BGP Part-1 | BGP & its Packet/Message Types

 Without Border Gateway Protocol (BGP) learning Networking is incomplete. In the world of the internet, BGP has the key role of connecting all devices, servers & people in the world.

BGP is used both inside & outside of the Autonomous system (AS) not like other routing protocols only used inside AS.

BGP Interview Questions & Answers:

 

• ·       BGP is working on which layer of the OSI Model?
• ·       How many message types are in BGP?
• ·       BGP is TCP or UDP?
• ·       What is an Open sent message?
• ·       What are the optional parameters in the Open sent message?
• ·       AS number is carrying in which BGP message?
• ·       How many types of BGP?
• ·       What is the hello & hold time of BGP?
• ·       Route-refresh is in which packet type?
• ·       What will be the reasons for sending Notification messages?
• ·       What is a withdrawn route in the update message of BGP?
• ·       Which parameters are in the Update message?

BGP is working on the Application layer of the OSI Model & using TCP before the neighbor establishment process with TCP port 179.

Types of BGP:

IBGP:

Internal border gateway protocol (IBGP) is used inside an autonomous system.

EBGP:

External border gateway protocol is used between different autonomous systems.

MP-BGP:

Multiprotocol BGP is used to carry VPNV4 routes in the MPLS VPN Network. I will explain this in detail in the MPLS VPN sections later.

During the BGP establishment process, different types of Packet b/w routers are sent. Today I will explain about BGP Packet types.

BGP Packet Types:

Below are BGP message types:

 

• §      Open message
• §      Keep-alive
• §      Update
• §      Notification

In some articles, the 5^th packet “Route-refresh” is also mentioned which I will explain in the below section.

 

These all message types are used for both IBGP & EBGP after the TCP connection.

Open Message:

 After the TCP process is completed then BGP will send an Open message to Discover & maintain the BGP peer relationship.

 

Open message contains Version, AS Number, Hold timer, Router ID & some optional parameters.

Version: The current version of BGP is 4.

Autonomous Number(AS): Here AS number of the router that sends this, by this way IBGP & EBGP connection will also be found like if the same AS number is on the local router then its IBGP & if different then EBGP.

Hold Time: The BGP peer will be up till the end of Hold time. The default hold of BGP is 180 seconds.

 

Router ID: This is part of the BGP identifier to identify the  Router in the BGP Domain. It can configured dynamically or manually. The larger loopback IP address will be Rouer ID, if no loopback then the highest IP Address of the physical interface.

Optional Parameters: if some new capability introduced in BGP then no need to make a new version, BGP uses the Optional part to adjust that, i-e Route-refresh, MP-BGP VPNV4 Route,4 Bytes AS Number.

 

Route-refresh as I explained above some vendors mentioned this as a 5^th type of BGP message but it actually its part of Open message optional parameters.

Keep alive Message:

To maintain active peer relationships keepalive messages are used. This message will send in 1/3 of the hold timer. The default hold is 180 sec & Keepalive is 60 sec but it can change.

Update Message:

This message type is used to share the updated prefix routes in BGP & to withdraw some routes that are deleted to update the peer router about that. BGP attributes are carrying an update message with prefixes.

Notification Message:

If some error occurs b/w two BGP peers then Notification message will send to turn down the BGP neighbor relationship.

 

The notification message will be sent when hold timers expire or manually reset the BGP session.

 

 

Next section of BGP I will explain the different States of BGP & troubleshooting steps of BGP.