Discover VRFs with SNMP

VRF info

This section describes how we discover L3VPN aka. “VRF” info.

This is implemented in the switches.connect.snmp.connector.SnmpConnector().get_my_vrfs() function, which is called from switches.connect.connector.Connector().get_hardware_details()

VRF information on a device is added to the Connector().vrfs{} dictionary, indexed by VRF name.

“Generic” SNMP

The Standard MIB “MPLS-L3VPN-STD-MIB” implements VRF data. (aka L3VPN). This is defined in https://www.rfc-editor.org/rfc/rfc4382.html A good reference is also https://mibs.observium.org/mib/MPLS-L3VPN-STD-MIB/

The parsing is implemented in switches.connect.snmp.connector._parse_mib_mpls_l3vpn()

The mib entry mplsL3VpnVrfEntry is a table with a series of rows below it. Each row is “indexed” by the VRF NAME encoded as an OID string. This is best explained with an example:

mplsL3VpnVrfName = “.1.3.6.1.2.1.10.166.11.1.2.2.1.1”

So the returned OID “.1.3.6.1.2.1.10.166.11.1.2.2.1.1.8.86.82.70.45.78.97.109.101” is mplsL3VpnVrfName.8.86.82.70.45.78.97.109.101.

This tells us the VRF name is a string of length 8, with the value “VRF-Name”. V is ascii code 86, R is 82, etc.

The other table row entries then give us the details about the VRF. Another example:

mplsL3VpnVrfDescription.8.86.82.70.45.78.97.109.101 = STRING “The description of the VRF” mplsL3VpnVrfRD.8.86.82.70.45.78.97.109.101 = STRING “64000:100”

Ie. here we have a description and the VRF “RD” value for the above VRF named “VRF-Name”

Arista Specific

Arista uses a vendor mib “ARISTA-VRF-MIB”, see https://www.arista.com/assets/data/docs/MIBS/ARISTA-VRF-MIB.txt

Similar to the standard mib, the VRF name is “OID encoded”. All entries below “aristaVrfEntry” use this ‘encoding’.

So, e.g. aristaVrfName.4.78.97.109.101 encodes a string of length 4 with value “Name”

Likewize for aristaVrfRouteDistinguisher, aristaVrfRoutingStatus , etc. where the returned value is the meaning of the MIB counter )(ie the RD, and IPv4/v6 status)

Parsing in handled in switches.connect.snmp.arista_eos.connector._parse_mib_arista_vrf_entries()

Interface VRF membership

On layer 3 devices, we may be able to find what VRF a specific interface is a member of. This is stored in the Interface().vrf attribute, as a string with the name of the VRF. Note this can be used as an index into the Connector().vrfs{} dictionary for more information.

“Generic” SNMP

See the standard MIB here: https://www.rfc-editor.org/rfc/rfc4382.html#page-4

The MPLS mib table mplsL3VpnIfConfEntry has a number of rows with information about VRF interfaces. Each entry has the VRF name as OID encoded, followed by the ifIndex. The value of that entry is indicative of the entry data.

This parsing is implemented in switches.connect.snmp.connector._parse_mib_mpls_vrf_members()

Again an example:

mplsL3VpnIfVpnClassification = “.1.3.6.1.2.1.10.166.11.1.2.1.1.2”

This entry specifies the type of VRF on an interface (carrier, enterprise or internet-provider). The value is not interesting to us, but the full OID is. It looks like:

mplsL3VpnIfVpnClassification.<vrf-name-as-oid-encoded>.<if_index> = value.

So the sub-oid gives us a VRF name, and an interface tied to it. We parse this for the vrf name, and lookup the interface from the index. Then we assign that interface to the Vrf() object, and set the Interface.vrf to point to that Vrf() object.

The same goes for the other table entries. Since we don’t care (mostly) about the returned values, we try one more entry (mplsL3VpnIfVpnRouteDistProtocol) if the device does not implement mplsL3VpnIfVpnClassification.

Arista Specific

Arista has a private MIB table aristaVrfIfTable that contains VRF Interface information.

In that table, the element aristaVrfIfMembership has what we need. An example:

aristaVrfIfMembership.999001 = STRING: “Management”

This means that the interface with ifIndex 099001 is member of VRF “Management”. So this is somewhat easier then the standard mib!

Displaying in HTML

The Interface() objects attribute vrf_name is the string that is set when an interface belong to a VRF. The template _tpl_if_name_icons.html will show the vrf icon and hover name if found.

{% if iface.vrf_name %}
&nbsp;
<i class="fa-solid fa-network-wired" aria-hidden="true"
    data-bs-toggle="tooltip"
    title="Member of VRF '{{ iface.vrf_name }}'">
</i>
{% endif %}