BGP Flashcards

Flashcards generated from lecture notes on Border Gateway Protocol (BGP) to help review key concepts and prepare for the exam.

What is Border Gateway Protocol (BGP)?

A standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet.

How is BGP classified as a routing protocol?

Path-vector routing protocol.

How does BGP make routing decisions?

Routing decisions based on paths, network policies, or rule-sets configured by a network administrator.

What is BGP used for routing within an autonomous system called?

Interior Border Gateway Protocol (IBGP).

What is the Internet application of the BGP protocol called?

Exterior Border Gateway Protocol (EBGP).

When was the Border Gateway Protocol sketched out?

1989.

In which RFC was BGP first described?

RFC 1105.

When has BGP been in use on the Internet since?

1994.

In which RFC was IPv6 BGP first defined?

RFC 1654.

In which RFC was IPv6 BGP improved?

RFC 2283.

What is the current version of BGP?

Version 4 (BGP4).

In which RFC was BGP4 first published?

RFC 1654.

In which RFC was BGP4 subsequently updated in 1995?

RFC 1771.

In which RFC was BGP4 subsequently updated in 2006?

RFC 4271.

What was the major enhancement of BGP4?

Support for Classless Inter-Domain Routing (CIDR) and use of route aggregation to decrease the size of routing tables.

What is RFC 4271 called which allows BGP4 to carry a wide range of IPv4 and IPv6 address families?

Multiprotocol Extensions or Multiprotocol BGP (MP-BGP).

What are BGP neighbors called?

Peers.

How are BGP neighbors established?

Manual configuration among routers.

What type of session and port do BGP neighbors create?

TCP session on port 179.

What does a BGP speaker send to maintain the connection between neighbors?

19-byte keep-alive messages every 30 seconds.

What transport protocol does BGP use?

TCP.

What is it called when BGP runs between two peers in the same autonomous system (AS)?

Internal BGP (iBGP or Interior Border Gateway Protocol).

What is it called when BGP runs between different autonomous systems?

External BGP (eBGP or Exterior Border Gateway Protocol).

What are routers on the boundary of one AS exchanging information with another AS called?

Border or edge routers or simply eBGP peers.

How are new routes learned from an eBGP peer propagated by default?

New routes learned from an eBGP peer are re-advertised to all iBGP and eBGP peers.

How are new routes learned from an iBGP peer propagated by default?

New routes learned from an iBGP peer are re-advertised to all eBGP peers only.

How can route propagation be controlled in detail?

Route-maps mechanism.

What action could be taken by the route-maps mechanism?

Drop the route, or modify some attributes of the route before inserting it in the routing table.

What uniquely identifies a network under a single technical administration that has a unique routing policy, or is multi-homed to the public internet?

An Autonomous System Number (AS number or just ASN).

By whom are Autonomous System Numbers assigned?

IANA.

When is an autonomous system number required?

If you are to run BGP and peer with your internet service provider and between internet service providers at peering points and Internet Exchanges (IX).

What does BGP use to determine the shortest path to a destination where a prefix is located?

Prefixes and Autonomous System Paths (AS Paths).

What are the two types of Autonomous System Numbers (ASN)?

16-bit and 32-bit.

The ASN field was extended the field from how many bits to how many bits?

16 to 32 bits.

What are the two types of ASN?

Public and private.

What is a public ASN?

Globally unique and may be announced on the global Internet to your ISP or at an internet exchange point (peering point) via BGP.

What is a private ASN?

Should not be seen on the global Internet (they shouldn't be announced via your exterior gateway routing protocol).

Private AS numbers are used by ISP's who use what?

Using BGP confederations or in private networks

Most ISPs utilize what to reject routes that contain private ASNs?

Route filters.

An AS-number must be obtained (leased) from where?

One of the Regional Internet Registries (RIR's) [ARIN, LACNIC, RIPE NCC, AFRINIC, APNIC].

Which registry you obtain your AS number from is based upon what?

Where in the world your network resides physically, or where your organization is headquartered, and where will be connecting.

ASN's are usually provided for which type of fee?

Container fee.

You will need to use an online registration page, or the downloadable form commonly called what to request your AS number (ASN)?

The ASN template.

What is one thing you will need to demonstrate to get an ASN?

That you have a connection to more than one ISP (or will in the next 30 days). This is called being 'multi-homed'.

What will you be asked to specify regarding your ISP to get an ASN?

You will be asked to specify the exterior routing protocol used to communicate with your ISP (this is exclusively BGP today).

To get an ASN You must demonstrate a need to utilize BGP with what?

You must demonstrate a need to utilize BGP with an organizationally unique route policy OR be multi-homed.

What is Internal Border Gateway Protocol (iBGP)?

Routing protocol used on the internet for exchanging routing information between two or more routers within an autonomous system (AS).

iBGP is used to do what?

Propagate routes within a single AS.

What is the purpose of iBGP?

Facilitate communication between multiple routers within an AS.

What is one of the primary functions of iBGP?

Exchange routing information between different routers in the same AS.

What is another function of iBGP?

Provide information to other BGP routers about available routes within the AS.

What does iBGP ensure for routers in the AS?

Ensure all routers in the AS have consistent routing information to make decisions about which paths to take when forwarding packets.

What type of capabilities does iBGP allow for?

Allow for redundancy and failover capabilities by allowing multiple paths to be used for a given route.

How does iBGP prevents routing loops?

Help prevent routing loops by ensuring that only one path is used for a given route at any given time.

How does iBGP work?

Each router sends its own routing table to its neighbors, which contains information about the networks it knows and how they can be reached from that router.

To configure iBGP you must first do what?

Configure the BGP process on each router and assign them unique AS numbers.

What type of relationships must you establish while configuring iBGP?

Establish neighbor relationships between the routers by specifying which IP addresses they should use for communication.

What additional parameters can you configure in iBGP?

Route filtering and authentication.

What commands can be used to verify an iBGP configuration?

“show ip bgp summary” and “show ip bgp neighbors.”

What is External Border Gateway Protocol (eBGP)?

Routing protocol used to exchange information between two autonomous systems (AS)

For what is eBGP used?

Connecting distinct networks, such as those belonging to different internet service providers.

Of what is eBGP responsible?

Exchanging network prefixes between autonomous systems, allowing them to communicate with each other.

What is the main difference between iBGP and eBGP?

iBGP is used to propagate routes within an autonomous system, eBGP is used for propagating routes between different autonomous systems.

How are routes advertised in iBGP concerning peers?

Routes received from a peer cannot be advertised to another peer but can be advertised to an eBGP peer.

How are routes advertised in eBGP concerning peers?

Routes received from a peer can be broadcast to both eBGP and iBGP peers.

Is a full mesh topology required for iBGP?

Required.

Is a full mesh topology required for eBGP?

Not Required.

Where is iBGP used?

Within the same organization.

Where is eBGP used?

Distinct networks (i.e., different organizations or internet service providers).

What does iBGP use for loop prevention?

Uses BGP split horizon.

What does eBGP use for loop prevention?

Uses path for loop prevention.

What is the default peer setup for iBGP?

TTL = 255.

What is the default peer setup for eBGP?

TTL = 1.

Are local preference attributes sent in iBGP?

Sent.

Are local preference attributes sent in eBGP?

Not Sent.

What is the behavior on route advertisement to a peer for iBGP?

The next hop remains unchanged.

What is the behavior on route advertisement to a peer for eBGP?

The next hop is changed to the local router.

When there are multiple paths, it’s better to use what type of interface with an IP address and to advertise that into your IGP?

Loopback interface.

Why is it best practice to use loopback interfaces when configuring IBGP?

A physical interface can go down, which means the IP address on the interface is no longer reachable.

What does eBGP require by default in order to establish a neighbor adjacency?

EBGP (external BGP) by default requires two Cisco IOS routers to be directly connected to each other in order to establish a neighbor adjacency.

What TTL do eBGP routers use for their BGP packets?

A TTL of one.

When two routers are not directly connected how can we still make it work?

Multihop.

What does the command 'show ip bgp neighbors | include External' return if the external BGP neighbor is not directly connected?

External BGP neighbor not directly connected.

What command disables the connected check so that R1 and R3 try to become eBGP neighbors?

neighbor [ip address] disable-connected-check.

What command is used to increase the TTL?

neighbor [ip address] ebgp-multihop [number].

What are the BGP states?

Idle, Connect, Active, OpenSent, OpenConfirm, Established.

What happens in the BGP Idle state?

Listens for an incoming connection request from its peer router.

For how long does the ConnectRetry timer allow for connection requests to be complete?

The ConnectRetry timer allows 120 seconds by default for connection requests between the routers to complete before shifting to the next state.

What happens in the BGP Connect state?

BGP tries to complete the three-way TCP handshake.

What happens in the BGP Active state?

BGP will kick off and attempt a new TCP connection.

What happens in the BGP OpenSent state?

BGP is listening for an Open message from its neighbor.

What are the most notable variables in the Open message?

BGP version, the autonomous system (AS) numbers, the source IP address of the configured neighbor, router-ID uniqueness, security parameters, capabilities.

If there are no errors in the OpenSent state, what does BGP start to do?

BGP starts to send Keepalive messages and negotiates the hold time.

What happens in the BGP OpenConfirm state?

BGP listens for Keepalive messages from its peer.

What happens in the BGP Established state?

BGP neighbor adjacency has been created.

Which command tells BGP which interface to source the BGP packets out of?

BGP update-source command.

What is a method used by distance vector protocols to prevent network routing loops?

Split horizon.

What is the basic principle of split horizon?

Never send routing information back in the direction from which it was received.

What does iBGP split horizon state?

A BGP router will not advertise prefixes from one iBGP neighbor to another iBGP neighbor.

What reflects learnt routes to their adjacent neighbors?

BGP route reflectors.