11. BGP Flashcards
What is the private range of BGP?
64512-65535
4.200.000.000-4.295
What are the 4 path attributes?
Well-known mandatory
Well-known discretionary
Optional transitive
Optional non-transative
What is the difference between well-known mandatory and well-known discretionary?
Must be recognized by al BGP implementations
Must be included with prefix advertisement
May or may not be included with a prefix advertisement
What is the difference between optional transitive and optional non-transative?
Do not have to be recognized by BPG implementations
Transitive will stay with the route advertisement
What is the NLRI?
Network layer reachability Information, a routing update that Contains: Prefix, prefix length and PAs
What is the AS_path attribute?
Well-known mandatory attribute that remembers the ASs is went through to prevent loops
What did multi protocol BGP added?
AFI - address family identifier - IPv4 of IPv6
SAFI - unicast multicast
How does MBGP achieves seperation?
With the attributes MP_REACH_NLRI and MP_UNREACH_NLRI
What port does BGP use?
TCP 179
Can BGP multihop session be esthablished?
Yes, but a underlaying routing protocol is needed
What are the 2 types if BGP sessions?
iBPG - within an AS, AD 200 TTL 255
eBGP - different AS, AD 20 TTL 1
What are the 4 BGP packet types?
Open
Update
Notification
Keepalive
What is the default BGP hold timer?
180 for Cisco
keepalive /3
What is a neighbor router called?
Peer
What is the Finite-State Mechanism?
Table BGP uses to maintain all peers and their operational status
What are the 6 states?
Idle Connect Active OpenSent OpenConfirm Established
What happens in the Idle state?
A TCP is initiated to a peer and it listen for a peer. If an error causes BGP to go back to Idle for the second time the ConnectRetryTimer is set to 60.
Will double every fail after this.
What happens in the Connect state?
After the three way handshake, CRt is reset and Open message is send to the peer —> OpenSent state
What happens in the Active state?
New three way handshake, hold timer set to 4 minutes
What happens in the OpenSent state?
- Version match
- AS match
- IP match
- RID unique
- Security parameters
If error –> Idle
What happens in the OpenConfirm state?
Waits for keepalive or Notification
Keepalive –> Established
Notification or hold timer –> Idle
What do keepalives an update messages do?
They reset the hold timer
What is the command for BGP routing activation?
router BPG as-number
neighbor ip remote-as as
What happens when you change the RID?
All BGP session are being reset
How to disable default address family?
no bgp default ipv4-unicast
What command is used to view active BGP sessions?
show bgp afi safi summary
What are the prefix types for the local RIB table?
Connected –> 0.0.0.0
Static/RP –> next hop IP
What are the three BGP tables?
Adj-rib-in
Local-rib
Adj-rib-out
What are the BGP route processing steps?
- Store the route in Adj-rib-in and apply inbound policy
- Update local-rib and clear Adj-rib-in
- Check global RIB if visible, if not leave in local-rib
- Identify best path
- Install in global RIB and store in Adj-rib-out
What is the command to show the adj-rib-out table?
show bpg afi safi neighbor ip advertised-routes
What is the command to pick up OSPF in BGP?
redistribute OSPF processid
What is the command to summarize in BGP
aggregate-address ip subnet summary-only
What is BGP?
A path vector routing protocol that does not contain a complete topology of the network
What does a UPDATE message contain?
The NLRI. Withdrawn NLRI contains only prefix. Can act as a keepalive.
When is a NOTIFICATION message sent?
When an error is detected or reset is requested
