Multicast Implementation On GÉANT2
The multicast topology within GÉANT2 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 GÉANT2 they are implemented in an almost identical format.
The PIM-SM topology is identical to the MBGP AS of GÉANT2, 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 routers are located in:
- Frankfurt
- Paris
- London.
The anycast address is not advertised outside of the GÉANT2 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 GÉANT2 do so using MBGP peerings.
MBGP is not a different protocol suite to BGP but rather 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 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 reverse path forwarding (RPF) check in MSDP and PIM functionality.
Within GÉANT2, 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 physical interfaces. Those peerings that have a session with a multicast-enabled network negotiate and facilitate the MBGP extensions of BGP.
To connect the GÉANT2 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 GÉANT2 BGP topology. Within GÉANT2, all internal MSDP (iMSDP) peering sessions between GÉANT2 routers follow the same configuration as iBGP; in both a full mesh topology is used, and for MSDP all GÉANT2 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 GÉANT2 network peers with other PIM domains by way of external MSDP (eMSDP) sessions. For more information on MSDP peering, see the MSDP Deployment page.
Monitoring
Monitoring services for the multicast service on GÉANT2 are available to the network operations staff of GÉANT2's partner NRENs (login is required). Monitoring is carried out using the following monitoring tools:
- Beacon tool for internal monitoring within GÉANT2
- Beacon tool to monitor external multicast connections
- Multicast traffic monitoring in access interfaces to GÉANT2
- MSDP peering and number of SA monitoring.
The network statistics portal also includes the multicast-enabled looking glass, multicast traffic through the Taksometro service, beacon tools for IPv4 and IPv6, and MSDP monitoring via the Cacti tool.
The main GÉANT2 website also provides more information on beacon deployment.
Deployment and Configuration Details
More information on the deployment and configuration on GÉANT2 of the protocols described above can be found on the following pages:
