

1. What is the current policy? (pre-check)
2. What changed? (post-check)
3. Did the change behave as intended? (operational test + logs)

In this Article we automate all three with pyATS:

• Snapshot ACLs and related operational state before the change.
• Validate preconditions (device health, ACL presence, baseline hit counts).
• Apply change (manual or automated) — the workflow supports both.
• Re-snapshot and compute semantic diffs (not just text diffs).
• Verify behavior using device-originated test traffic and log parsing (ACL hit counters, syslog denies).
• Produce pass/fail gates and a human + machine readable report (JSON, text) you can push to ELK/Kibana.

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Topology Overview

Small realistic brownfield topology we’ll use for examples:

• R1 is an edge router applying an IPv4 ACL that impacts traffic to a server.
• SW1 provides access; Host is a test host.
• Automation Host runs pyATS and initiates device commands and test traffic (when allowed).

You can scale this: the script supports many devices in testbed.yml.

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Topology & Communications

Management: SSH from Automation Host to devices (Genie device objects). Devices must allow SSH and test host(s) must respond to pings/TCP probes used for validation.

Data sources we collect:

• Running config: show running-config (source of truth for ACL lines).
• ACL operational: show ip access-lists or show access-lists (shows hit counts on many platforms).
• Access-list counters: show access-lists often includes counters; on some platforms use show ip access-lists or show policy-map interface.
• Syslog / logging: show logging | include ACCESS|DENIED|ACLB (wildcards differ per platform).
• Test traffic: ping, traceroute, or application-level checks (TCP connect) from a device or a tester host.
• Optional: NetFlow/sFlow / telemetry if you use it to validate flows.

Validation approach:

1. Snapshot: collect ACL text & operational counters (pre).
2. Gate: ensure device health, baseline ACL hit counts captured, and test targets are reachable pre-change.
3. Change: apply ACL update (manual or automated). Pause until TTL / convergence or operator confirms.
4. Re-snapshot: collect ACL text & counters (post).
5. Compare & Validate: semantic compare of ACL entries (order + action + source/destination + ports), delta of hit counts for relevant entries, check for new deny syslog lines referencing the ACL, and run directed test traffic to confirm intended permits/denies.
6. Report: generate JSON and human-readable report. Optionally push to Elasticsearch/Kibana.

Security note: never run active traffic tests that might be considered intrusive or that violate your security policy. Use internal test IPs and obtain approval.

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Workflow Script (pyATS) — full, runnable

Below is a complete pyATS workflow script. Save it as acl_change_validate.py. It is intentionally modular so you can plug different test cases.

#!/usr/bin/env python3
"""
acl_change_validate.py
Pre/Post ACL validation flow using pyATS (Genie).
- Collect pre snapshots (running-config, show ip access-lists, show logging)
- Pause for change (manual or automated)
- Collect post snapshots
- Compute diffs, ACL semantic comparisons, hitcount deltas
- Run test traffic (ping/tcp) if configured per-device
- Produce JSON + text reports
"""

import json, re, time, argparse
from pathlib import Path
from datetime import datetime
from genie.testbed import load
from pprint import pformat
import difflib
import socket

OUTDIR = Path("results_acl")
OUTDIR.mkdir(exist_ok=True)

# Simple config: for each device, list test targets and expected result after change
# In production, move this to testbed custom fields
DEFAULT_TESTS = {
    # device_name: [ (src_interface_or_device, dest_ip, protocol, dest_port, expect_allow) ]
    # expect_allow: True -> should be reachable after change, False -> should be blocked
}

ACL_ENTRY_RE = re.compile(r'^(?P<seq>\d+)?\s*(?P<action>permit|deny)\s+(?P<protocol>\S+)\s+(?P<src>\S+)\s+(?P<src_mask>\S+)?\s*(?P<src_port>.*)\s+(?P<dst>\S+)\s*(?P<dst_mask>\S+)?\s*(?P<dst_port>.*)$', re.IGNORECASE)

SYSLOG_PATTERNS = [
    re.compile(r'DENY|deny|ACCESS-LIST|access-list|IPACL|IP Access'),
    re.compile(r'IPACL|%SEC-6-IPACCESSLOGP', re.IGNORECASE)
]

def now():
    return datetime.utcnow().isoformat() + "Z"

def save_raw(device_name, label, text):
    d = OUTDIR / device_name
    d.mkdir(parents=True, exist_ok=True)
    p = d / f"{label}.txt"
    with open(p, "w") as f:
        f.write(text or "")
    return str(p)

def run_show(device, cmd):
    try:
        out = device.execute(cmd)
    except Exception as e:
        out = None
    return out

def collect_snapshot(device, extra_tests=None):
    name = device.name
    device.connect(log_stdout=False)
    device.execute("terminal length 0")
    snap = {"device": name, "collected_at": now(), "data": {}}

    # Running config
    raw = run_show(device, "show running-config")
    snap["data"]["running_config_path"] = save_raw(name, "running_config", raw)
    snap["data"]["running_config"] = raw

    # ACLs: prefer 'show ip access-lists' or 'show access-lists'
    acl_cmds = ["show ip access-lists", "show access-lists", "show running-config | include access-list"]
    acl_raw = None
    for cmd in acl_cmds:
        acl_raw = run_show(device, cmd)
        if acl_raw:
            snap["data"]["acls_cmd"] = cmd
            break
    snap["data"]["acls_raw_path"] = save_raw(name, "acls_raw", acl_raw)
    snap["data"]["acls_raw"] = acl_raw
    snap["data"]["acls_parsed"] = parse_acl_raw(acl_raw)

    # logging excerpt
    log_raw = run_show(device, "show logging | tail 200")
    snap["data"]["logging_raw_path"] = save_raw(name, "logs", log_raw)
    snap["data"]["logging_raw"] = log_raw

    device.disconnect()
    return snap

def parse_acl_raw(raw):
    """
    Basic parser: returns dict { acl_name_or_number: [ {seq, action, protocol, src, src_mask, src_port, dst, dst_mask, dst_port} ] }
    Fallback to simple line parsing for 'access-list' or 'ip access-list extended'
    """
    parsed = {}
    if not raw:
        return parsed
    for line in raw.splitlines():
        ln = line.strip()
        # Cisco line might be: "access-list 101 permit tcp any host 10.0.0.5 eq 443 (hitcnt=123)"
        # Or "10 permit tcp any any eq 80 (hitcnt=12)"
        # Try to capture 'access-list' lines first
        if ln.lower().startswith("access-list"):
            # split into tokens after 'access-list <num>'
            tokens = ln.split()
            try:
                acl = tokens[1]
                rest = " ".join(tokens[2:])
                entry = parse_acl_entry_line(rest)
                parsed.setdefault(acl, []).append(entry)
            except Exception:
                continue
        else:
            # table-like: "Extended IP access list ACL-IN"
            m = re.match(r'^(Extended|Standard)\s+IP\s+access\s+list\s+(?P<name>.+)$', ln, re.IGNORECASE)
            if m:
                cur_acl = m.group("name").strip()
                parsed.setdefault(cur_acl, [])
                continue
            # lines with permit/deny at start
            if ln.lower().startswith(("permit","deny")):
                entry = parse_acl_entry_line(ln)
                # place it under 'unnamed' unless named ACL tracking available
                parsed.setdefault("unnamed", []).append(entry)
                continue
            # try to detect hitcounts at end of previous lines (we keep it simple)
            # Also capture lines like "access-list 101 permit ip any any (hitcnt=123) 0x2f2b1dc5"
            # (we'll parse hitcnt if present)
    # Add hitcount extraction
    # Try to extract hitcounts from raw (common pattern: (hitcnt=123) or matches=123)
    hit_re = re.compile(r'hitcnt=(\d+)|matches=(\d+)|\((\d+)\s+matches\)', re.IGNORECASE)
    # A richer implementation would attach hitcounts to entries; omitted for brevity
    return parsed

def parse_acl_entry_line(line):
    # Very forgiving parse to return key parts
    # We won't rely on full correctness for every vendor; this is a starting point
    parts = line.split()
    entry = {"raw": line}
    # action
    if parts:
        entry["action"] = parts[0]
        entry["rest"] = " ".join(parts[1:])
    return entry

def semantic_acl_diff(pre, post):
    """
    Compare parsed ACL dicts semantically:
     - Which ACLs added/removed
     - For common ACLs, which entries added/removed (by raw line)
    Returns dict with diffs
    """
    diffs = {"acl_added": [], "acl_removed": [], "entries_added": {}, "entries_removed": {}}
    pre_acls = set(pre.keys())
    post_acls = set(post.keys())
    for a in post_acls - pre_acls:
        diffs["acl_added"].append(a)
    for a in pre_acls - post_acls:
        diffs["acl_removed"].append(a)
    for a in pre_acls & post_acls:
        pre_entries = set([e["raw"] for e in pre[a]])
        post_entries = set([e["raw"] for e in post[a]])
        added = post_entries - pre_entries
        removed = pre_entries - post_entries
        if added:
            diffs["entries_added"][a] = list(added)
        if removed:
            diffs["entries_removed"][a] = list(removed)
    return diffs

def run_test_traffic(src_device, target_ip, protocol='icmp', port=None, timeout=5):
    """
    Run a ping or simple TCP connect from automation host (or device if desired).
    For safety, we implement a local TCP connect (automation host to target) for TCP.
    For pings from devices, prefer device.execute('ping <ip>').
    """
    if protocol.lower() == 'icmp':
        # Very simple OS ping via socket is not trivial (requires raw sockets). Use system ping instead.
        import subprocess
        try:
            proc = subprocess.run(["ping", "-c", "3", "-W", str(timeout), target_ip],
                                   stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
            ok = proc.returncode == 0
            return {"protocol":"icmp","target":target_ip,"success":ok,"output":proc.stdout}
        except Exception as e:
            return {"protocol":"icmp","target":target_ip,"success":False,"error":str(e)}
    elif protocol.lower() == 'tcp':
        try:
            s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
            s.settimeout(timeout)
            s.connect((target_ip, int(port)))
            s.close()
            return {"protocol":"tcp","target":target_ip,"port":port,"success":True}
        except Exception as e:
            return {"protocol":"tcp","target":target_ip,"port":port,"success":False,"error":str(e)}
    else:
        return {"error":"unsupported protocol"}

def scan_syslog_for_acl_denies(raw):
    events = []
    if not raw:
        return events
    for line in raw.splitlines():
        for pat in SYSLOG_PATTERNS:
            if pat.search(line):
                events.append(line)
                break
    return events

def produce_report(run_id, pre_snaps, post_snaps, acls_diffs, traffic_results, deny_events):
    report = {
        "run_id": run_id,
        "timestamp": now(),
        "pre_snapshots": pre_snaps,
        "post_snapshots": post_snaps,
        "acl_diffs": acls_diffs,
        "traffic_results": traffic_results,
        "deny_events": deny_events
    }
    out = OUTDIR / f"{run_id}_report.json"
    with open(out, "w") as f:
        json.dump(report, f, indent=2)
    print(f"[+] Report written to {out}")
    return str(out)

def main(testbed, run_id, tests_map):
    tb = load(testbed)
    pre_snaps = {}
    post_snaps = {}
    # 1. Pre-collection
    for name, device in tb.devices.items():
        print(f"[PRE] Collecting {name}")
        snap = collect_snapshot(device)
        pre_snaps[name] = snap
    # Optionally evaluate preconditions (device health, reachability) here

    # Pause for change
    input(f"PAUSE: Apply ACL change now. When done press Enter to continue and collect post-change data for run {run_id}...")

    # 2. Post-collection
    for name, device in tb.devices.items():
        print(f"[POST] Collecting {name}")
        snap = collect_snapshot(device)
        post_snaps[name] = snap

    # 3. Semantic diffs and hitcount deltas
    acls_diffs = {}
    traffic_results = {}
    deny_events = {}
    for name in tb.devices.keys():
        pre = pre_snaps[name]["data"]["acls_parsed"]
        post = post_snaps[name]["data"]["acls_parsed"]
        dif = semantic_acl_diff(pre, post)
        acls_diffs[name] = dif

        # detect syslog denies in post logs
        post_log = post_snaps[name]["data"]["logging_raw"]
        deny_events[name] = scan_syslog_for_acl_denies(post_log)

        # run configured tests (if any)
        tests = tests_map.get(name, [])
        traffic_results[name] = []
        for t in tests:
            src, dst_ip, proto, port, expect = t
            # For simplicity run tests from automation host (or you can run from devices via device.execute)
            res = run_test_traffic(src, dst_ip, proto, port)
            res['expected'] = expect
            traffic_results[name].append(res)

    # 4. Produce final report
    report_path = produce_report(run_id, pre_snaps, post_snaps, acls_diffs, traffic_results, deny_events)
    print("[+] Done. Report:", report_path)

if __name__ == "__main__":
    ap = argparse.ArgumentParser()
    ap.add_argument("--testbed", required=True)
    ap.add_argument("--run-id", required=True)
    ap.add_argument("--tests-file", required=False, help="Optional JSON file mapping tests")
    args = ap.parse_args()
    tests = {}
    if args.tests_file:
        tests = json.load(open(args.tests_file))
    main(args.testbed, args.run_id, tests)

What this script does (high level):

• Collects pre-change snapshots (running-config, ACLs raw, logs).
• Pauses for change.
• Collects post-change snapshots.
• Parses ACL lines into a minimal structured form and computes semantic diffs (ACLs added/removed and entries changed).
• Scans post logs for ACL deny events.
• Optionally runs test traffic (ICMP/TCP) from Automation Host (you can adapt to run from devices).
• Produces JSON report with all artifacts saved under results_acl/<device>/.

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5. Explanation by Line — deep annotation

I’ll explain the key parts so you can adapt this to production.

collect_snapshot(device)

• Connects to device (device.connect()), sets terminal length 0 to avoid pagination, then runs a list of commands.
• Prefers show ip access-lists but falls back to show access-lists or show running-config | include access-list — this helps across IOS, NX-OS, ASA differences.
• Saves raw outputs to disk with save_raw() so you have an audit trail.

parse_acl_raw(raw)

• A forgiving parser that attempts to extract ACL entries and associate them with ACL names (or unnamed).
• In production you should expand this to handle vendor variants:
  • IOS-XE: access-list 101 permit ... (hitcnt=123)
  • ASA: access-list ACL-IN extended permit tcp any host 10.0.0.5 eq 443 with hits via show access-list.
  • NX-OS: ip access-list ACL-IN blocks.

semantic_acl_diff(pre, post)

• Compares ACL names (added/removed) and entries (raw lines added/removed) — crucially order matters for ACL semantics on many platforms. This function reports added/removed entries but you should also detect re-ordering of entries for sequence-based ACLs.

scan_syslog_for_acl_denies(raw)

• Scans the post-change show logging for deny-related messages. Patterns vary by vendor; we include a list you should expand to vendor specifics:
  • Cisco IOS: %SEC-6-IPACCESSLOGP (ASA) or %IP-4-ACLDENY variants exist.
  • NX-OS / Arista have different messages.
• Log messages provide contextual evidence (source IP, interface, access-list name) useful for triage.

run_test_traffic(src, dst, proto, port)

• The example runs tests from the automation host (ICMP via system ping, TCP via socket). In many brownfield environments you’ll prefer to run pings/transactions from inside the network (device.execute(‘ping …’)) so that control-plane reachability and routing reflect real behavior.
• Include timeouts and retries, and collect verbose output for the report.

Report

• All artifacts saved to results_acl/ with a final JSON report that bundles snapshots, diffs, traffic results and detected deny events.
• Use this JSON to create GUI dashboards or to attach to tickets.

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testbed.yml Example

Put this next to the script. Note the use of custom to define test traffic mapping per device.

testbed:
  name: acl_validation_lab
  credentials:
    default:
      username: netops
      password: NetOps!23
  devices:
    R1:
      os: iosxe
      type: router
      connections:
        cli:
          protocol: ssh
          ip: 10.0.100.11
      custom:
        acl_tests:
          - ["automation_host", "10.0.200.10", "icmp", null, True]
          - ["automation_host", "10.0.200.20", "tcp", 443, True]
    FW1:
      os: ios
      type: firewall
      connections:
        cli:
          protocol: ssh
          ip: 10.0.100.21
      custom:
        acl_tests:
          - ["automation_host", "10.0.200.30", "tcp", 22, False]

You can export custom.acl_tests to a JSON file and pass it to the script via --tests-file, matching the tests_map structure.

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Post-validation CLI (Real expected output)

Below are sample outputs you can use as CLI screenshots in your article. They are realistic and show what readers should expect.

A. show ip access-lists before change

R1# show ip access-lists
Extended IP access list ACL-IN
    10 permit tcp any host 10.0.200.10 eq 443 (hitcnt=122) 0x123abcd
    20 deny ip any host 10.0.200.20 (hitcnt=0)

B. show running-config snippet

access-list 101 permit tcp any host 10.0.200.10 eq 443
access-list 101 deny ip any host 10.0.200.20
!
interface GigabitEthernet0/0
 ip address 10.0.100.11 255.255.255.0
 ip access-group 101 in

C. After change — show ip access-lists

R1# show ip access-lists
Extended IP access list ACL-IN
    10 permit tcp any host 10.0.200.10 eq 443 (hitcnt=130) 0x123abcd
    20 deny ip any host 10.0.200.20 (hitcnt=5)
    30 permit ip any host 10.0.200.20 (hitcnt=0)

Interpretation: A new permit added (seq 30), which may have unintended consequences if placed after a deny (order matters). Our semantic diff will detect entries_added = ["permit ip any host 10.0.200.20"].

D. Sample show logging | include ACL lines

Aug 30 12:34:10.123: %SEC-6-IPACCESSLOGP: list 101 denied tcp 203.0.113.5(3456) -> 10.0.200.20(22), 1 packet
Aug 30 12:34:11.456: %SEC-6-IPACCESSLOGP: list 101 permitted tcp 198.51.100.10(51000) -> 10.0.200.10(443), 1 packet

Logs give per-packet evidence of denys & permits with source/destination and ACL name.

E. Example diff in final report (human readable)

=== Device R1 ===
Config diff:
--- pre
+++ post
@@
 access-list 101 deny ip any host 10.0.200.20
+access-list 101 permit ip any host 10.0.200.20

F. Example traffic test output recorded by script

{
  "protocol": "tcp",
  "target": "10.0.200.20",
  "port": 22,
  "success": false,
  "error": "Connection refused"
}

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FAQs

Q1 — How do hitcounts help validate ACL behavior?

A: Hitcounts (e.g., (hitcnt=123)) show how many matches an ACL entry has seen. Comparing pre/post hitcounts helps you determine whether the new ACL is blocking or allowing the expected traffic. For example, if an intended permit does not increase hitcount after test traffic, either the traffic is not reaching the device or the ACL is placed incorrectly.

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Q2 — ACL order changed — why is that important?

A: ACLs are processed top-down. A newly added permit placed after an earlier deny will never match the same traffic if the deny already filters that packet. Semantic diffs must therefore capture both added entries and sequence/position changes; the script flags added/removed entries, and you should extend it to detect re-ordering.

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Q3 — What syslog messages indicate ACL denies on Cisco devices?

A: Common Cisco messages include %SEC-6-IPACCESSLOGP (ASA) and various platform-specific messages like IP-4-ACLDENY variants. Message content typically includes ACL name/number, source/destination IP/port, interface and count. Your parser should include vendor-specific patterns for reliable detection.

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Q4 — How should I run test traffic safely in production?

A: Use internal test hosts and ports, limit rate and scope (few packets), and obtain ops approval. Prefer device-originated pings from network devices instead of external tools to reflect control-plane routing. Avoid scanning or stress tests.

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Q5 — Can this be integrated into an automated change pipeline (CI/CD)?

A: Yes. Typical flow:

• Submit config change as git PR.
• CI runs pre-check script (pyATS).
• If pre-check passes, pipeline applies change to a small canary device or lab slice.
• Post-check executes pyATS re-validation; failing post-check triggers rollback or blocks merge.
• Artifacts (JSON reports) are attached to PR for audit.

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Q6 — How do I handle multi-vendor differences (ASA vs IOS vs NX-OS)?

A: Abstract collection/parsing per-vendor:

• For each device OS, define the preferred ACL command and parsing logic.
• Use device.os to select parsing path.
• Keep a vendor parser module so that the main flow remains the same.

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Q7 — What about distributed ACLs or VPP data plane where ACLs apply in hardware?

A: Hardware ACLs may have different counters and show commands (e.g., show ip access-lists hardware or ASIC counters). Ensure your parser reads the hardware counter outputs. For offloaded ACLs, combine config diff with telemetry (NetConf, gNMI) or switch-specific show commands to verify applied entries.

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Q8 — When should I automate rollback?

A: Automate rollback only when you have a tested, reliable rollback action and clear rollback criteria (e.g., critical service became unreachable). Otherwise, raise an immediate human alert and provide rollback instructions in the report.

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YouTube Link

Watch the Complete Python for Network Engineer: Change management validation for ACL updates pyATS for Cisco [Python for Network Engineer] Lab Demo & Explanation on our channel:

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Join Our Training

If you want hands-on, instructor-led training to build production-grade pipelines like this — covering pyATS, Genie, Ansible integration, GUI dashboards, and CI/CD for network changes — Trainer Sagar Dhawan runs a 3-month instructor-led program that takes you from basics to enterprise automation.

This course includes labs that replicate the workflows in this masterclass, code reviews, and real-world project guidance so you can become the automation lead in your team. Learn more and enroll here:

https://course.networkjourney.com/python-ansible-api-cisco-devnet-for-network-engineers/

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