Week 10: BGP Flashcards
What are the two types of Routing?
- Interior Gateway Protocol (IGP)
- Exterior Gateway Protocol (EGP)
Where are IGP’s used?
Within autonomous systems
What are examples of IGP’s?
- RIP
- OSPF
- IS-IS
- EIGRP
Where are EGP’s used?
Between Autonomous Systems
What are some examples of EGP’s?
- BGP
- EGP
- ISO’s IDRP
- Novell’s NLSP
Why do we need BGP?
- Single public IP + NAT/PAT
- Redundancy
- Multihoming
What is BGP?
A popular Exterior Gateway Path Vector Protocol
What is unique about BGP?
BGP forms neighbour relationships before sending routing information
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
What port does BGP use for the TCP connection between neighbours?
Port 179
What list does BGP use to route a packet?
The list of Autonomous Systems
What is the list of Autonomous Systems that are associated with a BGP route called?
AS_PATH
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
what is the AS_PATH attribute?
A list of traversed Autonomous Systems?
How does BGP determine the shortest inter-AS path?
From a list of numbers known as the AS_PATH attribute
What is the metric used by BGP?
Hop Count
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
What is an Autonomous System (A.S.)?
A collection of networks under a single Administrative Domain
How many bits long are Autonomous System Numbers?
16 bit
What is the bit range for Globally Unique A.S. Numbers?
1 - 64,511
What is the bit range for Private A.S. Numbers?
64, 512 - 65,535
What is a Stub A.S.?
An A.S. where all packets coming from it are originated in it
What is a Transit A.S.?
An A.S. that forwards incoming packets to their destination en-route
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.
