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- Withdrawn routes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.
Can BGP be thought of as a Control Plane routing protocol or application?
Yes because it allows for the exchange of routes with a peer that is multiple hops away.
What is the administrative distance of iBGP?
200
What is iBGP?
- Internal BGP
- Within the same AS or in the same BGP Confederation
What is eBGP?
- External BGP
- Session established with a router in a different AS.
What is the administrative distance of eBGP?
20
What are 2 times iBGP might be needed?
- When multiple routing policies are needed
- When transit connectivity is provided through an AS (Company A connects to Company C through Company B)
Why is redistributing the BGP table into an IGP not a viable solution?
- Scalability (too many routes)
- Custom Routing - BGP uses PA’s to calculate path, IGPs use a metric. A BGP path might be suboptimal when compared with the IGP path
- An IGP can’t understand Path Attributes
What is one viable solution to providing connectivity in an AS using iBGP?
Establishing a full mesh where all routers are neighbors with each other
How many BGP message types are there?
4
What are the 4 BGP message types?
- OPEN - establishes adjacency
- UPDATE - advertises, updates, or withdraws routes
- NOTIFICATION - indicates an error condition to a BGP neighbor
- KEEPALIVE - ensures that the BGP neighbor is still alive
What 5 things are in an OPEN message?
- BGP version number
- Originating router’s ASN
- Hold time
- BGP Router ID
- Other optional parameters for session capabilities
What 3 routing changes happen by the local host if the neighbor’s Hold Timer reaches 0?
- BGP session is torn down
- Routes from that neighbor are removed
- An UPDATE route withdraw message sent to other BGP neighbors
How is the Hold Timer negotiated between two neighbors?
Both routers propose a Hold Time in the OPEN message. The shortest of the two is agreed upon.
What are the 2 criteria for Hold times?
- Must be at least 3 seconds
- Must be 3 times the Keepalive interval
What are 4 characteristics about the BGP Identifier?
- It is the router ID.
- 32-bit UNIQUE number
- Must be non-zero or neighborships won’t establish
- Can be set manually or dynamically
What is a heartbeat mechanism that ensures a neighbor is up and healthy?
The Hold timer.
How often are BGP KEEPALIVE messages sent?
By default they are exchanged every 60 seconds or 1/3 of the hold time.
When are 4 times when a NOTIFICATION message is sent?
- An error in the BGP session is detected
- Hold Timer expired
- Neighbor capabilities changed
- BGP session reset was requested
What does SAFI stand for?
Subsequent Address Family Identifier
What 2 facts are true about Well-known Mandatory attributes?
- Must be recognized by all BGP speakers
- Must be included in all update messages
What kind of BGP attribute is AS_PATH?
Well-known mandatory
What is contained in the AS_PATH attribute?
The list of all autonomous systems the prefixes contained in this update have passed through.
How does the AS_PATH attribute prevent loops?
If a router receives an NLRI with that router’s own AS listed the router assumes a loop and drops the UPDATE.
What happens if the HOLD timer is set to 0?
Keep-alives are disabled.
What does NLRI stand for?
Network Layer Reachability Information
What 3 things does an UPDATE message do?
- Advertises feasible routes
- Withdraws previously advertised routes (prefix only)
- Can function as a keep-alive to reduce bandwidth
What is the default HOLD timer interval?
- 180 seconds (3 times the KEEP-ALIVE value)
What is the HOLD timer?
A countdown timer for each neighbor.
What 2 things can reset the HOLD timer?
- An UPDATE message
- A KEEP-ALIVE
What is the purpose of the HOLD timer?
To make sure the neighbor is alive.
How can the Router ID be used to detect loops?
The Router ID appears in all UPDATE messages. If a router receives an UPDATE message with its own Router ID in it then there’s a loop.
How is eBGP different from iBGP in regards to their BGP peers?
eBGP router expects its peer to be in a different AS. iBGP router expects its peer to be in the same AS.
How is eBGP different than iBGP when it comes to TTL and what effect does it have?
- eBGP TTL is 1, iBGP TTL is 255
- eBGP’s TTL of 1 prevents neighborships over multiple hops.
What does an eBGP router do to prevent loops that an iBGP router does not?
- eBGP router prepends its own AS to any route it advertises.
- When the eBGP router receives a route that has its own AS number in the AS Path it discards it.
What does a BGP NOTIFICATION message do?
It causes the BGP session to close.
How many NLRIs and withdrawn routes in an update message?
Only 1 NLRI per update message but it can have multiple withdrawn routes
