Drain a site

Move traffic away from a server before maintenance, without a hard withdraw.

Draining (RFC 8326) takes a server out of an anycast set without a hard withdraw: our routes go out again with the GRACEFUL_SHUTDOWN community 65535:0, so routers move traffic to the other sites while this one still answers.

ansage drain --wait 30 && systemctl stop ansage

This drains for 30 s, then withdraws. drain_time does the same on every stop (a second SIGTERM ends the wait).

What changes on the wire

Session	ansage sends
iBGP	GRACEFUL_SHUTDOWN, LOCAL_PREF 0
eBGP	GRACEFUL_SHUTDOWN, plus `drain_prepend` extra prepends of our AS

Router side

FRRBIRD

FRR lowers routes with 65535:0 to LOCAL_PREF 0 by itself. Nothing to configure.

BIRD needs a filter, or set drain_prepend on the ansage side:

if (65535, 0) ~ bgp_community then bgp_local_pref = 0;

Settings

drain_time = 30        # on SIGTERM/SIGINT, drain this many seconds before closing (default 0)

[[neighbor]]
address = "10.0.0.1"
asn = 64501
drain_prepend = 2      # eBGP, for neighbors that ignore GRACEFUL_SHUTDOWN

Is it drained?

$ ansage show summary
...
Announcing: 192.0.2.0/24 2001:db8:1::/48
Drained for 42.0s: our routes carry GRACEFUL_SHUTDOWN (ansage undrain ends it)

The metric ansage_drained is 1 while drained; /api/summary has the drain start in drained.