Discover interface IP addresses

We call _get_my_ip_addresses() to read the device interface IP addresses in the default routing table. We do this in a number of ways, as follows.

IPv4

There is an OLD and DEPRECATED MIB entry in the IP-MIB (RFC-4293 at time of writing, started with RFC1213): ipAddrTable (.1.3.6.1.2.1.4.20)

This table is still supported on many devices, but can only be used to find IPv4 addresses of interfaces.

This is parsed in _parse_mibs_ip_addr_table()

Specifically,

ipAdEntIfIndex (.1.3.6.1.2.1.4.20.1.2) can be read to get both the IPv4 address for an ifIndex (647 in the example below).

The returned OID and values look like this:

ipAdEntIfIndex.10.163.188.2 = 647

The field ipAdEntNetMask (.1.3.6.1.2.1.4.20.1.3) is read to get the accompanying subnet mask value.

The returned OID and values look like this:

ipAdEntNetMask.10.163.188.2 = 255.255.254.0

IPv6

Likewise, there is an OLD and DEPRECATED MIB entry in the IPV6-MIB (ipv6MIB or .1.3.6.1.2.1.55): ipv6AddrPfxLength This returns IPv6 address and prefix length for interfaces. This tables is also still supported on some devices.

We read ipv6AddrPfxLength (.1.3.6.1.2.1.55.1.8.1.2) as this returns both the IPv6 address and prefix length for an ifIndex.

This is parsed in _parse_mibs_ipv6_interface_address()

Format is ipv6AddrPfxLength.<ifIndex>.<ipv6 in 16 decimals> = <prefix-len>

The returned OID and values look like this:

ipv6AddrPfxLength.18.254.128.0.0.0.0.0.0.2.204.52.255.254.168.110.119 INTEGER = 64

where ifIndex=18, IPv6 address = 254.128.0.0.0.0.0.0.2.204.52.255.254.168.110.119, which is fe80::2cc:34ff:fea8:6e77 (link-local), with prefix-length 64.

IPv6 and IPv4

The ‘modern’ way to get interface addresses for IPv4 or IPv6 is from the newer IP-MIB ipAddressTable (.1.3.6.1.2.1.4.34)

The table index at ipAddressIfIndex (.1.3.6.1.2.1.4.34.1.3) has a field indicating protocol and thus can be used for more multiple addresses.

We commonly see type 1(IPv4), type 2 (IPv6) and also type 4

Specifically, the field ipAddressPrefix contains the interface IP, the interface ifIndex, subnet ip and prefix all in one entry!

The returned OID is structured as shown here:

ipAddressPrefix.<address-type>.<length>.<interface-dotted-decimal-ip-address> =
        ipAddressPrefixOrigin.<ifIndex>.<add-type>.<addr-lenght>.<subnet-ip-in-dotted-decimal>.<subnet-prefix-length>

where ipAddressPrefixOrigin = “.1.3.6.1.2.1.4.32.1.5”

We parse this in _parse_mibs_ip_address_prefix(), and set interface IP for both v4 and v6.

Note

Standard SNMP MIBs described below do not include VRF context — they typically expose only the default routing table data. !

The above means that if an interface is assigned to a VRF, it will not appear in the above tables by default! Hence we will not know the IP address for those interfaces, unless a vendor MIB implementes this data and our driver supports it!

However, some vendors have implemented access to VRF-specific interface data using the ‘context’ field in an SNMP v3 query. Ie. set the context to the VRF name, and they will return ipAddressTable (newer) or ipAddrTable (older) data specific to that VRF. Some drivers support this (e.g. Arista Networks). See the vendor-specific entries for more.