Introduction to BGP

ISP Workshops

Last updated 4th September 2016 1

Border Gateway Protocol
p  A Routing Protocol used to exchange routing
information between different networks
n  Exterior gateway protocol
p  Described in RFC4271
n  RFC4276 gives an implementation report on BGP
n  RFC4277 describes operational experiences using BGP
p  The Autonomous System is the cornerstone of
BGP
n  It is used to uniquely identify networks with a common
routing policy

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BGP
p  Path Vector Protocol
p  Incremental Updates
p  Many options for policy enforcement
p  Classless Inter Domain Routing (CIDR)
p  Widely used for Internet backbone
p  Autonomous systems

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Path Vector Protocol
p  BGP is classified as a path vector routing
protocol (see RFC 1322)
n  A path vector protocol defines a route as a
pairing between a destination and the
attributes of the path to that destination.

12.6.126.0/24 207.126.96.43 1021 0 6461 7018 6337 11268 i

AS Path

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Path Vector Protocol
AS11268
AS6337

AS7018

AS500

AS6461

AS600
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Definitions
p  Transit – carrying traffic across a network
n  (Commercially: for a fee)
p  Peering– exchanging routing information
and traffic
n  (Commercially: between similar sized
networks, and for no fee)
p  Default– where to send traffic when there
is no explicit match in the routing table

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Default Free Zone
The default free zone is made
up of Internet routers which
have routing information about
the whole Internet, and
therefore do not need to use a
default route
NB: is not related to where an
ISP is in the hierarchy
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 Peering and Transit example
transit
provider A Backbone provider E
Provider C

peering IXP-West peering peering IXP-East

Backbone
provider B Provider D
provider F

transit

A and B peer for free, but need

transit arrangements with C and D
to get packets to/from E and F 8
Autonomous System (AS)
AS 100

p  Collection of networks with same routing policy

p  Single routing protocol
p  Usually under single ownership, trust and
administrative control
p  Identified by a unique 32-bit integer (ASN)
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Autonomous System Number
(ASN)
p  Two ranges
0-65535 (original 16-bit range)
65536-4294967295 (32-bit range – RFC6793)
p  Usage:
0 and 65535 (reserved)
1-64495 (public Internet)
64496-64511 (documentation – RFC5398)
64512-65534 (private use only)
23456 (represent 32-bit range in 16-bit world)
65536-65551 (documentation – RFC5398)
65552-4199999999 (public Internet)
4200000000-4294967295 (private use only – RFC6996)
p  32-bit range representation specified in RFC5396
n  Defines “asplain” (traditional format) as standard notation
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Autonomous System Number
(ASN)
p  ASNs are distributed by the Regional Internet
Registries
n  They are also available from upstream ISPs who are
members of one of the RIRs
p  The entire 16-bit ASN pool has been assigned to
the RIRs
n  Around 43000 16-bit ASNs are visible on the Internet
p  Each RIR has also received a block of 32-bit ASNs
n  Out of 15300 assignments, around 11800 are visible on
the Internet
p  See www.iana.org/assignments/as-numbers

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Configuring BGP in Cisco IOS
p  This command enables BGP in Cisco IOS:
router bgp 100
p  For ASNs > 65535, the AS number can be
entered in either plain or dot notation:
router bgp 131076
n Or
router bgp 2.4
p  IOS will display ASNs in plain notation by default
n  Dot notation is optional:

router bgp 2.4

bgp asnotation dot
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BGP Basics
Peering

A C

AS 100 AS 101

B D

E
p  Runs over TCP – port 179
p  Path vector protocol AS 102
p  Incremental updates
p  “Internal” & “External” BGP
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Demarcation Zone (DMZ)
A C
DMZ
AS 100 Network AS 101
B D

AS 102

p  DMZ is the link or network shared between ASes

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BGP General Operation
p  Learns multiple paths via internal and
external BGP speakers
p  Picks the best path and installs it in the
routing table (RIB)
p  Best path is sent to external BGP
neighbours
p  Policies are applied by influencing the best
path selection

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Constructing the Forwarding Table
p  BGP “in” process
n  Receives path information from peers
n  Results of BGP path selection placed in the BGP table
n  “best path” flagged
p  BGP “out” process
n  Announces “best path” information to peers
p  Best path stored in Routing Table (RIB) if:
n  Prefix and prefix length are unique, and
n  Lowest “protocol distance”
p  Best paths in the RIB are installed in forwarding
table (FIB)
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 Constructing the Forwarding Table
discarded
BGP in
in
process accepted
everything

bgp BGP routing

peer table table

best paths
BGP out forwarding
out table
process

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eBGP & iBGP
p  BGP is used
n  Internally (iBGP)
n  Externally (eBGP)

p  iBGP used to carry

n  Some/all Internet prefixes across ISP
backbone
n  ISP’s customer prefixes

p  eBGP used to
n  Exchange prefixes with other ASes
n  Implement routing policy
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BGP/IGP model used in ISP
networks
p  Model representation

eBGP eBGP eBGP

iBGP iBGP iBGP iBGP

IGP IGP IGP IGP

AS1 AS2 AS3 AS4

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External BGP Peering (eBGP)

AS 100 AS 101
C

p  Between BGP speakers in different AS

p  Should be directly connected
p  Never run an IGP between eBGP peers
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Configuring External BGP
p  Router A in AS100 ip address on
ethernet interface

interface FastEthernet 5/0

ip address 102.102.10.2 255.255.255.240
!
router bgp 100 Local ASN
network 100.100.8.0 mask 255.255.252.0
neighbor 102.102.10.1 remote-as 101 Remote ASN
neighbor 102.102.10.1 prefix-list RouterC-in in
neighbor 102.102.10.1 prefix-list RouterC-out out
!

Inbound and
ip address of Router C outbound filters
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ethernet interface

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Configuring External BGP
p  Router C in AS101 ip address on
ethernet interface

interface FastEthernet 1/1/0

ip address 102.102.10.1 255.255.255.240
!
router bgp 101 Local ASN
network 100.100.64.0 mask 255.255.248.0
neighbor 102.102.10.2 remote-as 100 Remote ASN
neighbor 102.102.10.2 prefix-list RouterA-in in
neighbor 102.102.10.2 prefix-list RouterA-out out
!

Inbound and
ip address of Router A outbound filters
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ethernet interface

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Internal BGP (iBGP)
p  BGP peer within the same AS
p  Not required to be directly connected
n  IGP takes care of inter-BGP speaker
connectivity
p  iBGP speakers must be fully meshed:
n  They originate connected networks
n  They pass on prefixes learned from outside the
ASN
n  They do not pass on prefixes learned from
other iBGP speakers
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Internal BGP Peering (iBGP)
AS 100
B
A
C

p  Topology independent
p  Each iBGP speaker must peer with every other
iBGP speaker in the AS 24
Peering between Loopback Interfaces
AS 100

C
A

p  Peer with loop-back interface

n  Loop-back interface does not go down – ever!
p  Do not want iBGP session to depend on state of
a single interface or the physical topology 25
Configuring Internal BGP
p  Router A in AS100 ip address on
loopback interface

interface loopback 0
ip address 105.3.7.1 255.255.255.255
!
router bgp 100 Local ASN
network 100.100.1.0
neighbor 105.3.7.2 remote-as 100 Local ASN
neighbor 105.3.7.2 update-source loopback0
neighbor 105.3.7.3 remote-as 100
neighbor 105.3.7.3 update-source loopback0
!
ip address of Router B
loopback interface

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Configuring Internal BGP
p  Router B in AS100 ip address on
loopback interface

interface loopback 0
ip address 105.3.7.2 255.255.255.255
!
router bgp 100 Local ASN
network 100.100.1.0
neighbor 105.3.7.1 remote-as 100 Local ASN
neighbor 105.3.7.1 update-source loopback0
neighbor 105.3.7.3 remote-as 100
neighbor 105.3.7.3 update-source loopback0
!
ip address of Router A
loopback interface

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Inserting prefixes into BGP
p  Two ways to insert prefixes into BGP
n  redistribute static
n  network command

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Inserting prefixes into BGP –
redistribute static
p  Configuration Example:
router bgp 100
redistribute static
ip route 102.10.32.0 255.255.254.0 serial0

p  Staticroute must exist before redistribute

command will work
p  Forces origin to be “incomplete”
p  Care required!

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Inserting prefixes into BGP –
redistribute static
p  Care required with redistribute!
n  redistribute <routing-protocol> means
everything in the <routing-protocol> will be
transferred into the current routing protocol
n  Will not scale if uncontrolled
n  Best avoided if at all possible
n  redistribute normally used with “route-maps”
and under tight administrative control

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Inserting prefixes into BGP –
network command
p  Configuration Example
router bgp 100
network 102.10.32.0 mask 255.255.254.0
ip route 102.10.32.0 255.255.254.0 serial0

p  A matching route must exist in the routing

table before the network is announced
p  Forces origin to be “IGP”

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Configuring Aggregation
p  Three ways to configure route aggregation
n  redistribute static
n  aggregate-address
n  network command

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Configuring Aggregation –
Redistributing Static
p  Configuration Example:
router bgp 100
redistribute static
ip route 102.10.0.0 255.255.0.0 null0

p  Static route to “null0” is called a pull up

route
n  Packets only sent here if there is no more
specific match in the routing table
n  Care required – see previously!

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Configuring Aggregation –
Network Command
p  Configuration Example
router bgp 100
network 102.10.0.0 mask 255.255.0.0
ip route 102.10.0.0 255.255.0.0 null0

p  A matching route must exist in the routing

table before the network is announced
p  Easiest and best way of generating an
aggregate

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Configuring Aggregation –
aggregate-address command
p  Configuration Example:
router bgp 100
network 102.10.32.0 mask 255.255.252.0
aggregate-address 102.10.0.0 255.255.0.0 [summary-only]
!
ip route 102.10.32.0 255.255.252.0 null 0

p  Requires more specific prefix in BGP table

before aggregate is announced
p  summary-only keyword
n  Optional keyword which ensures that only the
summary is announced (the more specific
routes are suppressed)
 Summary
BGP neighbour status
Router6>sh ip bgp sum
BGP router identifier 10.0.15.246, local AS number 10
BGP table version is 16, main routing table version 16
7 network entries using 819 bytes of memory
14 path entries using 728 bytes of memory
2/1 BGP path/bestpath attribute entries using 248 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1795 total bytes of memory
BGP activity 7/0 prefixes, 14/0 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

10.0.15.241 4 10 9 8 16 0 0 00:04:47 2
10.0.15.242 4 10 6 5 16 0 0 00:01:43 2
10.0.15.243 4 10 9 8 16 0 0 00:04:49 2
...

BGP Version Updates sent Updates waiting 36

and received
 Summary
BGP Table
Router6>sh ip bgp
BGP table version is 16, local router ID is 10.0.15.246
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path

*>i 10.0.0.0/26 10.0.15.241 0 100 0 i
*>i 10.0.0.64/26 10.0.15.242 0 100 0 i
*>i 10.0.0.128/26 10.0.15.243 0 100 0 i
*>i 10.0.0.192/26 10.0.15.244 0 100 0 i
*>i 10.0.1.0/26 10.0.15.245 0 100 0 i
*> 10.0.1.64/26 0.0.0.0 0 32768 i
*>i 10.0.1.128/26 10.0.15.247 0 100 0 i
*>i 10.0.1.192/26 10.0.15.248 0 100 0 i
*>i 10.0.2.0/26 10.0.15.249 0 100 0 i
*>i 10.0.2.64/26 10.0.15.250 0 100 0 i
... 37
Summary
p  BGP4 – path vector protocol
p  iBGP versus eBGP
p  Stable iBGP – peer with loopbacks
p  Announcing prefixes & aggregates

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Introduction to BGP
ISP Workshops

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