Chap 12 - Advanced BGP (part 2) Flashcards
List the 13 Best Path Decision Makers and path attributes in order that are examined to determine the best path.
- Weight * Local Pref
- Local Originated
- AIGP
- Shortest AS_Path
- Origin Type
- Lowest MED
- eBGP or iBGP
- Lowest IGP next hop
- If both paths are eBGP then the oldest route
- Prefer the route that comes from the Peer with the lowest RID
- Lowest Cluster List Length
- Lowest neighbor address
What is path attribute Weight?
- 16-bit value
- Assigned locally on the router
- Not advertised to other routers
- Higher weight is preferred
- Only influences outbound traffic
- It’s the first attribute examined so it over-rides other attributes.
If Weight is not assigned what is the default value?
0
What is path attribute Local_Pref?
- Well-known Discretionary
- iBGP only
- Included with advertisements throughout an AS
- Indicates the preference for exiting the AS
- Not advertised to eBGP peers
- Higher value preferred
- Default value is 100
- Can be set for specific routes with route-map
What is the path attribute Local Originated?
- Determines if the route originated locally
- Preference is given in the following order
- routes advertised locally
- networks aggregated locally
- routes received by BGP peers
What is the path attribute AIGP?
- Accumulated Interior Gateway Protocol
- Optional Non-transitive
- Used btw multiple AS’s under same routing domain
- When IGP is redistributed into BGP AIGP can accept the OSPF metric as this path attribute and advertise it to the other AS’s
What are the 4 AIGP guidelines?
- A path with an AIGP metric is preferred to one without
- AIGP metric includes the derived metric + the metric to the next hop
- Lowest metric wins
- If multiple AIGP paths exist and one next hop address contains an AIGP metric and the other doesn’t then the one without the AIGP metric will not be used
What is the path attribute AS_Path?
- A list of AS’s a packet has to go through to get to the destination
- Basically a hop count
- Shorter the list the better
- Typically the AS path is prepended with the network owner’s ASN
How is the AS_Path prepended to affect inbound traffic?
Create a route-map targeting a network being advertised outward and ‘set as-path prepend (ASN) (ASN) (ASN)’. Then apply the route-map to the BGP peer ending with keyword ‘out’
What is the path attribute Origin Type?
- Well-known Mandatory
- Networks advertised with the ‘network’ stmt assigned ‘I’ for IGP
- Networks redistributed into BGP assigned ‘incomplete’ or ‘?’ * Preference order:
- 1) IGP origin
- 2) EGP origin
- 3) Incomplete origin *
What is the path attribute MED ?
- Multiple-exit Discriminator
- Non-transitive
- Default is 0
- Used in iBGP only
- Should only be used if there are 2 links to the same AS
- Uses 32-bit value called metric
- Lower value is better
- Applied with a route map
What is the Best Path decision maker is eBGP over iBGP
- Prefer eBGP first
- Confederation members second
- iBGP peers last
What is the Best Path decision maker Lowest IGP metric to the next hop?
Prefer the path with the lowest IGP metric to the BGP next hop.
What is the Best Path decision maker Oldest EGP route
When 2 paths are eBGP prefer the path that is oldest.
What is the Best Path decision maker Peer with the lowest RID
Prefer the path that comes from the BGP peer with the lowest RID
