Description

When we use vrf-table-label in an instance for multicast, the BUD node (A bud node is a PE device that functions as both an egress and transit device) will generate different labels upstream, which causes duplicate traffic and possible traffic blackouts. The show rsvp session p2mp command will give egress and transit LSP sessions with different "label in" values.

This article explains that the behavior is expected and the recommendation is to use "vt-*" instead of "vrf-table-label" to avoid the problem.

 

Symptoms

When vrf-table-label is used in an MX-3 configuration, where MX-3 is a BUD node in the setup, traffic is replicated to 172.16.4.4 with a different label as shown in the following output:

user@MX-3_re> show rsvp session p2mp
Ingress RSVP: 3 sessions
Total 0 displayed, Up 0, Down 0

Egress RSVP: 3 sessions
P2MP name: 172.16.1.1:1:mvpn:L3VPN, P2MP branch count: 2
To              From               State   Rt Style     Labelin Labelout LSPname
172.16.3.3         172.16.1.1         Up       0  1 SE      16        - 172.16.3.3:172.16.1.1:1:mvpn:L3VPN
Total 1 displayed, Up 1, Down 0

Transit RSVP: 4 sessions
P2MP name: 172.16.1.1:1:mvpn:L3VPN, P2MP branch count: 2
To              From               State   Rt Style     Labelin Labelout LSPname
172.16.4.4         172.16.1.1         Up       0  1 SE      30       25 172.16.4.4:172.16.1.1:1:mvpn:L3VPN
Total 1 displayed, Up 1, Down 0

This will result in duplicated traffic, which may lead to traffic blackouts.

 

Solution

This is expected behavior while using the vrf-table-label knob.

 

The workaround to avoid such duplicate traffic scenario is to use a vt-* interface instead of the vrf-table-label knob.

Here is the BUD node showing the same label with vt-* for the same scenario.

user@MX-3_re# run show rsvp session p2mp

Ingress RSVP: 3 sessions
Total 0 displayed, Up 0, Down 0

Egress RSVP: 3 sessions
P2MP name: 172.16.1.1:1:mvpn:L3VPN, P2MP branch count: 2
To              From                  State   Rt Style Labelin Labelout LSPname
172.16.3.3         172.16.1.1         Up       0  1 SE      30        3 172.16.3.3:172.16.1.1:1:mvpn:L3VPN
Total 1 displayed, Up 1, Down 0

Transit RSVP: 4 sessions
P2MP name: 172.16.1.1:1:mvpn:L3VPN, P2MP branch count: 2
To              From                  State   Rt Style Labelin Labelout LSPname
172.16.4.4         172.16.1.1         Up       0  1 SE      30       25 172.16.4.4:172.16.1.1:1:mvpn:L3VPN
Total 1 displayed, Up 1, Down 0


Note: When a " vt " interface is used in the configuration, "ultimate-hop popping" of the label is automatically enabled.

 

Modification History

2022-06-27: Change IP address to be compliant with RFC block

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