Flashcards
SCC BGP (Boarder Gateway Protocol)
Cards in this deck (20)
- 1What is an autonomous system?An AS is a connected group of one or more Internet Protocol prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy.
- 2What do autonomous systems enable?Free flow of traffic
– Governed by Interior routing protocol (IGP) - 3How is internet accessed?Internet accessed via one or more autonomus system border routers (ASBR)?
- 4Why do we need Exterior routing (EGP)?Cannot have free flow of traffic outside
-Some AS may have performance or reliability problems
-Commercial traffic may be banned on some networks
-Could be political restrictions/sanctions in place
Interior protocols can't handle such contraints - 5What does BGP provide each AS?– eBGP: obtain subnet reachability information from neighboring ASes
– iBGP: propagate reachability information to all ASinternal routers.
– determine “good” routes to other networks based on reachability information and policy - 6What does BGP allow subnets to do?allows subnet to advertise its existence to rest of Internet: “I am here”
- 7Why is BGP not called distance vector protocol? Why is BGP not called link state protocol?– Not Distance-Vector as needs more than hop-count
– Not Link-State as doesn’t have full topology - 8• Routers receive reachability info from neighbours, as ‘Update message’, containing what?NLRI = Network Layer Reachability Information
– Attribute list
• AS Path - most important, mandatory attribute - 9What information does the AS path have?– a path-list of every Autonomous System between router and destination
- 10What route does BGP select and what is this dependent on?– BGP selects routes with shortest path
• Dependent on policies in force – i.e. Policy can override default behaviour - 11What does the BGP open message do?– Defines hold-time by which time an update must be sent
– An identifier, one of the sender’s IP addresses
– Parameters, including authentication type - 12What does update message do?– Lists unfeasible or withdrawn routes
– Network Layer reachability Information (NLRI) – the available prefixes - 13What does the notification message do?– An error code and error sub code
- sender then closes connection - 14What does the keep alive message do?– Contains no message part – just the header to act as a ‘heartbeat’
- 15How are each AS treated?Each AS treated as a ‘black box’
- 16How does BGP session work?:two BGP routers (“peers”) exchange BGP
messages over semi-permanent TCP connection:
• advertising paths to different destination network prefixes (BGP is a “path vector” protocol) - 17• What happens when AS3 gateway router 3a advertises path AS3,X to AS2 gateway router 2c:AS3 promises to AS2 it will forward datagrams towards X
- 18Difference between the eBGP and iBGP connectivity?eBGP - connection to external autonomous systems
iBGP - connection within internal autonomous systems - 19What does BGP select route based on?1. local preference value attribute: policy
decision
2. shortest AS-PATH
3. closest NEXT-HOP router: hot potato routing
4. additional criteria - 20—
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- Flashcards on SCC BGP (Boarder Gateway Protocol), created by Ayo Olutobi on 27/04/2017.
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- Ayo Olutobi
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