Routing

To begin, the BGP routing process needs to be initialised on the router with the command router bgp followed by the ASN, for example router bgp 65512 It is essential that a router ID (RID) is configured on the router, either statically or dynamically. The dynamic router ID uses the highest IP address of any…

BGP sessions with neighbours may report one of six states: Idle Connect Active OpenSent OpenConfirm Established Idle Idle is the first stage in the BGP neighbour state. BGP tries to initiate a TCP connection to the BGP peer and also listens for any incoming connections. If an error causes the BGP session to go the…

Type Name Functional Overview 1 OPEN Sets up the BGP adjacency 2 UPDATE Advertises, updates or removes routes 3 NOTIFICATION Indicates an error to a BGP neighbour 4 KEEPALIVE Ensures that the BGP neighbours are still reachable Summary table of BGP Packet Types OPEN This message is used to establish and set up the BGP…

There are two types of BGP Sessions, the configuration is similar expect eBGPs a unique AS in the remote-as suffix Internal BGP (iBGP) Sessions that established within the same AS are classified as internal BGP sessions. The administrative distance assigned upon placement in the routers table is 200. The TTL on packets from iBGP routers…

To communicate with other BGP routers, BGP unlike other routing protocols does not use hello packets or discover neighbours dynamically. BGP was designed to not support neighbours changing frequently, and configuration to bring up sessions between routers to be co-ordinated by their network administrators. To communicate with other routers, BGP uses TCP port 179 to…

BGP was originally intended just for IPv4 support. Multi Protocol BGP, or MP-BGP, was introduced in RFC 2858 with an extension called the address family identifier. The address family identifier allows multiple specific network protocols in BGP to be supported, such as IPv4 and IPv6. Each address family maintains its own database and configuration information…

As BGP is a path vector routing protocol, it does not keep a complete map of the network topology. BGP behaves like a distance vector protocol in ensuring that paths are loop free. The well-known mandatory attribute, AS_PATH, includes a complete list of all the ASNs that the prefix advertisement has transmitted through up to…

BGP will use path attributes with each network path. These path attributes provide control of routing policies within BGP, these path attributes are classified as: Optional non-transitive Optional transitive Well-known discretionary Well-known mandatory Well known attributes must be recognised by all BGP implementations, with mandatory attributes being included with every prefix advertisement. Discretionary attributes might…

In order for an organisation to connect to the rest of the internet directly; it must obtain a public Autonomous System Number, or AS Number. AS Numbers originally were up to 16-bits in length, but were expanded through RFC 4893 to allow up to 32-bits in length due to AS Number exhaustion. This increased the…

In BGP, a group of routers under a single organisations control is usually housed within a single autonomous system (AS) number, with optionally a number of other interior gateway protocols within that domain to route packets within that group of routers. BGP can be used as the only interior gateway protocol as well as the…