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Gurudath KS Tue, 06/10/2014 - 01:13
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BGP uses different source and destination ports other than 179 depending on who originates the session. BGP is essentially a standard TCP based protocol, which means that it is client and server based.

When a TCP client attempts to establish a connection to a TCP server it first sends a TCP SYN packet to the server with the destination port as the well known port. This first SYN essentially is a request to open a session. If the server permits the session it will respond with a TCP SYN ACK saying that it acknowledges the request to open the session, and that it also wants to open the session. In this SYN ACK response the server uses the well known port as the source port, and a randomly negotiated destination port. The last step of the three way handshake is the client responding to the server with a TCP ACK, which acknowledges the server’s response and completes the connection establishment.

Now from the perspective of BGP specifically the TCP clients and servers are routers. When the “client” router initiates the BGP session is sends a request to the server with a destination port of 179 and a random source port X. The server then responds with a source port of 179 and a destination port of X. Therefore all client to server traffic uses destination 179, while all server to client traffic uses source 179

Mitesh Manwatkar Tue, 06/10/2014 - 01:31
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What happens in case of ospf,rip.eigrp etc. How do they make their communication reliable?

Gurudath KS Tue, 06/10/2014 - 02:16
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Hi Mitesh,


OSPF and EIGRP are internal routing protocols and they use Multicast address for communication.

In BGP links has to be manually made up.



OSPF is classless and converges fairly quickly, using cost as it’s metric. A router running OSPF creates its own database which contains information on the entire OSPF network, not simply neighbor’s routes like EIGRP. This allows the router to make intelligent choices about path selection on its own instead of relying exclusively on neighbor information.

OSPF routers do form neighbor relationships though. They exchange hellos with neighboring routers and in the process learn their neighbor’s Router ID (RID) and cost. Those values are then sent to the adjacency table.

Every router is responsible for computing its own best paths to all destinations within an OSPF domain. Once the SPF algorithm selects the best paths, they are then eligible to be added to the routing table.



Fast Convergence

EIGRP uses the DUAL algorithm to converge very quickly. It does this by knowing neighbor router’s routing tables and predefining primary and secondary routes to every destination network.

Triggered Updates

EIGRP uses partial triggered updates to its directly connected neighbors rather than periodically sharing its entire routing table. This saves link bandwidth because updates are only sent if a change is incurred, only the changes are sent in the update, and lastly – the updates are only sent to a routers’s affected neighbors. Very efficient!

Protocol Independent

Enhanced Interior Gateway Routing Protocol supports more than just IPv4. It supports IPv4, IPv6, IPX, and AppleTalk.


EIGRP sends route updates, hellos, and queries to its neighbors using the multicast address so end hosts are not affected. Hellos are sent out every 5 seconds by default to learn about new neighbors and make sure existing neighbors are still available.


Gurudath K S


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