BGP Path Attributes
BGP Path Attributes
BGP (Border Gateway protocol is a path vector routing protocol, meaning it uses path attributes to determine the best routing path. There are eight (8) attributes that BGP uses in the determination of that best path.
| Mnemonic | First Letter | BGP Path Attribute |
| We | W | Weight |
| Love | L | Local_pref |
| Oranges | O | Originate |
| As | A | AS_path |
| Oranges | O | Origin type |
| Mean | M | MED (Multiple Exit Discriminator) |
| Pure | P | Paths |
| Refreshment | R | Router-ID |
This is not inclusive of all 13 path attributes but this covers the majority of them. particularly those that we often see used in practice.
WEIGHT
- Cisco specific parameter
- Local to the router
- Preference: highest weight
LOCAL PREFERENCE (Local_pref)
This parameter often gets used for route manipulation.
- Default value of 100
- Preference: highest local_pref
ORIGINATE
- How a path was sourced
- Preference: local paths from network or redistribute commands are preferred versus local aggregates via aggregate-address command
AUTONOMOUS SYSTEM PATH (AS_path)
- The number of autonomous systems in the path
- Preference: shortest AS_path
- It is the number of autonomous systems in the path NOT the number of routers in the path.
ORIGIN TYPE
- Preference: lowest origin type
- IGP
- EGP
- ? Incomplete
MED (Multi-exit Discriminator)
- Optional non-transitive attribute
- A hint to external neighbors about the preferred path into an autonomous system (AS) that has multiple entry points
- Preference: lowest MED
PATHS (eBGP vs iBGP)
- Preference: eBGP over iBGP
| eBGP | iBGP |
| Administrative distance: 20 | Administrative distance: 200 |
ROUTER-ID
- Preference: route from the lowest router ID
Summary of the Attributes
| Mnemonic | First Letter | BGP Path Attribute | Preference |
| We | W | Weight | Higher |
| Love | L | Local_pref | Higher |
| Oranges | O | Originate | Local vs Aggregate |
| As | A | AS_path | Lower |
| Oranges | O | Origin type | IGP vs EGP vs ? |
| Mean | M | MED | Lower |
| Pure | P | Paths | eBGP vs oBGP |
| Refreshment | R | Router-ID | Lower |
BGP Route Manipulation
Scenario: Make the preferred path for the 20.0.0.0 network go through another router
R4(config)#ip prefix-list PREF20 permit 20.0.0.0/24
R4(config)#route-map MAP20 permit 10
R4(config-route-map)#match ip address prefix-list PREF20
R4(config-route-map)#set ?