Attributes are one of the most important things concerning BGP. With attributes almost everything can be controlled.
There are diefferent types of attributes:
- Welll-known-attributes
- mandatory
- Origin
- AS_Path
- Next_Hop
- discretionary
- Local_Preference
- Atomic_Aggregate
- Operational-attributes
- transitiv
- Aggregator
- Community
- non-transitiv
- Multi_Exit_Descriminator
- Propriatary-attributes
- Weight (Cisco specific)
Description of Attributes:
Origin
- Type Code 1
- Well known, mandatory
- Identifies the route was originally learned from by BGP
- 0 – by an IGP
- 1 – by an EGP
- 2- not complete /unrelaiable
AS_Path
- Type Code 2
- Well known, mandatory
- Lists ASs between local and destination
Next_Hop
- Well known, mandatory
- Type Code 3
- Identifies next hop to destination
- Next_Hop = IP address of peer
MED – Multi-Exit Discremitnator (MED)
- Type Code 4
- Optional, non-transitiv
- AKA „metric“
- Indicats preference of one route into an AS over another
- Only shared with neighboring AS’s
- Lowest MED is preferred
Here in this example the MED 100 is preferred as the host with the better Path.
MED ist configured with the „default-metric [number]
Local_Preference
- Type Code 5
- Well known, discretionary
- Indicates preference of one route over another
- Within AS only
- Higher Local_Preferance preferred
- Cisco’s default is 100
- It’s setup with a route-map
If there are two path’s from an AS to another and you can setup the local preference for your AS, you can set the Local_Preference to a lower value to become the one route more preferable. So this can be used to optimize the way routers communicate.
Atomic_Aggregate
- Type Code 6
- Well known, discretionary
- Automatically sent when two overlapping routes are received
- Less specific route is chosen
- More specific route is not chosen
Aggregator
- Type Code 7
- Optional and transitiv
- Any router that aggregates routes can use this attribute
- Attribute contains
- Aggregators AS number
- Aggregators IP address
Community
- Definded in RFC 1997
- Type Code 8
- Optional and transitiv
- Allows for grouping of destinations
- Apply policies to the group
- Transitiv but dropped by default
- Predefined communities
- No_Export – Do not advertise to EBGP peers
- No_Advertise – Advertise to no one
- Internet – Advertise to everyone
Weight
- Cisco defined
- Non-transitiv
- Wight assigned on peer-by-peer basis
- Integer value (0-65535)
- Higher values are preferred
- Default is 32768 for originated local routes
- Default for other routes is 0
- BGP config command: neighbor [ip] weight [weight]