local CLUSTER_ID check in CLUSTER_LIST.

[kishiguro]

local CLUSTER_ID check in CLUSTER_LIST should be performed to non route reflector client as well.
otherwise multi cluster configuration may have routing loop.

[fujita]

local CLUSTER_ID check in CLUSTER_LIST should be performed to non route reflector client as well.

Hmm, with the following description of rfc4456, I thought that only RR uses this attribute:

When an RR reflects a route, it MUST prepend the local CLUSTER_ID to
the CLUSTER_LIST. If the CLUSTER_LIST is empty, it MUST create a new
one. Using this attribute an RR can identify if the routing
information has looped back to the same cluster due to
misconfiguration.

Looks like that bird does the same:
https://github.com/CZ-NIC/bird/blob/master/proto/bgp/attrs.c#L1376