Cisco routers running IS-IS in IP environments still listen to ISHs,
intermediate-system hellos, generated by ES-IS protocol, in conformance with
ISO 10589 requirements. When the physical and data-link layers are operational,
an ES-IS adjacency can be formed even if appropriate conditions do not exist
for establishing an IS-IS adjacency.
Figure
shows what the
output for the show clns neighbors command looks like when
an ES-IS adjacency is formed instead of an IS-IS adjacency. This is usually
because IS-IS hellos are not being processed, as a result of interface MTU
mismatch or misconfigured authentication.