BGP - BGP Fundamentals

17 important questions on BGP - BGP Fundamentals

What are the 4 BGP Path Attribute classes in BGP?

1. Well-known, Mandatory
    Well-known=RFC Compliant,
2. Well-known, Discretionary
    Well-known=RFC Compliant,
3. Optional, Transitive
    Optional=Can be proprietary
    Transitive=It will be passed to other routers
4. Optional, Non-Transitive
    Optional=Can be proprietary
    Non-Transitive=Doesn't have to be passed

What are 3 Well-known, mandatory BGP Path Attributes?

1. AS-Path: Which AS has the route passed through
2. Next-hop: What is the next-hop address
3. Origin: Where does the route come from?

What are 2 Optional, Transitive BGP Path Attributes?

1. Aggregator
2. Community
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What are 3 Optional, Non-Transitive BGP Path Attributes?

1. MED (Metric)
2. Originator ID
3. Cluster

How does BGP does loop prevention?

It uses the AS_Path attribute. Each Autonomous System has an AS number. When the route is propagated from one AS to another it's AS number is appended to the AS_Path attribute. When a receiving AS sees it's own AS number in the route it knows it is a looped route.

What are 5 characteristics of BGP Neighbors (Peers)?

1. There are 2 types of Neighbors
2. Neighbors do not have to be directly connected
3. BGP doesn't use 'Hello' messages
4. Uses TCP 179
5. Simple Communication (Open, Keepalive, Update, Notification)

Which 6 states does the BGP Neighbor Relationship pass through?

1. Idle
2. Connect
3. Active
4. OpenSent
5. OpenConfirm
6. Established

What two ranges are there for Autonomous System Numbers?

First range: 16 bit field, providing 65.535 ASNs
Extended range: 32 bit field, providing 4.294.967.294 ASNs

Which Autonomous System Numbers are available as private ASNs to use within every organization.

For the first range: 64.512 thru 65.535, a total of 1024
For the extended range: thru 4.294.967.294, for a total of 94.967.294

How does a BGP router provide the possibility of using multiple address families?

It uses extensions:
The AFI (Address Family Identifier) for IPv4, IPv6, etc.
The SAFI (Subsequent Address Family Identifier) for Unicast, Multicast, etc.
It was introduced with RFC-2858 and is called MultiProtocol-BGP

How does MP-BGP (MultiProtocol-BGP) achieve seperation of different address families?

It uses the following Path Attributes:
These are carried in BGP update messages and carry information about different address families.

What is important to note about the BGP Database and different address families and subsequent address families?

Every address family maintains a seperate database for each Sub-Address Family.
BGP includes an AFI and SAFI with every route advertisement.

What is important to note about multi-hop BGP sessions?

The BGP routers need some kind of route, either static or dynamic, to reach the neighbor.
A default route will NOT suffice.

What is different on eBGP sessions than on iBGP sessions?

1. Time to Live on eBGP is set to 1, so no multihop sessions. (iBGP TTL is 255).
2. Advertising router modifies BGP next-hop address to IP address  sourceing the BGP connection.
3. Advertising router prepends its ASN to existing AS_Path variable.
4. Receiving router verifies the AS_Path variable doesn't contain an ASN that matches the local routers.

What 4 message types are used by BGP?

1. Open (Set up and establish BGP adjacency)
2. Update (Advertises, Update, or Withdraws routes)
3. Notification (Indicates an error condition to a BGP Neighbor)
4. Keepalive (Ensures BGP Neighbors are still alive.)

What are the 6 BGP Neighbor states, or Finite-State Machine(FSM)?

1. Idle
2. Connect
3. Active
5. OpenConfirm
6. Established

In BGP, what is a Network Layer Reachability Information (NLRI) and what does it consists of?

It is a routing update send to BGP Neighbors.
It includes a Network Prefix, Prefix Length and any BGP Path Attributes for the specific route.

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