How does BGP guarantee to be loop free?
A router running BGP does not accept a routing update that already includes its AS in the path list, because the update has already passed through its AS, and accepting it again will result in a routing loop.
What restrictions exist for BGP in regards to the routes that it can advertise?
BGP specifies that a BGP router can advertise to its peers in neighboring ASs only those routes that it uses.
What port does BGP run on to guarantee reliable communication between peers?
BGP used TCP port 179 for reliable communication between peers
What type of updates are sent between BGP peers?
After a peer is brough up, full routing tables are exchanged. Afterwards, BGP routers need to send out changes (incremental updates) after that, and only when triggered to do so.
How many bytes of data can be sent out by a BGP router before it stops and waits for an acknowledgement?
BGP uses TCP, and is uses a dynamic window, which allows for up to 65576 bytes to be outstanding before it stops and waits for an acknowledgement.
How does IBGP avoid routing loops within an AS?
BGP specifies that routes learned through IBGP are never propagated to other IBGP peers. By default, each BGP speaker is assumed to have a neighbor statement for all other IBGP speakers in the AS.
How does BGP work with broadcasting and multicasting?
TCP sessions cannot be multicast or broadcast because TCP has to ensure the delivery of packets to each recipient. Because TCP cannot use broadcasting, BGP cannot use it either.
Describe the basic function of BGP synchonization.
BGP synchonization is the practice of redistributing BGP into a IGP. But because of the size of a standard BGP table, synchonization is no longer a viable option, so it is disabled by default.
When is the only good time that you should enable BGP synchonization?
Enable sybchonization if there are routers inthe BGP transit path in the AS that are not running BGP (and therefore the routers do not have full-mesh IBGP within the AS)
By default, how often does BGP send a keepalive message to one of its neighbors?
BGP sends BGP/TCP keepalives by default every 60 seconds.
What type of tables are maintained by BGP?
BGP maintains its own topology table and a neighbors table.
What are the four different message types used by BGP?

– Open


– Keepalive


– Update


– Notification 

Describe the use of the open message type by BGP.
After a TCP connection is established, the first message sent by each side is an open message. If the open message is acceptable, a keepalive message confiming the open message is sent back by the side that received the open message.
Describe the use of the keepalive message type by BGP.
The keepalive packets are sent to ensure that the connection is alive between the BGP peers.
Describe the use of the update message type by BGP.
An update message has information on one path only; multiple paths require multiple messages. All the attributes in the message refer to that path, and the networks are those that can be reached through that path.
Describe the use of the notification message type by BGP.
A BGP router sends a notification message when it detects an error condition. The BGP router closes the BGP connection immediately after sending the notification message.
What is the default hold time maintained by routers running BGP?
The default hold time is 180 seconds
What are the four separate categories that path attributes can fall under?

– Well-known mandatory


– Well-known discretionary


– Optional transitive


– Optional nontransitive 

What is the only type of attribute that may also be marked as a partial?
Only optional transitive attributes might be marked as partial.
Describe well-known attributes used by BGP.
A well-known attribute is one that all BGP implementaions must recognize and propagate to BGP neighbors. A well-known mandatory attribute must appear in all BGP updates. A well-known discretionary attribute does not have to be present in all BGP updates.
Describe the use of optional attributes used by BGP.
Attributes that are not well-known are called optional. BGP routers that implement an optional attribute might propagate it to other BGP neighbors, based on its meaning.
How does a BGP router handle optional transitive and nontransitive attributes?
BGP routers that do not implement an optional transitive attribute should pass it to other BGP routers untouched and mark the attribute as partial. BGP routers that do not implement an optional nontransitive attribute must delete the attribute and must not pass it to other BGP routers.
What are the well-known mandatory attributes defined by BGP?

– AS-path


– Next hop



What are the well-known discretionary attributes defined by BGP?

– Local preference


– Atomic aggregate 

What are the optional transitive attributes defined by BGP?

– Aggregator


– Community 

What are the optional nontransitive attributes defined by BGP?
– Multiexit-discriminator (MED)
Describe the AS-path attribute used by BGP.
The AS-path attribute is the list of AS numbers that a route has traversed to reach a destination with the number of the AS that originated the route at the end of the list. It is a well-known mandatory attribute.
Describe the next-hop attribute used by BGP.
The next-hop attribute indicates the next-hop IP address that is to be used to reach a destination (the neighbor that sent the update)
How does the next-hop attribute differ in BGP from other IGP next-hop attributes, like RIP?
In BGP, when a router advertises the next-hop to get to a specific network no connected to that router, it does not name itself as the next-hop, but names the actual IP address that is advertizing that specific network.
Describe the origin attribute used by BGP.

The origin attribute defines the origin of the path information. The 3 possible values are:


– IGP – The route is interior to theoriginating AS (indicated by an "i" in the BGP table)


– EGP – The route is learned via EGP (indicated by an "e" in the BGP table) 


– Incomplete – The route’s origin is unknown (like redistribution);