BGP – Introduction

What protocol and port does BGP use to communicate with its neighbors?
BGP used TCP on port 179 for communication between neighbors.
How big can a BGP message be?
A BGP message can be a small as 19 bytes and as large as 4096 bytes
What are the four different messages types used by BGP?

– Open – Both sides send an open message immediately after the TCP session has been established. It conveys important information about the BGP speaker’s configuration and abilities.

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-; Update – Update messages lists withdrawn and new routes.


-; Notification – A notification message is generated when a fatal error condition arises.After transmitting the notification, the sender tears down the TCP connection.


-; Keepalive – Keepalive messages are sent when the connection is otherwise idle, to make sure the hold timer doesn’t expire.

Briefly list the different BGP states that a session can be in.

– Idle

– Connect

– Active

– OpenSent

– OpenConfirm

– Established

Describe the "Idle" BGP state.
The router isn’t trying to set up a BGP session, and if the neighbor were to attempt to create a session, the TCP connection would be refused. The router waits for a ;start; event, typically the user enabling BGP or adding a neighbor or an interface coming up.
Describe the ;Connect; BGP state.
In this state, the router waits for its own TCP session establishment attempt to complete, and it listens for incoming TCP sessions.
Describe the ;Active; BGP state.
BGP is waiting for a TCP session.
Describe the ;OpenSent; BGP state.
The open message has been sent, but an open message hasn’t yet been received from the neighbor.
Describe the "OpenConfirm" BGP state.
The open message from the neighbor has been received, but not yet the initial keepalive message that completes the BGP session setup phase.
Describe the "Established" BGP state.
The initial keepalive message has been received, and the session is now ready for transmission of update, keepalive, and notification messages.
What does a router do with a route that it receives from an external AS, but contrains its own AS in the path?
To prevent routing loops, a router ignores any routes it receives from a router in a neighboring AS that contains its own AS number.
What’s the purpose of the ;hold-down; route in the IP routing table?
If there is no matching route in the routing table, the prefix won’t be announced. Routing an address range to the Null0 interface makes sure BGP will always announce the prefix.
What command shows you information about a specific route in the BGP table?
show ip bgp ip_address
What command enables the soft reconfiguration option for inbound routes from a specific neighbor?
neighbor neighbor_ip soft-reconfiguration inbound
What command configures a neighbor for BGP?
neighbor neighbor_ip remote-as neighbor_as
What command softly refreshes the routes in both directions?
clear ip bgp neighbor_ip soft
What needs to be enabled to make it possible to ask a peer to resend all its routes?
Route refresh needs to be supported on both ends of the connection. You can verify this with the show ip bgp neighbors command.
What’s the purpose of the next-hop-self command?
The next hop command instructs the router to replace the next hop attribute in the route updates it sends to the peer with its own IP address. Normally, the route updates would specify the IP address of the router which it learned the route from.
Describe what the no synchonization command does on a router using BGP.
By default, Cisco routers don’t include routes they have learned through iBGP in the regular routing table until they see those same routes present in an IGP. The no synchonization command disable this setting.
Describe the fast-external-failover feature enabled by default on a BGP router.
The fast-external-failover allows the router to terminate the session as soon as the interface it runs over goes down.
What command allows you to have a BGP neighbor more than 1 hop away?
neighbor neighbor-ip ebgpmultihop hop-count