BGP Flashcards
What is a BGP autonomous system
A series of routers under the same administrative domain
How does BGP fundamentally determine the shortest path to a prefix.
By using a route with the lowest amount of AS tags
How does BGP prevent routing loops?
If a route is learned containing the local AS number (aka looped), the route is discarded.
What is external BGP?
The peering of routers in different AS numbers
Which TCP ports does BGP use for session establishment?
TCP 179
What are the 6 states of BGP peering?
Idle Connect Active Opensent Openconfirm Established
What happens in the BGP ‘Idle’ state?
BGP listens and attempts to establish a connection over tcp 179
What happens in the BGP ‘Connect’ state?
BGP waits for the connection process to complete. Once finished the interface with the higher IP address will manage the connection state.
What happens in the BGP ‘Active’ state?
The higher ip router (active) Initiates a new tcp 179 connection. The passive router waits for this session to begin so routes can be shared.
What happens in the BGP ‘Opensent’ state?
BGP config is checked and verified between peers. Source IPs and AS numbers are checked and router ID is confirmed unique.
What happens in the BGP ‘Openconfirm’ state?
Both peers wait to receive their keep alive messages. The default keep alive time is 60 secs. Dead time is 180 secs.
What is the default keep alive and dead timer interval for BGP?
60 seconds keep alive
180 seconds dead timer
What happens in the BGP ‘Established’ state?
Peer keeps lives are flowing and routes are exchanged.
What weight does BGP assign a locally originated route?
32,768
What 8 variables determine BGP path selection?
Weight Local preference Shortest AS path Origin type Multi-exit discriminator eBGP over iBGP Lowest IGP cost to next BGP hop Oldest route Lowest RID
