BGP

BGP is exterior gate protocol & internet protocol.

BGP is distance vector routing protocol & classless routing protocol.

BGP metric is Attributes & TCP-179

BGP send multiple attribute for route to its neighbor router to reach the network.

It depend upon neighbor router for selecting attribute from update to select best path to reach in network.

If no attributes selected, then every attributes has own priority, on the priority basis attribute are selected.

Internet was built initially for communication in USA, between navy & intelligence bureau, was known as ARPANET.

There are 3 Tier for internet-

Tier-1 = Those company which are more than 70% of contribute in world fiber connectivity.

Tier-2= Company are those who have strong presence in particular region.

Tier-3= Company are those who have strong presence in particular country.

When to use BGP-

BGP is most appropriate when at least of the following exist.

An AS allow packet to transmit through it to reach other autonomous system.

An AS has multiple connection to other AS.

The flow of traffic entering & exiting your AS musty be manipulated.

When to not use BGP-

BGP is not always appropriate don’t use BGP if you have one of the following condition.

Single connection to internet or other AS.

Router policy & route selection are not a concern for your AS.

Lack of memory or processor power on BGP router to handle constant update.

Limited understanding of route filtering and BGP path selection process.

Low BW between AS.

Type of BGP

When BGP neighbor belong to different AS, its called as External BGP (EBGP).

AD value is 20 & Public IP address use in EBGP.

Neighbor should be directly connected in EBGP.

When BGP neighbor belong to same AS, its called as Internal BGP (IBGP).

AD value is 200 & Private IP address use in IBGP.

Neighbor not should be directly connected in IBGP.

Type of Attributes

Attributes is use to selected best path to reach the destination.

There are subcategory of attributes-

 

Well-known Mandatory – its standard attributes supported by all BGP and always included in every BGP update.

Well-known Discretionary – its standard attributes supported by all BGP and optionally included BGP updates.

Optional Transitive –This attribute is not supported to implement BGP. Transitive indicates that a noncompliant BGP router will forward the unsupported attribute unchanged, when sending updates to peers.

Optional Non-Transitive – This is not be supported to implement BGP. Non-Transitive indicates that a non-compliant BGP router will strip out the unsupported

Attribute when sending updates to peers.

 AS-Path = well-known mandatory

 Next-Hop =well-known mandatory 

 Origin =well-known mandatory

 Local Preference =well-known, discretionary

 Community =optional transitive

 Multi-Exit-Discriminator (MED) =optional non-transitive

  Weight =Cisco Proprietary

       1)    Weight 

                        

The weight attribute is only apply for inbound routes, is only seen in IBGP.

The highest value is most desirable in weight attribute.

Weight value does not send to any BGP neighbor.

Its Cisco Proprietary.

      2)    Local Preference

                        

The local preference is only apply for in bound routes & is only seen in IBGP.

By default local preference value is 100 & highest value is most desirable.

Its use to change decision of all router in your AS.

 

      3)    AS-Path

The AS-PATH attribute is actually list of AS nos that a route has traversed in order to reach router.

It only use for outbound routes.

Adding to existing AS-Path in longer AS-Path, which route should be less desirable in inbound route.

AS-Path information should not add to a route until it is advertised to an eBGP peer.

      4)    Origin

Origin is Mandatory Attribute & its has 3 code.

IGP - Originated from an interior gateway protocol, This mean that route was injected into BGP & origin code is “i”.

EGP - Originated from an external gateway protocol & its origin code is “e”.

Incomplete - Unknown origin. This mean the route was redistributed into BGP. origin code is  “?”.

      5)    MED (Multiexit Discriminator)

                  

MED is use for outbound route, default MED value is 0.

Path with lowest MED value are most desirable.

MED sent to external BGP neighbor only & always compare.

      6)    Community

Communities are mean of tagging router to ensure consistent filtering.

By default communities are stripped in outgoing BGP updates whether updates are sent to IBGP or EBGP neighbor.

Any BGP router can filter routes in incoming or outgoing updates or select preferred routes based on Communities.

      7)    Next-hops

For Example, Router A has AS-65000 & Router B and C is in Same AS-66000.

We Router A advertise to Router B keeping Router A as next-hop.

And While Router B advertise to Router C which are in same AS-66000, so keeping next-hop as a Router A.

BGP States-

Idle- 

Try to initiate a TCP connection to BGP peer.

Connect-

From remote peer BGP waits for a TCP connection. If its successful connected then its jump to OPEN sent. If its not successful connected then its goes to Active state.

Active- 

From remote peer BGP attempts to initiate a TCP connection. If its successful connectedthen its move to OPEN sent. If its not successful then BGP will wait for a ConnectRetry timer to expire, and after move back to Connect State.

OpenSent- 

Open message has been sent from one router and is await for Open message from the other router. After the one router receives the OPEN message from the other router. If error not found BGP peer will send KEEPALIVE message and move to openconfirm state or if error is found then move to idle state.

OpenConfirm-

BGP listens for a reply KEEPALIVE message. If receipt of a neighbor Keepalive, then state is moved to Established

 Established-

 BGP peer successfully established.