What are BGP - Optional Transitive attributes and what does that mean?
BGP speakers may or may not recognize the attribute. If transitive bit is set, BGP will pass these attributes along and mark them as “partial”.
• COMMUNITY
• AGGREGATOR
BGP - Well-Known Mandatory
MUST be recognized by ALL BPG speakers AND MUST be included in Update messages
• AS_PATH
• ORIGIN
• NEXT_HOP
BGP - Well-Known Discretionary
MUST be recognized by ALL BGP speakers, but may or may not be sent in Update messages.
• LOCAL_PREF
• Atomic Aggregate
BGP Communities
blah
BGP - Optional Non-Transitive
if the Transitive Bit is NOT set, the attribute will be ignored and NOT passed along to other BGP speakers:
• MED
• ORIGINATOR_ID
• CLUSTER_LIST
BGP - Best Path Algorithm ( we love oranges as oranges mean pure refreshment)
- Prefer the path with the highest WEIGHT. (Cisco Proprietary)
- Prefer the path with the highest LOCAL_PREF.
- Prefer the path that was locally originated (via a network or aggregate BGP subcommand or through redistribution from an IGP).
- Prefer the path with the shortest AS_PATH.
- Prefer the path with the lowest origin type. (IGP > EGP > Incomplete)
- Prefer the path with the lowest multi-exit discriminator (MED).
- Prefer eBGP over iBGP paths.
- Prefer the path with the lowest IGP metric to the BGP next hop. (Continue, even if bestpath is already selected.)
- Determine if multiple paths require installation in the routing table for BGP Multipath. (Continue, if bestpath is not yet selected.)
- When both paths are external, prefer the path that was received first (the oldest one).
- Prefer the route that comes from the BGP router with the lowest router ID.
- If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.
- Prefer the path that comes from the lowest neighbor address.
List the 4 categories for BGP attributes
- Well-known mandatory
- Well-known discretionary
- Optional transitive
- Optional nontransitive
Private AS BGP Range
AS 64512 to AS 65535
Weight Attribute
- Cisco Proprietary
- 1st in BGP algorithm
- Locally significant
- Influenced by updating received NLRI
iBPG Peering Rules - BGP Split Horizon rule
an iBGP peer can send NLRI to EXTERNAL peers, but CANNOT send NLRI to other iBGP peers. This is called the .
This is a loop prevention mechanism.
Exceptions include:
Route Reflection
Confederations
Can BGP peer with a neighbor that’s not in it’s routing table?
No.
And a default route DOES NOT count. BGP must have IP reach-ability outside of a default route to establish a peer
How is BGP Router-ID chosen?
- bgp router-id x.x.x.x command
- Highest numeric IP of any UP loopback interface.
- Highest numeric IP of any physical interface
Do eBGP peers have to be directly connected? Why or why not?
Yes, eBGP neighbors have to be directly connected because eBGP uses a TTL of 1 by default.
You can increase the TTL with the ‘ebgp-multihop x’ command.
You can also use ‘disable-connected-check’ if the peer IP is on a directly connected router, but not on the directly connected link.
Do iBGP peers have to be directly connected?
No.
iBGP peers do NOT have to be directly connected to establish peering.
BGP State - Idle
The BGP process is either administratively down or awaiting the next retry attempt.
BGP State - Connect
The BGP process has detected an incoming TCP connection request and is waiting for the TCP connection to be completed.
BGP State - Active
BGP has initiated an outbound TCP connection request and is waiting for the 3-way handshake to complete. BGP can enter this state either because:
- My router was the first router to initiate a connection (from Idle-to-Active)
- My router received an initial, inbound connection request that failed to complete the TCP handshake (Idle-Connect-Active)
BGP State - Open Sent
The TCP handshake was successful, and a BGP Open message has been sent to the peer, but the matching Open message has not yet been received yet.
BGP State - Open Confirm
A BGP Open message has been both sent to and received from the other router.
Does an advertising eBGP Neighbor change the next hop address for an advertisement?
What about an advertising iBGP neighbor?
• eBGP: next-hop IP address is changed by the advertising
router.
• iBGP: next-hop IP address is not changed by the
advertising router (configurable)
What is BGPs Loop Prevention Mechanism?
AS PATH
What are the states in the BGP Finite State Machine?
Idle
Connect
Active
OpenSent
OpenConfirm
Established
Adj-RIBs-In contains what kind of BGP routing information?
Processed or unprocessed?
How many Adj-RIBs-In are there?
How to see it? (command)
contains unprocessed received BGP routing information
» One (1) per neighbor/peer
» The BGP Best Path Selection Algorithm uses
the information in this table to determine “the
best” route
» Can be seen with the command:
• show ip bgp neighbor x.x.x.x received-routes
• show bgp ipv4 unicast neighbor x.x.x.x received-routes
Adj-RIBs-Out contains what kind of BGP routing information?
Processed or unprocessed?
How many Adj-RIBs-Out are there?
How to see it? (command)
contains all the
processed outgoing BGP routing
information
» One (1) per neighbor
» Can be seen by running commands:
• show ip bgp neighbor x.x.x.x advertised-routes
• show bgp ipv4 uni neighbor x.x.x.x advertised-routes
