bgpd: bgpd-update-delay.patch

bgpd: bgpd-update-delay.patch

COMMAND:

'update-delay <max-delay in seconds> [<establish-wait in seconds>]'

DESCRIPTION:

This feature is used to enable read-only mode on BGP process restart or when
BGP process is cleared using 'clear ip bgp *'. When applicable, read-only mode
would begin as soon as the first peer reaches Established state and a timer
for <max-delay> seconds is started.

During this mode BGP doesn't run any best-path or generate any updates to its
peers. This mode continues until:

1. All the configured peers, except the shutdown peers, have sent explicit EOR
(End-Of-RIB) or an implicit-EOR. The first keep-alive after BGP has reached
Established is considered an implicit-EOR.
   If the <establish-wait> optional value is given, then BGP will wait for
   peers to reach establish from the begining of the update-delay till the
   establish-wait period is over, i.e. the minimum set of established peers for
   which EOR is expected would be peers established during the establish-wait
   window, not necessarily all the configured neighbors.
2. max-delay period is over.

On hitting any of the above two conditions, BGP resumes the decision process
and generates updates to its peers.

Default <max-delay> is 0, i.e. the feature is off by default.

This feature can be useful in reducing CPU/network used as BGP restarts/clears.
Particularly useful in the topologies where BGP learns a prefix from many peers.
Intermediate bestpaths are possible for the same prefix as peers get established
and start receiving updates at different times. This feature should offer a
value-add if the network has a high number of such prefixes.

IMPLEMENTATION OBJECTIVES:

Given this is an optional feature, minimized the code-churn. Used existing
constructs wherever possible (existing queue-plug/unplug were used to achieve
delay and resume of best-paths/update-generation). As a result, no new
data-structure(s) had to be defined and allocated. When the feature is disabled,
the new node is not exercised for the most part.

Signed-off-by: Vipin Kumar <vipin@cumulusnetworks.com>
Reviewed-by: Pradosh Mohapatra <pmohapat@cumulusnetworks.com>
             Dinesh Dutt <ddutt@cumulusnetworks.com>