Shulou(Shulou.com)06/01 Report--

BGP: Border Gateway Protocol (BGP) is a routing protocol that runs between TCP Autonomous Systems (AS) regions.

Why use BGP:

When an enterprise is connected to multiple ISPs (Internet Service Providers), BGP policy routing can be used to select the best route for some services and improve resource utilization.

When an enterprise wants to differentiate between its own traffic (i.e., corporate data within the enterprise network) and traffic from ISPs, BGP uses powerful policy routing to achieve this.

Some small partners may wonder why OSPF should be used if branch offices and headquarters use MPLS/×× connections.

Because a. As mentioned above, BGP has powerful routing filtering and routing policy functions, which OSPF cannot match.

BGP has many extended attributes, and can flexibly optimize various routes according to business requirements, and even the same route can enjoy different routing policies in different enterprises.

BGP is suitable for large enterprise network connections. Due to IGP's limited processing power (OSPF is believed to handle up to 1W routes, ISIS can handle up to 3W routes).

Things to note in practice:

When BGP protocol is used for interconnection in large enterprise networks, all routers on the transit path should run IBGP simultaneously in the transit autonomous system, which is called IBGP to realize full interconnection IBGP. Because if nothing is done to implement fully interconnected IBGP, although the AS advertises that the network is OK, if the two ASs want to transmit data through the transit AS, it will cause packet loss or loop in the transit AS due to unreachable routing.

if BGP route are few and can be redistributed to IGPs run within that AS, there is no nee to run IBGP on kitty routers in the transit path. In this case, however, BGP synchronization needs to be turned on to ensure that packets are not lost. However, if fully interconnected IBGP is implemented, BGP synchronization does not need to be enabled.

Routing decision process. When a route path needs to be selected, it can be selected by adjusting weight, local priority, AS path length, WED, etc.

There are two ways to advertise a network to BGP: using the command network; and redistributing routes from IGP to BGP. However, it is not recommended to redistribute routes from IGP to BGP because any changes to IGP routes, such as link failures, will cause BGP updates, resulting in unstable BGP tables.

Configure commands in BGP speaker Note that the neighbor command tells BGP who to advertise to, while the network command tells BGP what to advertise.

If the neighbor command uses the IP address of the loopback interface instead of the physical interface address as the original address, you must add a corresponding command: neighbor {ip-address| peer-group-name} update-source loopback interface-number。

When routing to EBGP, the next hop changes to its own update source. When routing to IBGP, the next hop remains unchanged. If B receives A's advertisement and sends it to C, if next-hop-self is not used, the update source of this route is still A's IP source, which will cause unreachability. Example:

A-B-C, A and B are EBGP neighbors, B and C are IBGP neighbors.

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