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Multicast in L3VPNs

Multicast in L3VPNs. draft-ietf-l3vpn-2547bis-mcast-03.txt. Bruce Davie 1 bsd@cisco.com. 1. Not a draft co-author, or a multicast expert. Overview. Aiming to encourage more involvement in multicast L3VPN work by providing user-friendly overview of problem space

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Multicast in L3VPNs

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  1. Multicast in L3VPNs draft-ietf-l3vpn-2547bis-mcast-03.txt Bruce Davie1 bsd@cisco.com 1. Not a draft co-author, or a multicast expert

  2. Overview • Aiming to encourage more involvement in multicast L3VPN work by providing user-friendly overview of problem space • Focus more on problems that the current proposed solutions • Hard questions will be deflected to draft authors • Likewise questions such as “why did you choose to design it that way?” • Attempts to make me look ignorant will be frowned upon

  3. Agenda • Recap of current (deployed) state (draft-rosen1) • See also draft-raggarwa-l3vpn-2547-mvpn-00.txt, draft-ycai-mboned-mvpn-deploy-00.txt • Enhancements/changes in L3VPN WG draft • Supporting multiple tree types • Aggregation • Carrying customer multicast routing in BGP • Inter-AS improvements • Note Well: WG draft is a superset of draft-rosen • i.e. deployed solutions will not be obsoleted by new draft 1draft-rosen-vpn-mcast-08.txt

  4. L3VPN Multicast—Motivation • Customers with IP multicast traffic would like to use MPLS VPN services • RFC 2547/4364 only addresses unicast • As usual, multicast makes the problem harder • Difficult to achieve same scalability as unicast

  5. Multicast VPN—Current Deployments • Based on draft-rosen-vpn-mcast-08.txt • Many similarities to unicast, and some differences • CE-routers maintain PIM adjacency with PE-router only • Similar concept to 2547/4364 VPNs • P-routers do not hold (S, G) state for individual customers • Unlike unicast, there is some per-customer state in P-routers • PE-routers exchange customer routing information using PIM • Contrast to BGP for unicast • Customer multicast group addresses need not be unique • Same as 2547/4364

  6. Multicast VPN—Current State (2) • Multicast domain is a set of multicast-enabled VRFs (mVRFs) that can send multicast traffic to each other • e.g., VRFs associated with a single customer • Maps all (S, G) that can exist in a particular VPN to a single (S, G) group in the P-network • This is the Multicast Distribution Tree (MDT) • Amount of P-state is a function of # of VPNs rather than # of (S, G)s of all customers • This is not as good as unicast, but better than the alternative • Mapping is achieved by encapsulating C-packet into P-packet using GRE

  7. Customer BDefault MDT239.192.10.2 Default Multicast Distribution Tree • PE routers build a default MDT in the global table for each mVRF using standard PIM procedures • All PEs participating in the same mVPN join the same Default MDT • Every mVRF must have a Default MDT • MDT group addresses are defined by the provider • Unrelated to the groups used by the customer • Data MDTs may be created for high BW sources PE PE PE Customer BCE Customer BCE Customer BCE

  8. Customer BDefault MDT239.192.10.2 Customer BDefault MDT239.192.10.2 RootLeaf Multicast TunnelInterface Default Multicast Distribution Tree PE PE PE Customer BCE Customer BCE Customer BCE • Default MDT is used as a permanent channel for PIM control messages and low bandwidth streams • Access to the Default MDT is via a Multicast Tunnel Interface • A PE is always a root (source) of the MDT • A PE is also a leaf (receiver) to the MDT rooted on remote PEs

  9. Limitations of draft-rosen • At least one multicast tree per customer in core • No option to aggregate multicast customers on one tree • Multicast traffic is GRE (not MPLS) encapsulated • Bandwidth and encaps/decaps cost • Operational cost—different mcast and unicast data planes • PIM the only fully described way to build core trees • Can’t leverage p2mp RSVP-TE, mLDP • PE-PE exchange of C-routes using per-customer PIM instances • Inter-AS challenges

  10. PMSI: P-Multicast Service Interface • New terms introduced to decouple tree from service • A PE delivers packet to PMSI, then all the required PEs will get than packet and known which MVPN it belongs to • Three types of PMSI • MI-PMSI: Multipoint Inclusive, all → all • All PEs of an MVPN can transmit to all PEs • UI-PMSI: Unidirectional Inclusive, some → all • Unidirectional, selected PEs can transmit to all PEs • Selective: S-PMSI, some → some • Unidirectional, selected PEs can transmit to selected PEs

  11. Types of Multicast Trees • Inclusive: contains all the PEs for a given MVPN • Selective: contains only a subset of PEs of a given MVPN • Aggregate • Carries traffic for more than one MVPN • May be either inclusive or selective

  12. Supporting Multiple Tree Types • A PMSI is scoped to a single MVPN • PMSI is instantiated using a tunnel (or set of tunnels) • Tunnels may be: • PIM (any flavor) • MPLS (mLDP or p2mp RSVP-TE) • Unicast tunnels with ingress PE replication • Can map multiple PMSIs onto one tunnel (aggregation) • Encaps a function of tunnel, not service • Single provider can mix and match tunnel types

  13. Mappings to Old Terminology • Default MDT • MI-PMSI, instantiated by PIM Shared Tree or set of PIM Source Trees • Data MDT • S-PMSI, instantiated by PIM Source Tree • New terminology helpful in: • Describing the complete set of options • Allowing multiple instantiations of same service, without changing service spec

  14. Autodiscovery • The process of discovering all the PEs with members in a given MVPN • Similar to RFC4364, but: • New address family MCAST-VPN • Contains address of the originating PE • Contains tunnel attribute to specify what sort of tunnel (e.g. PIM-SSM, mLDP, etc.) can be supported by this PE, and whether aggregation is supported • May contain a tunnel ID • Can also be used to discover set of PEs interested in a given group (to enable S-PMSI creation)

  15. Aggregation • Conflicting goals • Scale: Minimize P-router state → Use as few trees as possible • Optimality: Send traffic at most once on each link, and only to PEs that need it → Use a tree for each customer multicast group • Solution: lots of options • Draft-rosen has one MDT per VPN, and optional data MDT for high BW or sparse customer groups • New draft also allows a tunnel to be shared among multiple mVPNs • Better aggregation, less optimality • Requires a de-multiplexing field (e.g., MPLS label) • Utility of aggregation depends on amount of “congruence” and traffic volume

  16. PE-PE Routing Exchange • In draft-rosen, C-PIM instances exchange PIM messages over the MDT as if it were a LAN • Per-customer PIM peering among the PEs • By contrast, one BGP instance carries all customer unicast routes among PEs • PIM Hellos can be eliminated, but Join/Prunes remain • In new draft, BGP is proposed, as in unicast • New AFI/SAFI • Advertisement contains essentially the same info as a PIM join or prune (source, group, PE sending the message) • RDs are used to disambiguate customer multicast group and source addresses • BGP route reflectors may be used

  17. Inter-AS • Old (draft-rosen) approach: tunnel spans multiple ASes • Undesirable fate-sharing, must agree on tunnel type • New draft allows another approach: may “splice” tunnels from different ASes • Allows each AS to build its tunnels independently of other ASes • Scaling now independent of number of PEs in other ASes

  18. Inter-AS Overlay Tunnel • For a given MVPN, each AS independently builds an intra-AS tunnel • In addition, an “overlay tunnel” that spans multiple ASes is built • Each PE announces its MVPN membership info to the ASBRs with iBGP • An ASBR announces the AS MVPN membership to other ASBRs (in other ASes) using eBGP • This enables an AS-level spanning tree to be built among the set of ASes with members in this MVPN • Inter-AS tunnels spliced to intra-AS tunnels

  19. Inter-AS Tunnels Customer A Customer A ASBR1 ASBR2 Customer A ASBR3 Intra-AS tunnels Customer A Inter-AS tunnels

  20. Other issues • RPF establishment • PEs need to know who their RPF PE is for a given route • Duplicate detection • Multihomed sites and switching from shared to source tree can lead to a PE getting duplicate packets • draft proposes means to address this • C-RP Engineering • RP location in customer sites may cause “hairpin” • PEs may act as “outsourced” C-RPs • PEs may speak MSDP to C-RPs

  21. Conclusions • WG draft builds on rosen draft without obsoleting it: • Support for more tree types, including PIM variants, mLDP, RSVP-TE • Separation of service and mechanism • Aggregation support • More Inter-AS options with improved independence • Possibility to use BGP for C-route exchange

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