Lecture 3: BGP Breakdown Flashcards
How is a TCP connection established between 2 routers?
- Routers send an open message (a hello)
What information do routers exchange in an open message?
- ASN
- BGP version and Router ID
- The connection keepalive time interval
What are the 4 types of BGP messaging?
Open, Keepalive, Update, and Notification
What is a Keepalive message?
- Contains no data
- Determines if a BGP router is still “alive”
- Timing of these messages based on time interval in the open message
it is common to use a window of
___ the stated keepalive interval before declaring a router “dead”
3-4x
What is the Update message?
It is used to share path
information
Each update message informs a neighbour about:
- A path to some block of IP addresses
- The attributes of that path
- Any no-longer-valid routes to that particular block
What are the 3 parts of update messages?
- The Network Layer Reachability Information (NLRI)
- The path attributes
- Any withdrawn routes
What is the Network Layer Reachability Information (NLRI)
Defines where a particular path connects to, in terms of network (IP) address
Typically takes the form of an IP address in CIDR notation
What are Path Attributes?
Important data values related to a particular path
What are the 3 classes that path attributes can be broadly split into?
- Mandatory
- Well-Known
- Transitive
What is the Notifiation message?
A notification is sent when the router has detected an error
What happens when a router receives a notification message?
Receiing router immediately terminates the BGP connection between routers to prevent further errors
What 2 classes do path attributes fall into?
- Well known
- Mandatory
What are well-known attributes?
Attributes must be recognized
by all routers
What are mandatory attributes?
Attributes must accompany any BGP update – they cannot be omitted
What is the Origin Attribute?
- Well known & mandatory
- Describes how a particular path
was learnt - 2 valid values: IGP, Incomplete
What is the Next Hop Attribute?
- Well known & mandatory
- Describes the next external IP address to connect to in the path
What happens to the next hop attribute when a route is advertised from one gateway router to another (in a different AS)?
The sender updates the Next Hop attribute to point to themselves
When the path is shared internally the next hop is ___
Not updated
What is a looking glass server?
- Operated by a network provider
- Allows you to see current routing tables
