BGP Border Gateway Protocol

By Amir and David

What Is BGP ? Exterior gateway protocols are designed to route

between autonomous systems. AS’s : A set of routers that operate under the same

administration and share a common routing strategy. Bgp can be used for routing packets both within and

between AS’s Routers that belong to different AS’s and exchange BGP

updates are running External BGP.These routers are usually adjacent to each other and share the same physical media and subnet.

Routers that belong to the same AS and exchange BGP updates are running Internal BGP.

BGP Operations

BGP neighbors communicating between AS’s must reside on the same physical network.

Before it exchanges information with an external AS,BGP ensures that networks within the AS’s are reachable.

It does this by a combination of internal peering among routers within the same As.

It redistributes BGP routing information to interior gateway protocols such as RIP and OSPF.

BGP Operations

BGP uses TCP When 2 routers running BGP form a TCP

connection they exchange messages to open and confirm the connection parameters

Initial dat exchange between 2 bgp routers is the entire BGP routing table.This is known as an open message.

An open message must be confirmed before updates notifications and keepalives can be exchanged.

BGP Operations

BGP does not require a periodic refresh of the entire routing table.

Routers instead receive incremental updates that contain the latest version of each peers routing table.

Incremental updates are sent out as the routing tables change.

BGP update messages contain network numbers and path pairs

BGP Operations

Bgp maintains a routing table with all feasible paths to a particular network.

It advertises only the optimal path in its update messages based on metrics assigned by the network admin through the use of config files.

BGP metrics may be based on any number of criteria including AS hop count,type of link,bandwidth,and path cost..

Basic BGP Configuration

Verifying BGP Operation

Configuring the ,….

Verifying BGP Configuration

BGP Configuration Steps activates the BGP protocol and identifies the

local autonomous systemLABA(config)#router bgp 100 .

permits BGP to advertise a networkLABA(config-router)#network 192.168.5.0

(network network-number)

The network command creates the route in the BGP table only if the routes are already present in the IP routing table)

BGP Configuration StepsThe first step to activate BGP

Start in config mode.

LABA(config)#router bgp 100 (router bgp autonomous-system)

This activates the BGP protocol and identifiesthe local autonomous system the router belongs to.

The second step

LABA(config-router)#network 192.168.5.0 (network network-number)

This command permits BGP to advertise a network if the network is listed in the IP routing table.

The network command creates the route in the BGP table only if the routes are already present in the IP routing table)

Third step

Define BGP neighbors

LABA(config-router)#neighbor 172.16.66.1 remote-as 200

(neighbor ip-address remote-as autonomous system)

The neighbor part of the command indicates the routers to which we are trying to connect to using BGP.The IP address is the next hop directly connected address.(This is usually the IP address of the interface at the other end of the connection) .

The remote-as autonomous system identifies the AS numberof the router to which we are trying to connect using BGP.

The value placed in the autonomous-system part of the command determines whether the communication between the neighbor is an EBGP or an IBGP session.

LABA(config)#router bgp 100

LABA(config-router)#network 192.168.5.0

LABA(config-router)#neighbor 172.16.66.1 remote-as 200

LABA(config-router)#exit

LABB (config)#router bgp 200

LABB(config-router)#network 172.16.0.0

LABB(config-router)#neighbor 172.16.66.2 remote-as 200

LABB(config-router)#exit

Verifying BGP Configuration

LABA#show ip bgp

This command shows the BGP routing table.

LABA#show ip route

This command will show the ip routing table and should show the route that was learned through BGP.

To check that a neighbor connection is up use the show ip bgp neighbors command

LABA#show ip bgp neighbors

The BGP = Established field indicates a connection was made between router LABA and LABB.