BGP

Question 1

Autonomous System (AS)

Answer 1

Collection of networks with same routing policy:
The Autonomous System is used to uniquely identify networks with a common
routing policy
Usually under single ownership, trust and administrative
control
Identified by a unique 32-bit integer

Question 2

Autonomous System Number (ASN)

Answer 2

ASNs are distributed by the Regional Internet Registries(RIR)
They are also available from upstream ISPs who are members of one
of the RIRs

Question 3

Border Gateway Protocol (RFC4271)

Answer 3

BGP is an exterior gateway protocol (EGP)
that is used to exchange routing information
among routers in different autonomous
systems (ASs).

BGP routing information includes the
complete route to each destination.

BGP uses the routing information to
maintain a database of network reachability
information, which it exchanges with other
BGP systems.

Multiprotocol BGP (MBGP) extensions
enable BGP to support IP version 6 (IPv6).

Question 4

Internet & BGP

Answer 4

• Hierarchical structure
• At center: “tier-1” ISPs or Internet backbone networks (e.g., MCI, Sprint, AT&T, Cable and
Wireless), national/international coverage, connect to large tier-2 ISPs and to all tier-1 ISPs and
many customer networks.

Question 5

BGP Path-Vector Routing

Answer 5

IGPs announce networks and describe the metric to reach those networks.
BGP announces paths and the networks that are reachable at the end of the
path.

BGP describes the path by using attributes, which are similar to metrics.

BGP allows administrators to define policies or rules for how data will flow
through the autonomous systems.

While routing protocols such as RIP, OSPF, and EIGRP each have their own
metrics used to pick the “best” path to a destination network, BGP uses a
collection of path attributes (PAs).

Question 6

BGP Routing Policies

Answer 6

The “hop-by-hop” routing paradigm rule: an AS advertises to its neighbor ASs only those routes that it itself uses.

This rule reflects the “hop-by-hop” routing paradigm generally used
throughout the current Internet.

BGP can support any policy conforming to the “hop-by-hop” routing
paradigm.

Question 7

BGP Characteristics

Answer 7

BGP is most appropriate when at least one of the following conditions exists:
1.An AS allows packets to transit through it to reach other autonomous systems (for example, it is a service provider).
2.An AS has multiple connections to other autonomous systems.
3.Routing policy and route selection for traffic entering and leaving the AS must be
manipulated.

BGP is not always appropriate. You do not have to use BGP if you have one of the following conditions:
1.Limited understanding of route filtering and BGP path-selection process
2.A single connection to the Internet or another AS
3.Lack of memory or processor power to handle constant updates on BGP routers

Question 8

Types of BGP

Answer 8

iBGP (Internal BGP) peers are two routers talking BGP that are in the same AS. This means they are managed by the same entity (e.g. owned by the same company)

eBGP (External BGP) peers are two routers talking BGP that are in two different AS. So, they are managed by different entities.

Question 9

BGP Message Types

Answer 9

BGP defines the following message types:
Open
Includes hold time and BGP router ID
Keepalive
Periodic keepalive messages to verify TCP connectivity
Update
Incremental, triggered updates only
Information for one path only (could be to multiple networks)
Includes path attributes and networks
Notification
When error is detected
BGP connection closed after message is sent

Question 10

BGP States

Answer 10

1. Idle: Router is searching routing table to see
   whether a route exists to reach the
   neighbor.
2. Connect: Router found a route to the
   neighbor and has completed the three-way
   TCP handshake.
3. Active: router goes into active state if no
   response to open message
4. Open sent: Open message sent, with the
   parameters for the BGP session.
5. Open confirm: Router received agreement
   on the parameters for establishing session.
6. Established: Peering is established; routing
   begins.

Question 11

Next Hop Attribute

Answer 11

Well-known, mandatory attribute
Next hop to reach a network
Usually a local network is the next hop in eBGP session
Allows IGP to make intelligent forwarding decision

Question 12

Origin Attribute

Answer 12

• The origin attribute informs all autonomous systems in the internetwork how the prefixes were introduced into BGP.
• The origin attribute is well-known, mandatory.

Question 13

Local Preference Attribute

Answer 13

Paths with highest local preference value are preferred:
Local preference is used to advertise to IBGP neighbors about how to leave
their AS (determines best path for outbound traffic)
The local preference is sent to IBGP neighbors only (that is, within the AS
only).
The local preference attribute is well-known and discretionary.
Default value is 100.

Question 14

MED Attribute

Answer 14

• The paths with the lowest MED (also called the metric) value are the most
desirable: – MED is used to advertise to EBGP neighbors how to exit their AS to reach
networks owned by this AS (determines best path for inbound traffic) – AS 65000 advertise two diffrent MED values to neighbours
• The MED attribute is optional and nontransitive.
• Influence inbound traffic

Question 15

Weight Attribute (Cisco Only)

Answer 15

Paths with the highest weight value are preferred
• Weight not sent to any BGP neighbors; local to this router only
• Influence outbound routing decision

Question 16

Definitions

Answer 16

Transit – carrying traffic across a network, usually for a fee
Peering – exchanging routing information and traffic
Default – where to send traffic when there is no explicit match
in the routing table

Question 17

Internal BGP Peering (iBGP
)

Answer 17

•BGP peer within the same AS
•Not required to be directly connected
Topology independent
Each iBGP speaker must peer with every other iBGP speaker
in the AS