Routing

MPLS L3 VPN optionB, Label Allocation and Substitution.

  • 1.  MPLS L3 VPN optionB, Label Allocation and Substitution.

    Posted 06-04-2021 03:57
    Hello, all!

    I have interprovider mpls L3 VPN option b. Topology has classic design and looks like here:

    PE  <---ospf/rsvp/mp-ibgp(inet-vpn)-----> ASBR_1<---mp-ebgp(inet-vpn)---> ASBR_2

    PE has :
     - vrf 'one' with route distinguisher (RD) 100:10 and  route target export (RT-exp)/ route target import (RT-imp): 100:100, 100:200

    ASBR_1 has:
     - vrf 'two' with RD 100:20 and RT-imp 100:100
    - vrf 'three' with RD 100:30 and RT-imp 100:200

    When ASBR_1 received from PE prefixes with RT 100:100, 100:200 (vrf 'one' at PE1) it placing this routes to vrfs (tables) 'two' and 'three'.

    As described at Junos OS VPNs Configuration Guide    (p.196 ) ASBR_1 should reallocate/substitute mpls label for vpn routes when it will advertise routes to ASBR_2:

    For a VPN optionB ASBR, labels for transit routes are substituted for a local virtual tunnel label or vrf-table-label label. When a VRF table is configured on the ASBR (this type ofconfiguration is uncommonfor the optionB model),the ASBR does not generate MPLSswap or swap and push state for transit routes. Instead, the ASBR re-advertises a localvirtual-tunnelor vrf-table-label label and forwards that transit traffic based on IPforwarding tables. The label substitutionhelps to conserve labels on JuniperNetworksrouters.

    The question is what label does Junos OS will choose for bgp 'update' to ASBR_2? Label from vrf 'two' or 'three'  and why?

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    STANISLAV RYABUKHIN
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