Chap 11 - BGP (part 1) Flashcards
From a BGP perspective what is an Autonomous System?
- A collection of routers under a single organization’s control using one or more IGPs and common metrics to route packets within the AS.
- If multiple IGPs or metrics are used within the AS then the routing policy must appear consistent to external ASs.
What are the 2 blocks of private AS numbers?
- 64,512 to 65,535
- 4,200,000,000 to 4,294,967,294
Who assigns public AS numbers?
IANA
What 3 items does IANA require before assigning a public ASN?
- Proof of a publicly allocated network range
- Proof that Internet connectivity is provided through multiple connections
- Need for a unique routing policy from providers
If an organization cannot provide the 3 items IANA requires to get an AS number what should they do?
Use the ASN from their service provider.
What are PA’s?
Path Attributes
What are the 4 classes of BGP Prefix Path Attributes?
- Well-known mandatory
- Well-known discretionary
- Optional Transitive
- Optional non-transitive
What is the difference between Well-known Mandatory and Well-known Discretionary Path Attributes?
- Well-known mandatory PA’s must be included with every prefix advertisement
- Well-known discretionary PA’s may or may not be included
What are 2 characteristics of Optional Path Attributes?
- Recognition by other BGP implementations is optional
- Can be transitive or non-transitive
What does an NLRI include?
- Prefix length
- Prefix
What is contained in a BGP Update message?
- Withdrawn routes
- A single feasible route in the form of an NLRI
- Path Attributes
Does BGP contain a complete topology of the network?
No
How does AS_Path prevent loops?
If a router receives a prefix advertisement with its own AS listed in the AS_Path attribute it discards the prefix thinking that it forms a loop
What does AFI stand for and what one thing does it do?
- Address Family Identifier
- Enables BGP to support specific network protocols such as IPv4 and IPv6 in the same BGP session.
What does MP-BGP stand for and what 2 extensions does it add?
- Multi-protocol BGP
- Adds AFI and SAFI extensions
What 2 things does a SAFI do?
- Provides additional granularity to MP-BGP
- Identifies multicast or unicast
What 5 things does MP-BGP do to achieve AFI separation?
- Each AFI has its own config and database
- Each AFI has its own routing policy
- Each AFI has its own NLRI
- 2 additional PA’s are added for granularity
- MP_REACH_NLRI
- MP_UNREACH_NLRI
- AFI’s and SAFI’s are carried inside every BGP Update messages
What port does BGP use?
TCP 179
What advantages does TCP bring to BGP?
- TCP allows for fragmentation and sequencing
- Reliability through acknowledgements and retransmissions
- Neighbors can be multiple hops away
Do recent implementations of BGP allow fragmentation?
No
If fragmentation is not enabled what does BGP do instead?
Relies on MTU discovery
What is a BGP session?
The established adjacency between 2 BGP routers.
What is required for a BGP session that is multiple hops away?
It requires a route to the neighbor in its RIB from static routes or other routing protocols.