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zebra: Allow longer prefix matches for nexthops
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Zebra currently does a shortest prefix match for
resolving nexthops for a prefix.  This is typically
an ok thing to do but fails in several specific scenarios.
If a nexthop matches to a route that is not usable, nexthop
resolution just gives up and refuses to use that particular
route.  For example if zebra currently has a covering prefix
say a 10.0.0.0/8.  And about the same time it receives a
10.1.0.0/16 ( a more specific than the /8 ) and another
route A, who's nexthop is 10.1.1.1.  Imagine the 10.1.0.0/16
is processed enough to know we want to install it and the
prefix is sent to the dataplane for installation( it is queued )
and then route A is processed, nexthop resolution will fail
and the route A will be left in limbo as uninstallable.

Let's modify the nexthop resolution code in zebra such that
if a nexthop's most specific match is unusable, continue looking
up the table till we get to the 0.0.0.0/0 route( if it's even
installed ).  If we find a usable route for the nexthop accept
it and use it.

The bgp_default_originate topology test is frequently failing
with this exact problem:

B>* 0.0.0.0/0 [200/0] via 192.168.1.1, r2-r1-eth0, weight 1, 00:00:21
B   1.0.1.17/32 [200/0] via 192.168.0.1 inactive, weight 1, 00:00:21
B>* 1.0.2.17/32 [200/0] via 192.168.1.1, r2-r1-eth0, weight 1, 00:00:21
C>* 1.0.3.17/32 is directly connected, lo, 00:02:00
B>* 1.0.5.17/32 [20/0] via 192.168.2.2, r2-r3-eth1, weight 1, 00:00:32
B>* 192.168.0.0/24 [200/0] via 192.168.1.1, r2-r1-eth0, weight 1, 00:00:21
B   192.168.1.0/24 [200/0] via 192.168.1.1 inactive, weight 1, 00:00:21
C>* 192.168.1.0/24 is directly connected, r2-r1-eth0, 00:02:00
C>* 192.168.2.0/24 is directly connected, r2-r3-eth1, 00:02:00
B>* 192.168.3.0/24 [20/0] via 192.168.2.2, r2-r3-eth1, weight 1, 00:00:32
B   198.51.1.1/32 [200/0] via 192.168.0.1 inactive, weight 1, 00:00:21
B>* 198.51.1.2/32 [20/0] via 192.168.2.2, r2-r3-eth1, weight 1, 00:00:32

Notice that the 1.0.1.17/32 route is inactive but the nexthop
192.168.0.1 is covered by both the 192.168.0.0/24 prefix( shortest match )
*and* the 0.0.0.0/0 route ( longest match ).  When looking at the logs
the 1.0.1.17/32 route was not being installed because the matching
route was not in a usable state, which is because the 192.168.0.0/24
route was in the process of being installed.

Signed-off-by: Donald Sharp <[email protected]>
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@donaldsharp
donaldsharp committed Oct 23, 2023
1 parent a681f52 commit a272a2b
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Showing 2 changed files with 39 additions and 49 deletions.
6 changes: 3 additions & 3 deletions tests/topotests/all_protocol_startup/test_all_protocol_startup.py
View file
Original file line number Diff line number Diff line change
Expand Up @@ -599,16 +599,16 @@ def test_nexthop_groups():
count = 0
dups = []
nhg_id = route_get_nhg_id("6.6.6.1/32")
while (len(dups) != 3) and count < 10:
while (len(dups) != 4) and count < 10:
output = net["r1"].cmd('vtysh -c "show nexthop-group rib %d"' % nhg_id)

dups = re.findall(r"(via 1\.1\.1\.1)", output)
if len(dups) != 3:
if len(dups) != 4:
count += 1
sleep(1)

# Should find 3, itself is inactive
assert len(dups) == 3, (
assert len(dups) == 4, (
"Route 6.6.6.1/32 with Nexthop Group ID=%d has wrong number of resolved nexthops"
% nhg_id
)
Expand Down
82 changes: 36 additions & 46 deletions zebra/zebra_nhg.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2248,20 +2248,6 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
return 1;
}

if (top &&
((top->family == AF_INET && top->prefixlen == IPV4_MAX_BITLEN &&
nexthop->gate.ipv4.s_addr == top->u.prefix4.s_addr) ||
(top->family == AF_INET6 && top->prefixlen == IPV6_MAX_BITLEN &&
memcmp(&nexthop->gate.ipv6, &top->u.prefix6, IPV6_MAX_BYTELEN) ==
0)) &&
nexthop->vrf_id == vrf_id) {
if (IS_ZEBRA_DEBUG_RIB_DETAILED)
zlog_debug(
" :%s: Attempting to install a max prefixlength route through itself",
__func__);
return 0;
}

/* Validation for ipv4 mapped ipv6 nexthop. */
if (IS_MAPPED_IPV6(&nexthop->gate.ipv6)) {
afi = AFI_IP;
Expand Down Expand Up @@ -2364,7 +2350,7 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
zlog_debug(
" %s: Matched against ourself and prefix length is not max bit length",
__func__);
return 0;
goto continue_up_tree;
}

/* Pick up selected route. */
Expand All @@ -2391,20 +2377,12 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
/* If there is no selected route or matched route is EGP, go up
* tree.
*/
if (!match) {
do {
rn = rn->parent;
} while (rn && rn->info == NULL);
if (rn)
route_lock_node(rn);
continue;
}

/* If the candidate match's type is considered "connected",
* we consider it first.
*/
if (RIB_CONNECTED_ROUTE(match) ||
(RIB_SYSTEM_ROUTE(match) && RSYSTEM_ROUTE(type))) {
if (match && (RIB_CONNECTED_ROUTE(match) ||
(RIB_SYSTEM_ROUTE(match) && RSYSTEM_ROUTE(type)))) {
match = zebra_nhg_connected_ifindex(rn, match,
nexthop->ifindex);

Expand All @@ -2420,11 +2398,7 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
zlog_debug(
"%s: %pNHv given ifindex does not match nexthops ifindex found: %pNHv",
__func__, nexthop, newhop);
/*
* NEXTHOP_TYPE_*_IFINDEX but ifindex
* doesn't match what we found.
*/
return 0;
goto continue_up_tree;
}

/* NHRP special case: need to indicate onlink */
Expand All @@ -2437,7 +2411,7 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
__func__, match, match->nhe, newhop);

return 1;
} else if (CHECK_FLAG(flags, ZEBRA_FLAG_ALLOW_RECURSION)) {
} else if (match && CHECK_FLAG(flags, ZEBRA_FLAG_ALLOW_RECURSION)) {
struct nexthop_group *nhg;
struct nexthop *resolver;
struct backup_nh_map_s map = {};
Expand Down Expand Up @@ -2473,6 +2447,10 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
"%s: match %p (%pNG) not installed or being Route Replaced",
__func__, match, match->nhe);

if (CHECK_FLAG(match->status,
ROUTE_ENTRY_QUEUED))
goto continue_up_tree;

goto done_with_match;
}

Expand Down Expand Up @@ -2541,25 +2519,37 @@ static int nexthop_active(struct nexthop *nexthop, struct nhg_hash_entry *nhe,
if (pmtu)
*pmtu = match->mtu;

} else if (IS_ZEBRA_DEBUG_RIB_DETAILED)
zlog_debug(
" %s: Recursion failed to find",
__func__);

return resolved;
} else {
if (IS_ZEBRA_DEBUG_RIB_DETAILED) {
zlog_debug(
" %s: Route Type %s has not turned on recursion",
__func__, zebra_route_string(type));
if (type == ZEBRA_ROUTE_BGP
&& !CHECK_FLAG(flags, ZEBRA_FLAG_IBGP))
} else {
if (IS_ZEBRA_DEBUG_RIB_DETAILED)
zlog_debug(
" EBGP: see \"disable-ebgp-connected-route-check\" or \"disable-connected-check\"");
" %s: Recursion failed to find while looking at %pRN",
__func__, rn);
goto continue_up_tree;
}
return 0;

return 1;
} else if (IS_ZEBRA_DEBUG_RIB_DETAILED) {
zlog_debug(
" %s: Route Type %s has not turned on recursion %pRN failed to match",
__func__, zebra_route_string(type), rn);
if (type == ZEBRA_ROUTE_BGP
&& !CHECK_FLAG(flags, ZEBRA_FLAG_IBGP))
zlog_debug(
" EBGP: see \"disable-ebgp-connected-route-check\" or \"disable-connected-check\"");
}

continue_up_tree:
/*
* If there is no selected route or matched route is EGP, go up
* tree.
*/
do {
rn = rn->parent;
} while (rn && rn->info == NULL);
if (rn)
route_lock_node(rn);
}

if (IS_ZEBRA_DEBUG_RIB_DETAILED)
zlog_debug(" %s: Nexthop did not lookup in table",
__func__);
Expand Down

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