The link layer encapsulation type of this entry.
'Snap(2)' is to be specified only if IEEE 802.2
encapsulation is to be used over CSMACD or DS1
links.
For 'pdn(4)', X.121 address is stored in
wfIpv6AdjNodeWanAddr.
For 'wan(3)', X.121 or E.164 FR address is stored
in wfIpv6AdjNodeWanAddr.