BGP Flashcards
BGP ASN Ranges
test - test ack
- 16-bit integers btwn 1-65535
- 32-bit AS numbers created recently when 16-bit was full
- 1-64495 - Public ASNs
- 64496-64511 - reserved for documentation
- 64512-65535 - Private ASNs
BGP Traits
(5 points)
1) uses TCP 179
2) incremental, triggered updates
3) slow to converge
4) exchange network reachability info, called path vectors
5) advertises path of ASNs and the networks at the end of that path
BGP Data Structures
(Table Names)
1) BGP Neighbor Table
2) BGP Table
3) IP Routing Table
BGP Administrative Distance
1) EBGP AD = 20
2) IBGP AD = 200
BGP Message Types
1) Open
2) Keepalive
3) Update
4) Notification
BGP Open Message Contents
1) Version number - highest common version is used; most use BGPv4 now
2) AS Number - local router’s ASN. if it’s not what the peer expects, the BGP session is ended
3) Hold Time - time to wait between the successive keepalive and update msg from the sender. smaller hold time is accepted
4) BGP Router ID - 32-bit field that ID’s the sender. Determined the same way as in OSPF
5) Optional Parameters - Type, Length, Value (TLV) encoded.
BGP Update Message Contents
1) Withdrawn routes
2) Path Attributes
3) Network Layer Reachability Information (NLRI)
BGP Keepalive Messages
- If the negotiated hold time is 0, then periodic keepalives aren’t sent
- consists only of a message header
EBGP Neighbor Requirements
1) Different ASNs
2) Defined neighbors - a TCP session must be estabilished before starting BGP routing update exchanges
3) Reachability - by default, EBGP neighbors must be directly connected, and the IPs on that link must be reachable from each AS
BGP Route Selection Process
(High level)
1) Highest weight attribute (local to router)
2) Highest local preference attribute (global with AS)
3) Route originated by the local router (next hop = 0.0.0.0)
4) Shortest AS Path (Least number of AS in AS-Path attribute)
5) Lowest Origin attribute (IGP < EGP < incomplete)
6) Lowest MED attribute (exchanged between AS’s)
7) EBGP path over an IBGP path
8) (IBGP route) the path through the closest IGP neighbor (best IGP metric)
9) (EBGP route) Oldest EBGP path (neighbor with the longest uptime)
10) Lowest neighbor BGP router ID
11) Lowest neighbor IP address (multiple paths to the same neighbor)
BGP Path Attribute Types
- Well-known Mandatory
- Well-known Discretionary
- Optional Transitive
- Optional Non-Transitive
BGP Well-Known Mandatory Path Attributes
- required to be present for every route in every update
- Origin
- AS-Path
- Next-Hop
BGP Path Attribute - Origin
1) Well-Known Mandatory
2) set to “i” for IGP if injected with network command or via aggregation
3) set to “?” if injected with redistribution
4) “e” is deprecated
BGP Path Attribute - AS-Path
1) Well-Known Mandatory
2) sequence of AS numbers through which the network is accessed
3) local AS number is prepended each time the route crosses an AS boundary
4) the AS that originally injected the route into BGP is found at the right-most end of the AS path
5) router chooses the path with the least number of AS hops
6) used to avoid loops - the route is ignored if the router sees its own AS in the AS Path
BGP Path Attribute - Next-Hop
1) Well-Known Mandatory
2) indicates the IP address of the next-hop router
3) each router modifies the next-hop attribute as the route passes through the network