Multicast Implementation On GÉANT

The multicast topology within GEANT consists of independent Protocol Independent Multicast-Sparse Mode (PIM-SM), Multicast Source Discovery Protocol (MSDP) and Multi-Protocol Border Gateway Protocol (MBGP) domains. While the three separate protocols can work in mutually exclusive topologies, in GÉANT they are implemented in an almost identical format.

The PIM-SM topology is identical to the MBGP AS of GÉANT, specifically using PIM-SM version 2 as described in RFC2362, because PIM-SMv2 only allows for a single Rendezvous Point (RP). Anycast RP has been implemented to provide redundancy and a degree of load balancing within the network. This is achieved by assigning the same IP address to the second loopback interface on multiple routers, so that a designated router would forward PIM join messages to the administratively closest RP. In GÉANT the Anycast RPs are strategically chosen for this reason; these router are:

de1.de.geant.net
fr1.fr.geant.net
uk1.uk.geant.net

The anycast address is not advertised outside of the GÉANT AS and as such identifies the PIM domain as being identical to the MBGP AS.

To facilitate the exchange of multicast routes between ASes it is necessary to take advantage of the multi-protocol extension of BGP (MBGP) as described in RFC 2858. Those multicast enabled NRENs that peer with GÉANT do so using MBGP peerings.

MBGP is not a different protocol suite to BGP rbut ather an extension to BGP allowing the exchange of both unicast and multicast routing information over the same peering session. This is because MBGP allows for the exchange of any protocol that has a prefix-mask hierarchical address format. Specific protocols are denoted by address family identifiers (AFIs); IPv4 is represented by an AFI of 1 and IPv6 by an AFI of 2. Unicast and multicast routes are then identified within the IPv4 address family by the use of subsequent address family identifiers (SAFIs) as follows:

SAFI = 1: Denotes routes that are to be included in the unicast routing table only.
SAFI = 2: Denotes routes that are to be included in the multicast routing table only.
SAFI = 3: Denotes routes that are to be included in the unicast and multicast routing tables.

If a route is received with SAFI=1, it is placed into the unicast routing table (inet.0). If a route is received with SAFI=2 then it is placed into the multicast routing table (inet.2). If a route is received with SAFI=3 then an identical entry is placed in both the unicast and multicast routing tables, inet.0 and inet.2.

It is important to note that it is the routes in the multicast routing table (inet.2) that are used for the RPF check in MSDP and PIM functionality.

Within GÉANT a fully meshed iBGP topology is used with routers only forwarding those routes they have learned from external ASes. All internal BGP peerings negotiate and use the MBGP functionality described above. The eBGP peerings between GÉANT and external ASes are configured between directly connected physcial interfaces. Those peerings that have a session with a multicast enabled network negotiate and facilitate the MBGP extensions of BGP.

To connect the GEANT multicast domain to other peered domains it is necessary to incorporate MSDP for the exchanging of multicast group source information, in the form of MSDP Source Announcements (SAs).

MSDP peering sessions are much like BGP and the implementation of MSDP is much like the GÉANT BGP topology. Within GÉANT all internal MSDP (iMSDP) peering sessions between GÉANT routers follow the same configuration as iBGP, in both a full mesh topology is used and for MSDP all GÉANT routers are within the same ‘meshgroup’. This has the advantage of reducing MSDP SA flooding within the network because when a router receives a SA from another router in its mesh group it will only forward the SA to a router that is not within the mesh group. As such SAs received from a mesh group peer are always accepted and are not subject to an RPF check.

The GÉANT network peers with other PIM domains by way of external MSDP (eMSDP) sessions. The peering sessions are configured in one of three ways:

Single Access circuit
A single eMSDP session is configured between physical interfaces for all instances where the GEANT network peers with another PIM domain by way of a single point of access.
Singly Homed Dual Access circuits
In cases where there are two circuits (primary and back-up) between two routers a single eMSDP session is configured between the loopback interfaces of the two routers.
Dual Homed Access Circuit
In cases where GÉANT has multiple access circuits with a PIM domain that terminate on different routers, then multiple eMSDP peerings are established between the loopback interfaces of the GÉANT routers and those in the external domain.

The RPF check for SAs learned from another PIM domain is done purely on the AS path to the originator of the SA.