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[PTX] RSVP node-id based hello messages sent over bypass if available in Junos OS 16.1R1



Article ID: KB35559 KB Last Updated: 17 Apr 2020Version: 1.0

This article discusses the change in RSVP node-hello behavior from Junos OS release 16.1R1 onward due to which RSVP node-hello packets from PTX devices are seen to use the bypass-lsp even when a direct link to the adjacent router is available.

Note: Due to this, RSVP node-neighborship failure or flaps may be seen if RSVP authentication is not enabled globally on all links.



The customer network device PTX1 sees continuous RSVP node-neighborship flaps with the adjacent router (Cisco1). Packet capture on the Cisco1 router shows that the RSVP node-hello sent by PTX-1 to Cisco1 is being received on Cisco1 on the Cisco1-Cisco2 cross-link that has RSVP authentication enabled.

Lab setup

                                Bypass-LSP and RSVP node-hello path
|                                                                                          |
|                                                                                          |
|                                                                                          |
|                    +----------+                     +------------+                       |
|                    |          | |            |                       |
|                    | PTX-2    |                     |  Cisco2    |                       |
|                    |          +-------------------->+            |                       |
|                    |          |.52               .53|            |                       |
|                    +-----^----+                     +-------+----+                       |
|                      .229|                              .33 |                            |
|                          |                                  | RSVP auth on this link     |
|                          |                                  |                            |
|                          |                                  |        |
| |                                  |                            |
|                          |                                  |                            |
|                          |                                  |                            |
|                      .228|et-7/0/0                       .32|   xe-1/1/0                 |
|                     +---------+                     +-------v----+                       |
|                     |         |                     |            |                       |
+-------------------+ |         |.108            .109 |   Cisco1   | <---------------------+
                      | PTX-1   <---------------------+            |
                      |         | et-7/0/1    xe-1/0/1|            |
            Lo0       +---------+                     +------------+   Lo0            
                           +                                 +
                           |                                 |
                           |                                 |
                           |                                 |
                           |      1 hop LSP                  |

A lab simulation shows that the node-hello packet is being sent on the bypass LSP.

On PTX-1:

labroot@jtac-ptx5000dc-r2006# run show route
Jan 09 22:04:21

inet.0: 17 destinations, 17 routes (16 active, 0 holddown, 1 hidden)
+ = Active Route, - = Last Active, * = Both  *[IS-IS/18] 02:12:19, metric 10023310
                    > to via et-7/0/1.0, label-switched-path MA-MA:TL1RES-LCR21RONKVA-1:LCR
                      to via et-7/0/0.0, label-switched-path Bypass->

inet.3: 10 destinations, 10 routes (10 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both  *[RSVP/7/1] 02:12:44, metric 10023310
                    > to via et-7/0/1.0, label-switched-path MA-MA:TL1RES-LCR21RONKVA-1:LCR
                      to via et-7/0/0.0, label-switched-path Bypass->

labroot@jtac-ptx5000dc-r2006#  show protocols mpls label-switched-path MA-MA:TL1RES-LCR21RONKVA-1:LCR |display inheritance no-comments
priority 3 3;
auto-bandwidth {
    adjust-interval 10800;
    adjust-threshold 20;
    minimum-bandwidth 100m;
    maximum-bandwidth 25g;
    adjust-threshold-overflow-limit 4;
    adjust-threshold-underflow-limit 18;

labroot@jtac-ptx5000dc-r2006# show protocols rsvp |display inheritance no-comments
keep-multiplier 3;
interface all {
    hello-interval 2;
    subscription 85;
    link-protection {
        optimize-timer 1200;

On the MX router that is simulating the Cisco2 router, the node-hello is being received on the cross-link:

labroot@jtac-mx480-r2011-re1# run monitor traffic interface xe-1/1/0.0 matching "host and proto 46" no-resolve
Jan 09 22:05:20
verbose output suppressed, use <detail> or <extensive> for full protocol decode
Address resolution is OFF.
Listening on xe-1/1/0.0, capture size 96 bytes

22:05:21.216031  In IP 10. > RSVPv1 Hello Message, length: 32
22:05:30.217398  In IP > RSVPv1 Hello Message, length: 32
22:05:39.215720  In IP > RSVPv1 Hello Message, length: 32
22:05:48.215600  In IP > RSVPv1 Hello Message, length: 32
22:05:57.222703  In IP > RSVPv1 Hello Message, length: 32
22:06:06.214866  In IP > RSVPv1 Hello Message, length: 32



The cause for the RSVP node-neighborship to fail/flap on PTX-1 is that the node-hello that is sent as a labeled packet (that is, on the bypass-LSP between PTX-1 and Cisco1) does not have authentication and it traverses a cross-link between the Cisco1 and Cisco2 routers, which has authentication configured. But the node-hello from Cisco1 to PTX-1 is received on PTX-1 on the direct link interface facing Cisco-1.

Disabling RSVP authentication on the Cisco1-Cisco2 cross-link will fix this flapping issue.

However, the glaring question is why the RSVP node-hello from PTX-1 is traversing the cross-link/bypass-path when the direct link is available. This is explained in the Solution.



RSVP node-id based hello messages are sent over a bypass link if it is available, and are directly injected onto the bypass link without any IP route lookup in the kernel (dont-route flag is set). This makes the RSVP node-id hello sessions to remain immune to any system-wide churn after any failure event where a large number of route-next-hop adds/deletes/changes are downloaded from the RPD down to the PFE via the kernel.

This is done by default from Junos OS release 16.1R1 onward.

However, users can switch off this behavior by using the knob protocols rsvp no-node-hello-on-bypass. If the above knob has been turned on or if there is no bypass, then the node-id hello messages are pushed to the kernel without the dont-route flag so that the kernel decides how to forward the packet.


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