BGP (Border Gateway Protocol)

2.  BGP is a routing protocol that provides exchanging routes
between different autonomous systems. It is known as the
"routing protocol of internet" as well.
 If ISP-1 is down, Office users can reach web server over ISP-
2. Or even if they have same Bandwidth, BGP can find the
best path.

3.  BGP uses TCP port 179.
 Sends triggered updates (Triggered updates are sent if
only there is a change in the network.)
 BGP has a very high metric value. It is the routing
protocol that converges slowest in the world.

4. 1)Default Route Only
 155.5.5.0/24 networks is advertised to both ISPs. If a
problem occurs on ISP-1, customers of ISP-3 are
delivered to ISP-2 via slowly converge. That provides
redundancy on the network. (Because of 150.x.x.x
network won't be advertised from ISP-1)

5. 2)Partial Updates
 If branch offices have better connection to ISPs, it is useful
that taking partial updates from ISPs. For instances routes of
Russia and Australia are advertised and delivering these
specific routes to corporate office can be provided via BGP.
After preferred route for Russia can be managed over ISP-1,
preferred route for Australia can be managed over ISP-2. If
the link is down which goes to ISP-1, traffic can go ahead
over ISP-2.

6. 3)Full Updates
 All routing table of internet can be gathered from ISP-1
and ISP-2 but that requires two routing table which
needs over 100 MB of memory. Corporate office router
can't put all of these paths to its routing table and keeps
them on BGP table. If a path is down, corporate office
router finds this path from BGP table. This method
requires too much sources.

7.  If BGP is not tuned, it behaves just like RIP ! It is a
distance vector protocol but as known as "path vector"
too.
 ISP-1 and ISP-2 send full routing tables to corporate
office router and corporate office router defines the best
path via hop count ( ISP count ).

8. Tables :
• Neighbor table : Includes all neighbors which consists of BGP
configuration. ( Neighbors are configured manually for BGP )
• BGP table : Includes all BGP routes ( Can be so big )
• Routing table : Includes best path for the target network.
Packets :
• Open : Starts session between routers like a hello packet.
• Keep alive : Controls if the neighbor is up or not.
• Update : Update packet is sent if there is a change in the network.
• Notification: This packet is sent if there is an unwanted situation and
neighbor ship is deleted.