bgpd: With suppress-fib-pending ensure withdrawal is sent #17971
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When you have suppress-fib-pending turned on it is possible to end up in a situation where the prefix is not withdrawn from downstream peers. Here is the timing that I believe is happening: a) have 2 paths to a peer. b) receive a withdrawal from 1 path, set BGP_NODE_FIB_INSTALL_PENDING and send the route install to zebra. c) receive a withdrawal from the other path. d) At this point we have a dest->flags set BGP_NODE_FIB_INSTALL_PENDING old_select the path_info going away, new_select is NULL e) A bit further down we call group_announce_route() which calls the code to see if we should advertise the path. It sees the BGP_NODE_FIB_INSTALL_PENDING flag and says, nope. f) the route is sent to zebra to withdraw, which unsets the BGP_NODE_FIB_INSTALL_PENDING. g) This function winds up and deletes the path_info. Dest now has no path infos. h) BGP receives the route install(from step b) and unsets the BGP_NODE_FIB_INSTALL_PENDING flag i) BGP receives the route removed from zebra (from step f) and unsets the flag again. We know if there is no new_select, let's go ahead and just unset the PENDING flag to allow the withdrawal to go out at the time when the second withdrawal is received. Signed-off-by: Donald Sharp <[email protected]>
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@Mergifyio backport stable/10.0 stable/10.1 stable/10.2 |
✅ Backports have been created
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This was referenced Feb 1, 2025
ton31337
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Feb 2, 2025
bgpd: With suppress-fib-pending ensure withdrawal is sent (backport #17971)
ton31337
added a commit
that referenced
this pull request
Feb 2, 2025
bgpd: With suppress-fib-pending ensure withdrawal is sent (backport #17971)
ton31337
added a commit
that referenced
this pull request
Feb 2, 2025
bgpd: With suppress-fib-pending ensure withdrawal is sent (backport #17971)
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When you have suppress-fib-pending turned on it is possible to end up in a situation where the prefix is not withdrawn from downstream peers.
Here is the timing that I believe is happening:
a) have 2 paths to a peer.
b) receive a withdrawal from 1 path, set BGP_NODE_FIB_INSTALL_PENDING
and send the route install to zebra.
c) receive a withdrawal from the other path.
d) At this point we have a dest->flags set BGP_NODE_FIB_INSTALL_PENDING
old_select the path_info going away, new_select is NULL
e) A bit further down we call group_announce_route() which calls
the code to see if we should advertise the path. It sees the
BGP_NODE_FIB_INSTALL_PENDING flag and says, nope.
f) the route is sent to zebra to withdraw, which unsets the
BGP_NODE_FIB_INSTALL_PENDING.
g) This function winds up and deletes the path_info. Dest now
has no path infos.
h) BGP receives the route install(from step b) and unsets the
BGP_NODE_FIB_INSTALL_PENDING flag
i) BGP receives the route removed from zebra (from step f) and
unsets the flag again.
We know if there is no new_select, let's go ahead and just unset the PENDING flag to allow the withdrawal to go out at the time when the second withdrawal is received.