Best Effort On GÉANT2
Best Effort is the default service on the GÉANT2 network. BE delivery is simple and scalable, and is a cornerstone principle that has enabled the expansion of the Internet to the far reaches of the world.
The terms of this service dictate that the best attempt will be made to deliver packets to their destination. However, no special resources or priority are allocated to any packets, and no guarantees are given. 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 GÉANT2 network are:
- Interior Gateway Protocol (IGP)
- External Gateway Protocol (EGP)
The Interior Gateway Protocol
Within the GÉANT2 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 GÉANT2 network.
The IGP currently used in GÉANT is the ISO IGP IS-IS (Intermediate System to Intermediate System), 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 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 GÉANT2 router interfaces have been set up manually.
The metrics used on GÉANT2 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 GÉANT2.
The External Gateway Protocol
GÉANT2 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 GÉANT2 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 GÉANT2 core, route reflectors are not needed. All routers in GÉANT carry a full internet routing table.
Each GÉANT2 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 GÉANT2 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, ESnet and SINET) receive only the routes belonging to the non-European research networks. By default GÉANT2 sends the prefixes belonging to the NRENs, unless the NRENs tag routes with a specific community, which will instruct GÉANT2 to either block the route or take further action with the prefix (for example, as-path prepending).
