IS-IS Flooding Timers (LSP Interval, Retransmission, CSNP / PSNP)
When the topology changes, IS-IS regenerates its LSP and sends it to its neighbors. How fast those LSPs go out, how many seconds pass before one is sent again when there is no sign it arrived, and how often the index (CSNP) is handed out on a LAN are each governed by a separate timer.
This article lays out those four intervals from the standard and the IOS XR configuration, then checks them on real hardware from packet timestamps, by pushing 900 routes at once and by adding packet loss to a link.
How flooding itself works (never sending back out the receiving circuit, the SRM and SSN flags, how SNPs differ per medium) is covered in IS-IS Flooding and LSDB Synchronization, and LSP fragmentation and lifetime in IS-IS LSPs and the Link State Database.
Four intervals defined by the standard
ISO/IEC 10589:2002 (2nd edition) defines four intervals for flooding.
| Parameter | Meaning | Default |
|---|---|---|
minimumBroadcastLSPTransmissionInterval | Minimum interval between LSP transmissions on a broadcast circuit | 33 ms |
minimumLSPTransmissionInterval | Time to wait before sending the same LSP again (point-to-point) | 5 s |
completeSNPInterval | Interval at which the DIS sends CSNPs on a LAN | 10 s |
partialSNPInterval | Interval at which PSNPs are sent | 2 s |
On a LAN, one LSP at a time
Section 7.3.15.6 states that on a broadcast circuit setting the flag does not send the LSP right away.
Setting SRMflags on an LSP for a broadcast circuit does not cause the LSP to be transmitted immediately. Instead the Intermediate system shall scan the LSP database every minimumBroadcastLSPTransmissionInterval (with jitter applied as described in 10.1), and from the set of LSPs which have SRMflags set for this circuit, one LSP shall be chosen at random. This LSP shall be multicast on the circuit, and SRMflags cleared.
Note 35 allows up to ten LSPs to go back to back, as long as no more than 1000 / minimumBroadcastLSPTransmissionInterval LSPs are sent in any one second — about 30 per second at the default of 33 ms. The limit exists so that the rate matches the slowest router on the LAN. It applies per transmitting interface, so along a path where LSPs are relayed onward, the slowest interface sets the pace for everything downstream.
On point-to-point links, keep sending until it arrives
Section 7.3.15.5 covers resending on point-to-point circuits.
An IS shall perform the following action every minimumLSPTransmissionInterval with jitter applied as described in 10.1: For all point-to-point circuits C (including non-DA DED circuits and virtual links) transmit all LSPs that have SRMflag set on circuit C, but do not clear the SRMflag. The SRMflag will subsequently be cleared by receipt of a Complete or Partial Sequence Numbers PDU.
The interval between two consecutive transmissions of the same LSP is at least minimumLSPTransmissionInterval, and the standard accepts “any technique which ensures the interval will be between minimumLSPTransmissionInterval and 2 × minimumLSPTransmissionInterval”. In other words, the sender keeps going until a PSNP comes back.
CSNPs come from the DIS, PSNPs are the acknowledgment
On a LAN the DIS sends a CSNP (the index of the database) every completeSNPInterval, and routers that are missing something ask for it with a PSNP. Point-to-point links carry no periodic CSNP; the receiver acknowledges with a PSNP instead. PSNPs carry the entries whose SSNflag is set at that moment and are sent every partialSNPInterval (section 7.3.15.4).
Configuration on IOS XR
| Command | Standard parameter | Default | Mode |
|---|---|---|---|
lsp-interval | minimumBroadcastLSPTransmissionInterval | 33 ms | Interface |
retransmit-interval | minimumLSPTransmissionInterval | 5 s | Interface |
csnp-interval | completeSNPInterval | 10 s (not shown in the help) | Interface |
lsp-mtu | originatingL2LSPBufferSize | 1492 bytes | Router |
RP/0/RP0/CPU0:R1(config-isis-if)#lsp-interval ?
<1-4294967295> LSP transmission interval in milliseconds, default is 33RP/0/RP0/CPU0:R1(config-isis-if)#retransmit-interval ?
<0-65535> Interval in seconds, default is 5The Cisco command reference says csnp-interval “applies only to the designated router”, with a default of 10 seconds. The IOS XR help does not print a default, and show isis interface has no CSNP line, so this article checks it against the CSNPs actually on the wire.
Note that retransmit-throttle-interval, the millisecond setting that throttles retransmissions, appears in the command reference but does not exist under router isis on the XRd 26.1.1 used here.
Periodic LSP regeneration (lsp-refresh-interval, default 900 s) and lifetime (max-lsp-lifetime, default 1200 s) are covered in IS-IS LSPs and the Link State Database, and the SPF and LSP generation delays in IS-IS Convergence Timers.
Lab verification
Lab setup
R1 to R4 are level-2-only (wide) in the single area 49.0001, and every link metric is 10. R1 and R2 are point-to-point, R2, R3 and R4 share one LAN, and R3 is the LAN DIS (pinned with priority 100). The software is XRd 26.1.1.
| Router | Lo0 | Role |
|---|---|---|
| R1 | 1.1.1.1/32 | Originates many routes. lsp-mtu is lowered to split the LSP into fragments |
| R2 | 2.2.2.2/32 | Relays the LSPs received over point-to-point onto the LAN |
| R3 | 3.3.3.3/32 | The LAN DIS, which sends CSNPs |
| R4 | 4.4.4.4/32 | A non-DIS on the LAN |
To send many LSPs at once, 900 static routes are configured on R1 and advertised with redistribute static, and lsp-mtu 256 splits the result into 51 fragments. To observe retransmission, 15% packet loss is applied to the R1 - R2 point-to-point link.
Each STEP is captured in the order clear the counters, ping, then show. clear isis statistics runs per STEP and is logged to syslog as well, so the values in show isis statistics stay within that STEP.
RP/0/RP0/CPU0:R1#show clock
Wed Sep 16 23:23:44.646 UTC
23:23:44.750 UTC Wed Sep 16 2026
RP/0/RP0/CPU0:R1#logmsg informational STEP0-CLEAR: clear isis statistics
Wed Sep 16 23:23:44.840 UTC
RP/0/RP0/CPU0:R1#clear isis statistics
Wed Sep 16 23:23:44.933 UTC
RP/0/RP0/CPU0:R1#show clock
Wed Sep 16 23:23:45.062 UTC
23:23:45.157 UTC Wed Sep 16 2026Lab STEPs
| STEP | Change | What to check |
|---|---|---|
| 0 | Defaults | No periodic CSNP on point-to-point. On the LAN only the DIS sends CSNPs every 7.5 to 10 s |
| 1 | Add lsp-mtu 256, 900 static routes and redistribute static on R1 | The LSP splits into 51 fragments and goes out in bursts of ten |
| 2 | Set lsp-interval 100 on the R1 point-to-point interface | The wait between bursts grows roughly threefold |
| 3 | Set lsp-interval 10 on the R1 point-to-point interface | R1 sends faster, but the LAN side does not |
| 4 | Revert R1 and set lsp-interval 200 on the R2 LAN interface | The relaying side governs the interval seen on the LAN |
| 5 | Revert the R2 LAN and set lsp-gen-interval on R1 | Changing the generation delay does not change the sending interval |
| 6 | Revert lsp-gen-interval and set csnp-interval 3 on the DIS R3 | LAN CSNPs come every 2.25 to 3 s |
| 7 | Set csnp-interval 3 on the non-DIS R2 as well | Only the DIS sends CSNPs; the non-DIS setting has no effect |
| 8 | Revert csnp-interval, add 15% loss on R1 - R2 and regenerate the LSPs | The same sequence number is sent again about every 5 s |
| 9 | Set retransmit-interval 1 on R1 | The shortest gap between resends becomes 1.00 s |
| 10 | Remove the loss and revert retransmit-interval | The settings return to their defaults |
| 11 | Remove the statics and lsp-mtu (final state) | Matches STEP 0 |
Only the DIS sends CSNPs (STEPs 0, 6 and 7)
On the LAN with defaults, only the DIS R3 sends CSNPs. The 22 gaps range from 7.83 to 9.99 seconds — the 10-second value with 25% jitter, so 7.5 to 10 seconds.
5 2026-09-17 08:22:36.094719 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
11 2026-09-17 08:22:45.334925 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
17 2026-09-17 08:22:53.165649 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
23 2026-09-17 08:23:02.065148 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
29 2026-09-17 08:23:10.509856 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
35 2026-09-17 08:23:19.493583 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
41 2026-09-17 08:23:28.623430 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
47 2026-09-17 08:23:37.841210 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ffWith csnp-interval 3 on the DIS, the gaps become 2.26 to 2.98 seconds. The first two (7.9 s and 9.8 s) are the last of the default 10-second intervals, before the change took effect; from the third onward they sit inside the 3-second value with 25% jitter.
1 2026-09-17 08:49:18.234139 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
7 2026-09-17 08:49:26.132582 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
14 2026-09-17 08:49:35.953167 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
18 2026-09-17 08:49:38.564312 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
19 2026-09-17 08:49:41.288439 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
22 2026-09-17 08:49:44.213265 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
25 2026-09-17 08:49:46.793571 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
27 2026-09-17 08:49:49.259534 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
29 2026-09-17 08:49:52.187155 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ff
32 2026-09-17 08:49:55.022162 52:54:00:4c:94:fd L2 CSNP, Source-ID: 0030.0300.3003.00, Start LSP-ID: 0000.0000.0000.00-00, End LSP-ID: ffff.ffff.ffff.ff-ffThe same setting was then added to the non-DIS R2, yet all 77 CSNPs still came from the DIS R3 and none from R2 or R4, even though the configuration is present on R2.
RP/0/RP0/CPU0:R2#show configuration commit changes last 1
Wed Sep 16 23:54:23.349 UTC
!! Building configuration...
!! IOS XR Configuration 26.1.1
router isis 1
interface GigabitEthernet0/0/0/1
csnp-interval 3
!
!
endOn the point-to-point link, not a single periodic CSNP appeared throughout the lab. The receiver acknowledges with a PSNP instead.
LSPs go out in bursts of ten (STEPs 1 to 5)
With all 51 fragments flooded at once, the LSPs go out in bursts of ten with a wait between bursts. At the default lsp-interval of 33 ms that wait was 388 to 409 ms.
19 2026-09-17 08:26:59.636038 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-09, Sequence: 0x00000001, Lifetime: 1199s
20 2026-09-17 08:26:59.636228 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0a, Sequence: 0x00000001, Lifetime: 1199s
21 2026-09-17 08:26:59.636326 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0b, Sequence: 0x00000002, Lifetime: 1199s
22 2026-09-17 08:26:59.636444 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0c, Sequence: 0x00000001, Lifetime: 1199s
23 2026-09-17 08:26:59.636527 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0d, Sequence: 0x00000001, Lifetime: 1199s
24 2026-09-17 08:26:59.636631 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0e, Sequence: 0x00000001, Lifetime: 1199s
25 2026-09-17 08:26:59.636713 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0f, Sequence: 0x00000001, Lifetime: 1199s
26 2026-09-17 08:26:59.636934 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-10, Sequence: 0x00000001, Lifetime: 1199s
27 2026-09-17 08:26:59.637122 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-11, Sequence: 0x00000001, Lifetime: 1199s
28 2026-09-17 08:26:59.637237 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-12, Sequence: 0x00000001, Lifetime: 1199s
30 2026-09-17 08:27:00.046443 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-13, Sequence: 0x00000002, Lifetime: 1199s
31 2026-09-17 08:27:00.046701 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-14, Sequence: 0x00000001, Lifetime: 1199s
32 2026-09-17 08:27:00.046782 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-15, Sequence: 0x00000001, Lifetime: 1199s
33 2026-09-17 08:27:00.046926 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-16, Sequence: 0x00000001, Lifetime: 1199s
34 2026-09-17 08:27:00.047012 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-17, Sequence: 0x00000001, Lifetime: 1199s
35 2026-09-17 08:27:00.047089 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-18, Sequence: 0x00000001, Lifetime: 1199s
36 2026-09-17 08:27:00.047194 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-19, Sequence: 0x00000001, Lifetime: 1199s
37 2026-09-17 08:27:00.047312 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-1a, Sequence: 0x00000001, Lifetime: 1199s
38 2026-09-17 08:27:00.047495 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-1b, Sequence: 0x00000001, Lifetime: 1199s
39 2026-09-17 08:27:00.047594 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-1c, Sequence: 0x00000001, Lifetime: 1199sAt lsp-interval 100 ms the bursts still break at ten, and only the wait grows, to 1020 to 1176 ms.
10 2026-09-17 08:31:42.238137 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-00, Sequence: 0x00000007, Lifetime: 1200s
11 2026-09-17 08:31:42.238388 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-01, Sequence: 0x00000001, Lifetime: 0s
12 2026-09-17 08:31:42.238481 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-02, Sequence: 0x00000001, Lifetime: 0s
13 2026-09-17 08:31:42.238609 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-03, Sequence: 0x00000001, Lifetime: 0s
14 2026-09-17 08:31:42.238678 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-04, Sequence: 0x00000001, Lifetime: 0s
15 2026-09-17 08:31:42.238753 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-05, Sequence: 0x00000001, Lifetime: 0s
16 2026-09-17 08:31:42.238860 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-06, Sequence: 0x00000002, Lifetime: 0s
17 2026-09-17 08:31:42.238935 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-07, Sequence: 0x00000001, Lifetime: 0s
18 2026-09-17 08:31:42.238992 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-08, Sequence: 0x00000001, Lifetime: 0s
19 2026-09-17 08:31:42.239066 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-09, Sequence: 0x00000001, Lifetime: 0s
21 2026-09-17 08:31:43.333601 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0a, Sequence: 0x00000003, Lifetime: 1199s
22 2026-09-17 08:31:43.333843 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0b, Sequence: 0x00000004, Lifetime: 1199s
23 2026-09-17 08:31:43.333937 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0c, Sequence: 0x00000002, Lifetime: 1199s
24 2026-09-17 08:31:43.334371 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0d, Sequence: 0x00000002, Lifetime: 1199s
25 2026-09-17 08:31:43.334494 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0e, Sequence: 0x00000002, Lifetime: 1199s
26 2026-09-17 08:31:43.334626 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-0f, Sequence: 0x00000002, Lifetime: 1199s
27 2026-09-17 08:31:43.334853 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-10, Sequence: 0x00000002, Lifetime: 1199s
28 2026-09-17 08:31:43.334965 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-11, Sequence: 0x00000002, Lifetime: 1199s
29 2026-09-17 08:31:43.335058 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-12, Sequence: 0x00000002, Lifetime: 1199s
30 2026-09-17 08:31:43.335153 52:54:00:b1:9c:d7 L2 LSP, LSP-ID: 0010.0100.1001.00-13, Sequence: 0x00000004, Lifetime: 1199sThis is the rate cap from note 35 showing through. Converted to LSPs per second, every setting stays inside the cap.
lsp-interval | Cap from the standard (1000 / value) | Measured (10 LSPs / wait) |
|---|---|---|
| 33 ms (default) | 30.3 per second | 25.6 per second |
| 100 ms | 10 per second | 9.2 per second |
| 10 ms | 100 per second | 100 per second |
| 200 ms | 5 per second | 4.5 per second |
The cap applies per transmitting interface. With lsp-interval 200 on the R2 LAN side, R1 kept sending at its default rate (385 to 403 ms), yet the wait on the LAN grew to 2137 to 2441 ms.
28 2026-09-17 08:40:22.941384 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-00, Sequence: 0x0000000b, Lifetime: 1200s
29 2026-09-17 08:40:22.942026 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-01, Sequence: 0x00000003, Lifetime: 0s
30 2026-09-17 08:40:22.942185 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-02, Sequence: 0x00000003, Lifetime: 0s
31 2026-09-17 08:40:22.942263 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-03, Sequence: 0x00000003, Lifetime: 0s
32 2026-09-17 08:40:22.942340 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-04, Sequence: 0x00000003, Lifetime: 0s
33 2026-09-17 08:40:22.942424 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-05, Sequence: 0x00000004, Lifetime: 0s
34 2026-09-17 08:40:22.942505 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-06, Sequence: 0x00000004, Lifetime: 0s
35 2026-09-17 08:40:22.942571 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-07, Sequence: 0x00000004, Lifetime: 0s
36 2026-09-17 08:40:22.942661 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-08, Sequence: 0x00000003, Lifetime: 0s
37 2026-09-17 08:40:22.942727 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-09, Sequence: 0x00000005, Lifetime: 1200s
40 2026-09-17 08:40:25.205456 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-0a, Sequence: 0x00000008, Lifetime: 1198s
41 2026-09-17 08:40:25.205701 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-0b, Sequence: 0x00000006, Lifetime: 1198s
42 2026-09-17 08:40:25.205794 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-0c, Sequence: 0x00000004, Lifetime: 1198s
43 2026-09-17 08:40:25.205968 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-0d, Sequence: 0x00000004, Lifetime: 1198s
44 2026-09-17 08:40:25.206067 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-0e, Sequence: 0x00000005, Lifetime: 1198s
45 2026-09-17 08:40:25.206159 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-0f, Sequence: 0x00000005, Lifetime: 1198s
46 2026-09-17 08:40:25.206333 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-10, Sequence: 0x00000004, Lifetime: 1198s
47 2026-09-17 08:40:25.206467 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-11, Sequence: 0x00000004, Lifetime: 1198s
48 2026-09-17 08:40:25.206633 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-12, Sequence: 0x00000004, Lifetime: 1198s
49 2026-09-17 08:40:25.206761 52:54:00:38:bd:c1 L2 LSP, LSP-ID: 0010.0100.1001.00-13, Sequence: 0x00000006, Lifetime: 1198sThe reverse case is STEP 3: R1 sped up to 88 to 112 ms, while the LAN stayed at 334 to 386 ms, because R2 relays at its own default of 33 ms.
| STEP | R1 point-to-point | R2 LAN | R1 sending | Relayed on the LAN |
|---|---|---|---|---|
| 1 | 33 ms (default) | 33 ms (default) | 388-409 ms | 388-409 ms |
| 2 | 100 ms | 33 ms (default) | 1020-1176 ms | 1019-1165 ms |
| 3 | 10 ms | 33 ms (default) | 88-112 ms | 334-386 ms |
| 4 | 33 ms (default) | 200 ms | 385-403 ms | 2137-2441 ms |
The interval of relayed LSPs is set by the slowest interface along the path. Looking only at the LAN, there is no way to tell whether the value comes from the local setting or from an upstream one.
Changing lsp-gen-interval on the generating side from the default 50/200/5000 ms to 500/1000/5000 ms left the burst spacing at 322 to 400 ms (STEP 5). What sets the sending interval is lsp-interval, not the generation delay.
LSPs that do not arrive are sent again (STEPs 8 to 10)
With 15% loss on the R1 - R2 link and the LSPs regenerated, the same LSP ID with the same sequence number appears more than once.
35 2026-09-17 09:00:09.132738 0010.0100.1001.00-28 0x00000009
36 2026-09-17 09:00:09.132957 0010.0100.1001.00-29 0x00000007
37 2026-09-17 09:00:09.133061 0010.0100.1001.00-2a 0x00000008
38 2026-09-17 09:00:09.133126 0010.0100.1001.00-2b 0x00000008
39 2026-09-17 09:00:09.133183 0010.0100.1001.00-2c 0x00000008
40 2026-09-17 09:00:09.133244 0010.0100.1001.00-2d 0x00000008
61 2026-09-17 09:00:14.132923 0010.0100.1001.00-29 0x00000007
62 2026-09-17 09:00:14.133115 0010.0100.1001.00-2a 0x00000008
63 2026-09-17 09:00:14.133257 0010.0100.1001.00-2b 0x00000008
64 2026-09-17 09:00:14.133389 0010.0100.1001.00-2d 0x00000008
79 2026-09-17 09:00:19.133305 0010.0100.1001.00-28 0x00000009
83 2026-09-17 09:00:19.133759 0010.0100.1001.00-2c 0x00000008There were 28 resends: 21 of them 5.00 to 5.04 seconds apart, and seven at 9.99 to 10.04 seconds (where the resend was lost as well and took another period). That is the 5-second default of retransmit-interval, the minimumLSPTransmissionInterval of the standard.
With retransmit-interval 1 there were 35 resends and the shortest gap was 1.00 second.
57 2026-09-17 09:13:07.180255 0010.0100.1001.00-02 0x00000009
58 2026-09-17 09:13:07.180350 0010.0100.1001.00-04 0x0000000a
77 2026-09-17 09:13:08.180227 0010.0100.1001.00-02 0x00000009
78 2026-09-17 09:13:08.180394 0010.0100.1001.00-04 0x0000000a| Setting | Resends | Gaps |
|---|---|---|
| Default 5 s | 28 | 5.00-5.04 s for 21 of them, 9.99-10.04 s for seven |
retransmit-interval 1 | 35 | Shortest 1.00 s, median 1.48 s, longest 3.89 s |
The default of 5 seconds lands neatly on multiples of five, while at 1 second the values scatter, because the sending pace — roughly 390 ms of waiting every ten LSPs — is on the same time scale as the resend timer.
One caveat about reading the capture. A CML link capture is taken after the loss has been applied, so an LSP that was dropped leaves no record. Where the output above shows the same LSP twice, the LSP did reach R2; it was the acknowledging PSNP that was lost, so R1 sent the LSP again.
Without any loss, across the five floods of 51 to 66 LSPs in STEPs 1 to 5, four had zero duplicates and one had a single duplicate (a CML link is not perfectly lossless under load). That is an order of magnitude away from the 28 and 35 seen with loss applied.
Verification configs and show output
Each STEP was captured from all four routers as separate files per router. The verification config is the ..._run.txt (the final state is the one from the last STEP).
| File | Contents |
|---|---|
..._show.txt | show version, show interface description, show route, show route isis, the show isis set (interface, neighbors, neighbors detail, database, database detail, statistics, adjacency-log), show running-config router isis and show configuration commit list |
..._log.txt | show logging narrowed to that STEP. A logmsg marker is placed at the start of each STEP and its timestamp passed to show logging start. R2 commit times (%MGBL-CONFIG-6-DB_COMMIT) are here too |
..._run.txt | show running-config at that STEP (the verification config for that STEP) |
..._trace.txt | show isis trace all | include UPD_LSP_SET_SRM | utility tail -n 60 (which LSPs had SRM set for which circuit). Empty in STEPs with no such event |
..._clear.txt | The counters cleared before the capture of that STEP (clear isis statistics), also logged as STEP<N>-CLEAR |
..._trigger.txt | The SSH session on R1 that regenerated the LSPs (toggling redistribute static metric). Only in STEPs with a trigger |
..._ping.txt | From R1: ping 4.4.4.4 and traceroute 4.4.4.4 (source 1.1.1.1). None for R2 to R4 |
..._commit.cfg | What was committed in that STEP (show configuration commit changes last 1). Absent for routers that changed nothing |
STEP 0: Defaults
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | - | - |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 1: Add lsp-mtu 256, 900 static routes and redistribute static on R1 (51 fragments)
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | - | - |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 2: Set lsp-interval 100 on the R1 point-to-point interface
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | trigger | commit |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 3: Set lsp-interval 10 on the R1 point-to-point interface
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | trigger | commit |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 4: Revert R1 and set lsp-interval 200 on the R2 LAN interface
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | trigger | commit |
| R2 | show | log | run | trace | clear | - | - | commit |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 5: Revert the R2 LAN and set lsp-gen-interval on R1
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | trigger | commit |
| R2 | show | log | run | trace | clear | - | - | commit |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 6: Revert lsp-gen-interval and set csnp-interval 3 on the DIS R3
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | - | commit |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | commit |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 7: Set csnp-interval 3 on the non-DIS R2 as well
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | - | - |
| R2 | show | log | run | trace | clear | - | - | commit |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 8: Revert csnp-interval, add 15% loss on R1 - R2 and regenerate the LSPs
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | trigger | - |
| R2 | show | log | run | trace | clear | - | - | commit |
| R3 | show | log | run | trace | clear | - | - | commit |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 9: Set retransmit-interval 1 on R1
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | trigger | commit |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 10: Remove the loss and revert retransmit-interval
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | - | commit |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
STEP 11: Remove the statics and lsp-mtu (final state)
| Router | show | syslog | running-config | trace | clear | ping | trigger | commit |
|---|---|---|---|---|---|---|---|---|
| R1 | show | log | run | trace | clear | ping | - | commit |
| R2 | show | log | run | trace | clear | - | - | - |
| R3 | show | log | run | trace | clear | - | - | - |
| R4 | show | log | run | trace | clear | - | - | - |
Packet captures were taken per STEP on R1 - R2 (point-to-point) and R2 - LAN-SW (LAN). They show the timing of LSPs, CSNPs and PSNPs.
| STEP | R1 - R2 (P2P) | R1 - LAN-SW (LAN) |
|---|---|---|
| 0 | pcap | pcap |
| 1 | pcap | pcap |
| 2 | pcap | pcap |
| 3 | pcap | pcap |
| 4 | pcap | pcap |
| 5 | pcap | pcap |
| 6 | pcap | pcap |
| 7 | pcap | pcap |
| 8 | pcap | pcap |
| 9 | pcap | pcap |
| 10 | pcap | pcap |
| 11 | pcap | pcap |
Help taken beforehand in a candidate configuration and discarded with abort (nothing committed): R1 help
References
| Standard | Title | Summary |
|---|---|---|
| ISO/IEC 10589:2002 (2nd edition) | Intermediate System to Intermediate System intra-domain routeing information exchange protocol | Section 7.3.15.4 covers sending PSNPs (partialSNPInterval), section 7.3.15.5 resending on point-to-point circuits (minimumLSPTransmissionInterval, with SRMflag cleared by a CSNP or PSNP), section 7.3.15.6 pacing on broadcast circuits (minimumBroadcastLSPTransmissionInterval, one LSP at a time, the limit in note 35), and section 7.3.21 with the annex the defaults (33 ms, 5 s, 10 s, 2 s). |
| Routing Command Reference for Cisco ASR 9000 Series Routers, IOS XR Release 6.6.x - IS-IS Commands | lsp-interval / retransmit-interval / csnp-interval | States that csnp-interval “applies only to the designated router” and that retransmit-interval is for point-to-point links. |
Related articles
- What Is IS-IS
- IS-IS NSAP Addresses and the NET (System ID)
- IS-IS Level 1 and Level 2 (the hierarchy)
- IS-IS Packet Types and Header Format
- IS-IS Adjacency Formation and States
- IS-IS DIS and the Pseudonode
- IS-IS Network Types (broadcast / point-to-point)
- IS-IS Metrics (narrow and wide)
- IS-IS Authentication (hello-password and lsp-password)
- IS-IS LSPs and the Link-State Database
- The Main IS-IS TLVs
- IS-IS Flooding and LSDB Synchronisation
- IS-IS SPF Computation and Route Selection
- The IS-IS ATT Bit and the Level 1 Default Route
- IS-IS Route Leaking and the Up/Down Bit
- IS-IS Route Summarization
- The IS-IS Overload Bit
- IS-IS Hello and Holding Time
- IS-IS Convergence Timers (SPF / LSP Generation)
- IS-IS Flooding Timers (LSP Interval, Retransmission, CSNP / PSNP)
- IS-IS Redistribution (connected / static)
- IS-IS Redistribution of BGP Routes