- Single public IP + NAT/PAT - Redundancy - Multihoming

Week 10: BGP Flashcards by Mark Philips

What are the two types of Routing?

Interior Gateway Protocol (IGP)

- Exterior Gateway Protocol (EGP)

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Where are IGP’s used?

Within autonomous systems

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What are examples of IGP’s?

RIP
OSPF
IS-IS
EIGRP

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Where are EGP’s used?

Between Autonomous Systems

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What are some examples of EGP’s?

BGP
EGP
ISO’s IDRP
Novell’s NLSP

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Why do we need BGP?

Single public IP + NAT/PAT
Redundancy
Multihoming

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What is BGP?

A popular Exterior Gateway Path Vector Protocol

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What is unique about BGP?

BGP forms neighbour relationships before sending routing information

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What does BGP do when it forms neighbour relationships for transmitting routing information?

Neighbour IP address is explicitly configured & maye not be on the same subnet
Uses TCP connection b/w neighbours

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What port does BGP use for the TCP connection between neighbours?

Port 179

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What list does BGP use to route a packet?

The list of Autonomous Systems

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What is the list of Autonomous Systems that are associated with a BGP route called?

AS_PATH

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True or False: BGP doesn’t rely on neighbours to pass along routes from their routing tables

False, BGP relies on neighbours to pass along routes from their routing tables

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what is the AS_PATH attribute?

A list of traversed Autonomous Systems?

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How does BGP determine the shortest inter-AS path?

From a list of numbers known as the AS_PATH attribute

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What is the metric used by BGP?

Hop Count

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True or False: BGP uses a different metric than Hop Count for loop avoidance?

False, BGP uses Hop Count as a metric & a Loop Avoidance Device

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What is an Autonomous System (A.S.)?

A collection of networks under a single Administrative Domain

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How many bits long are Autonomous System Numbers?

16 bit

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What is the bit range for Globally Unique A.S. Numbers?

1 - 64,511

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What is the bit range for Private A.S. Numbers?

64, 512 - 65,535

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What is a Stub A.S.?

An A.S. where all packets coming from it are originated in it

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What is a Transit A.S.?

An A.S. that forwards incoming packets to their destination en-route

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What is a Multihomed A.S.?

An A.S. with more than one external connection

True or False: A Transit A.S. doesn't have to by multihomed for packet transit

False, A Transit A.S. must by multihomed by definition for a packet to transit the A.S.

What are the two types of Peering Relationships?

User/Customer Peering | Private Peering

What is Use/Customer Peering?

Using Transit A.S. to get to other networks

What is Private Peering?

Service providers agree to share routes

Where do Public Peering connections take place?

At well-known Internet Exchange Points (IXP)

What is an Internet Exchange Point?

A physical location through which Internet infrastructure companies (ISP's) & CDN's connect with each other

What are the two types of BGP?

Internal & External

What is External BGP (EBGP)?

A BGP session established between two neighbours in different Autonomous Systems

What is Internal BGP (IBGP)?

A BGP session that is established between two neighbours in the same AS

What is the sequence of operations for BGP?

- Establish session on TCP Port 179 - Exchange all active routes - Exchange incremental updates

What are the 4 types of BGP packets that are exchanged during operation?

- Open - Notification - Update - Keepalive

What is the function of Open BGP Message Type?

Establish a peering session - Announces A.S. of of the originating router - Determine hold timer interval

What is the function of a Keep_Alive BGP Message Type?

Handshake at regular intervals

What is function of a Notification BGP Message Type?

Shuts down a peering session

What is the function of an Update BGP Message Type?

Announcing new routes or withdrawing previously announced routes

What is the Open BGP Message Type used for?

- Announcing A.S. of the originating router | - Determines holddown timer interval between keep_alive or update messages (0 implies no keep_alive)

What is the keep_alive BGP message type used for?

- Sent periodically (but before hold timer expires) to peers to ensure connectivity - Send in place of an UPDATE message

What is the Notification Message Type used for?

- Error Notification | - TCP Conncetion is closed immediately after notification

What is the Update BGP Message Type used for?

- NLRI (Network Layer Reachability Information) - Path Attributes - Withdrawn Routes

What is Policy Based Routing (PBR)?

Access control is placed on some routes

What is Policy Based Routing used for?

- Avoid transit situations | - Enforce national laws/security concerns/content issues

True or False: BGP doesn't provide the capability for enforcing various policies

False, BGP provides capability for enforcing various policies

True or False: Policies are part of BGP

False, Policies are not part of the BGP Protocol, they are provided to BGP as config information

How does BGP enforce policies?

By choosing paths from multiple alternatives & controlling advertisement to other A.S.'s

List some examples of BGP Policies

- A multi-homed A.S. refuses to act as transit - A multi-homed A.S. can become transit for some A.S.'s - An A.S. can favour of disfavour certain A.S.'s for traffic transit from itself - A.S can enforce national laws or security concerns or content issues

What are the BGP Path Attributes?

- Origin - AS_PATH - NEXT_HOP

What is a BGP Path Attribute?

A characteristic of an advertised BGP route

what are the Types of BGP Path Attributes?

- Well-known Mandatory - Well-Known Discretionary - Optional Transitive - Optional Nontransitive

What are some Well-Known Mandatory BGP Path Attributes?

- ORIGIN - AS_PATH - NEXT_HOP

What are some Well-Known Discretionary BGP Path Attributes?

- LOCAL_PREF | - ATOMIC_AGGREGATE

What is an Optional Transitive BGP Path Attributes?

AGGREGATOR

What is an Optional Nontransitive BGP Path Attribute?

- MULTI_EXIT_DISC | - ORIGINATOR_ID

True or False: LOCAL_PREF is used externally to an A.S. to tell routers which path to take

False, LOCAL_PREF is used locally within an A.S. to tell routers which exit path to take out from an A.S.

True or False: LOCAL_PREF selects the lowest local preference

False, LOCAL_PREF selects the highest local preference

True or False: local preference is advertised between eBGP and iBGP peers

False, local preference is only advertised between iBGP peers LOCAL_PREF attribute is never advertised outside of an A.S.