BGP Fundamentals Flashcards
Autonomous System Numbers (ASNs)
An organization requiring connectivity to the Internet must obtain an ASN.
ASNs were originally 2 bytes (in the 16-bit range), which made 65,535 ASNs possible.
Due to exhaustion, RFC 4893 expanded the ASN field to accommodate 4 bytes (in the 32-bit range). This allows for 4,294,967,295 unique ASNs, providing quite an increase from the original 65,535 ASNs.
Private ASNs
Two blocks of private ASNs are available for any organization to use as long as they are
never exchanged publicly on the Internet.
ASNs 64,512–65,535 (within the 16-bit ASN range)
ASNs 4,200,000,000–4,294,967,294 (within the extended 32-bit range)
Path Attributes
BGP attaches path attributes (PA) associated with each network path.
The PAs provide BGP with granularity and control of routing policies within BGP.
- Well-known mandatory
- Well-known discretionary
- Optional transitive
- Optional non-transitive
Well-known attributes must be recognized by all BGP implementations.
Well-known mandatory attributes must be included with every prefix advertisement,
Well-known discretionary attributes may or may not be included with the prefix
advertisement.
Optional attributes do not have to be recognized by all BGP implementations.
Transitive stays with the route advertisement from AS to AS.
Non-Transitive cannot be shared from AS to AS.
NLRI
The Network Layer Reachability Information (NLRI) is the routing update that consists of
the network prefix, prefix length, and any BGP PAs for that specific route.
Loop Prevention
BGP is a path vector routing protocol and does not contain a complete topology of
the network like link-state routing protocols. BGP behaves similar to distance vector
protocols to ensure a path is loop free.
The BGP attribute AS_PATH is a well-known mandatory attribute and includes a
complete listing of all the ASNs that the prefix advertisement has traversed from its
source AS.
The AS_PATH is used as a loop prevention mechanism in the BGP protocol.
If a BGP router receives a prefix advertisement with its AS listed in the AS_PATH, it
discards the prefix because the router thinks the advertisement forms a loop.
Address Families
BGP was intended for routing of IPv4 prefixes between organizations, but RFC 2858 added Multi-Protocol BGP (MP-BGP) capability by adding extensions called address-family identifier (AFI) and additional granularity through a subsequent address-family identifier (SAFI).
An address-family (AFI) correlates to a specific network protocol, such as IPv4, IPv6.
A subsequent address-family identifier (SAFI), such as unicast and multicast.
Every address-family maintains a separate database and configuration for each proto-
col (address-family + subaddress family) in BGP. This allows for a routing policy in one
address-family to be different from a routing policy in a different address family even
though the router uses the same BGP session to the other router
iBGP / eBGP Sessions
iBGP : Sessions established with an IBGP router that are in the same
AS or participate in the same BGP confederation. AD of 200.
eBGP: Sessions established with a BGP router that are in a different AS. AD of 20.
Administrative distance (AD) is a rating of the trustworthiness of a routing
information source. If a router learns about a route to a destination from more than one
routing protocol, and they all have the same prefix length, AD is compared. The preference
is given to the route with the lower AD.
Inter-Router Communication
BGP does not use hello packets to discover neighbors like IGP protocols and cannot
discover neighbors dynamically.
BGP uses TCP port 179 to communicate with other routers. TCP allows for handling
of fragmentation, sequencing, and reliability (acknowledgement and retransmission) of
communication packets.
IGP protocols follow the physical topology because the sessions are formed with hellos
that cannot cross network boundaries (that is, single hop only). BGP uses TCP, which is
capable of crossing network boundaries (that is, multihop capable).
While BGP can form neighbor adjacencies that are directly connected, it can also form adjacencies that are multiple hops away. Multihop sessions require that the router use an underlying route installed in the RIB (static or from any IGP) to establish the TCP session with the remote endpoint.
Direct and Multihop Sessions
R2 is able to form a BGP session with R4, even though it passes through R3.
R1 and R2 use a directly connected route to locate each other.
R2 uses a static route to reach the 10.1.34.0/24 network, R4 has a static route to reach the 10.1.23.0/24 network.
R3 is unaware that R2 and R4 have established a BGP session, even though the packets flow
through R3.
BGP neighbors connected via the same network use the ARP table to locate the
Layer 2 address of the peer. Multihop BGP sessions require route table information for
finding the IP address of the peer.
