Function to calculate the MST Digest from the MST instances and VLANs protected.

mst_digest.py

#! /usr/bin/python

import re
import hmac
import base64

# Key specified by 802.1Q-2011, table 13-1
# Key is encoded in Base64
MAGIC_KEY = 'E6wGpi5H/VH5XSuiQ80DRg=='


def get_list_from_range_string(range_str):
    """Parse the String and return a list of integers.

    Use '-' to specify a range, or ',' to separate VLAN IDs in a list.

    Spaces and Negative numbers are not permitted. Zero is fine.

    Examples:

        '1' => [1]
        '1-5' => [1, 2, 3, 4, 5]
        '1-2,4-5' => [1, 2, 4, 5]
    """
    final_list = set()
    # Use split to separate groups
    for item in range_str.split(','):
        try:
            # Use RegEx to find ranges whitin the Groups
            match = re.match('(\d+)-(\d+)', item)
            if match:
                values = range(int(match.group(1)), int(match.group(2))+1)
                # Use the |= operator (union) to add the range to the set
                final_list |= set(values)
            else:
                final_list.add(int(item))
        except ValueError:
            raise RuntimeError('Could not parse the string correctly.')

    return list(final_list)


def calculate_mst_digest(mst_config):
    """Calculate the MST Configuration Digest of a Switch.

    VLANs must be specified as a dictionary in which the keys are MST
    instances and the value a vlan range string with every vlan the
    instance contains.

        ex:. mst_config = {
            1: '1,10-19,100',
            2: '2,20-29,200',
            3: '3,30-39,300',
        }

    Based on http://www.fragmentationneeded.net/2015/01/manually-calculating-mst-digests.html  # noqa
    """
    # Convert the Range Strings to a list of VLANs
    for mst_instance, range_string in mst_config.iteritems():
        mst_config[mst_instance] = get_list_from_range_string(range_string)

    # Create a list with 4096 positions
    instance_table = [0] * 4096

    # Each position in the instance-table represents a VLAN and the value in
    # that position is the MST instance protecting that VLAN
    for mst_instance, vlans in mst_config.iteritems():
        for vlan in vlans:
            instance_table[vlan] = int(mst_instance)

    # Convert MST instance numbers into one big bit string (each instance is 16
    # bits)
    bit_string = ''
    for instance in instance_table:
        # First most significant 8bits
        bit_string += '{0:02}'.format(instance / 256).decode('hex')
        # Least significant 8 bits
        bit_string += '{0:02}'.format(instance % 256).decode('hex')

    # Perform HMAC MD5 hash using the key from 802.1Q
    digest = hmac.new(base64.b64decode(MAGIC_KEY), bit_string)

    return digest.hexdigest()


def main():
    """Executes the function using the same values as the example."""
    vlans = {
        1: '1,10-19,100',
        2: '2,20-29,200',
        3: '3,30-39,300',
    }
    print('MST Digest: {}'.format(calculate_mst_digest(vlans)))


if __name__ == '__main__':
    main()

I see that my blog inspired this code. I'm happy to know you found it useful!

/chris

Indeed, Chris! It helped me a lot in a previous job. Thanks for sharing the knowledge!