Best Effort Implementation On GÉANT
Best Effort is the default service on the GÉANT network. When a router receives a packet, it will first determine where to send it (the ‘next-hop’). It will then look up the destination address in the forwarding table, using IP routing protocols.
The routing protocols used on the GÉANT network are:
- Interior Gateway Protocol (IGP)
- External Gateway Protocol (EGP)
The Interior Gateway Protocol
Within the GÉANT network, every router must know how to reach any other router, regardless of whether the target is a physical or virtual interface. The IGP makes this possible by providing a detailed map of the GÉANT network.
The IGP currently used in GÉANT is the ISO IGP IS-IS, which provides support for both IPv4 and IPv6. The ISO family is configured on all interfaces on routers on which IS-IS is running. These include:
- Trunk interfaces
- NREN access interfaces
- Peering interfaces
- PoP local Ethernet interfaces
- Loopback interfaces
A NET address is assigned to each router’s loopback interface. This address is then stored in the IS-IS routing table, which is used to locate individual routers.
The decision process in the IS-IS uses metrics to determine a preferred path. These metrics are normally assigned to interfaces by default according to their bandwidths. However, this can lead to less than optimum routing in the core. To avoid this, the metrics on the GÉANT router interfaces have been set up manually.
The metrics used on GÉANT are the IS-IS wide metrics. These were chosen because they can take a wider range of values than the default metrics. Instead of taking values between 1 and 63, wide metrics can take values from 1 to 16,777,215. This allows for a more granular traffic engineering setup in GÉANT.
The External Gateway Protocol
GÉANT currently uses Border Gateway Protocol version 4 with Multiprotocol extension (BGP4+) for inter-domain routing. As part of the BGP routing two behaviours can be defined in the GÉANT network:
- Interior BGP (iBGP)
- Exterior BGP (eBGP)
A full mesh, between all GÉANT routers, represents the iBGP configuration. As there are only 22 routers in the GÉANT core, route reflectors are not needed. All routers in GÉANT carry a full internet routing table.
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Each GÉANT router that is connected to an NREN has an eBGP peering with that NREN, receiving the routes announced by the NREN. These routes can be tagged by the NREN with different communities, telling the GÉANT network to act according to a particular community plan.
Connections with external networks are also developed using BGP. The gateways to DANTE World Service (DWS) providers accept a full routing table from them, and announce only DWS-registered customer routes.
The gateways to other research networks (such as ABILENE, CA*NET, NACSIS, ESNET) receive only the routes belonging to the non-European research networks. By default GÉANT sends the prefixes belonging to the NRENs, unless the NRENs tag routes with a specific community, which will instruct GÉANT to either block the route or take further action with the prefix (for example, as-path prepending).
