Transcript Document
Routing: Exterior Gateway Protocols
and Autonomous Systems
Border Gateway Protocol (BGP)
Reference
D. E. Comer, Internetworking with TCP/IP, ISBN 1-13018380-6, 4th Ed., Vol. 1, Ch. 15.
Network Architecture and Design
1
Autonomous System (AS)
AS = collection of networks under a single technical administration &
sharing the same routing policy
AS# can be 1 to 65535 (64512 – 65535 private)
Internal Gateway Protocols (IGPs) operate within an AS to ensure IP
connectivity within it
Exterior Gateway Protocols (EGPs) run between ASs to enable
routing policies between them
Network Architecture and Design
2
BGP Necessity
AS 20
(network X)
AS 10
(serv ice prov ider)
6
b
4k
Company A
(AS 1)
Q:
Q:
Q:
2M
ps
2 Mbps
bp
s
Company B
(AS 2)
how will the traffic between AS1 and AS20 flow
will AS 2 allow this traffic
how would you solve this problem with OSPF or EIGRP
Network Architecture and Design
3
BGP Characteristics
Distance-vector protocol with enhancements:
Reliable updates (TCP port 179)
Incremental, triggered updates only
Full BGP tables exchanged after conn.
setup
Only changes (delta) sent afterwards
Rich metrics (called path attributes)
Periodic keepalives to verify TCP connectivity
Designed to scale to huge internetworks
Full Internet Routing Table (FIRT) = 100.000
routes, 7000 ASs, 40MB approx.
Network Architecture and Design
4
BGP Characteristics
BGP session = TCP connection port 179
Two routers with BGP session established are
called peers or neighbors
No periodic updates
Triggered updates are batched and rate-limited
(every 5 seconds for internal peer, every 30
seconds for external peer)
Needs an IGP to provide TCP connectivity between
BGP peers
Network Architecture and Design
5
BGP Table and Routing Table
IGP/Static Routing
Table
BGP
Table
BGP
BGP has its own table, in addition to the IP routing
table
Information can be exchanged between the two
Network Architecture and Design
6
Sample BGP configurations
Customer connected to Internet Service Provider
(ISP)
Customer connected to several Service Providers
(multi-homed)
Service Provider networks (transit autonomous
systems)
Service Providers exchanging traffic at an exchange
point (e.g. AIX, MAE-East,…)
Backbone of large enterprise networks
Network Architecture and Design
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Configuration #1
Large customer or small ISP connecting
to the Internet
Leaf
autonom ous
sy stem
BGP
Network Architecture and Design
Internet
8
Configuration #2
Customer connecting to several service-providers
(multi-homed customer)
BGP is mandatory in this case
Public AS# needed for the customer
Provider-independent address space for the customer
BG
Internet
serv ice
prov ider #2
P
Internet
C ustom er
BG
P
Internet
serv ice
prov ider #1
Network Architecture and Design
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Configuration #3
Service providers exchanging traffic at exchange points
(*IX)
Internet
serv ice
prov ider #1
BG
P
P
BG
Internet
serv ice
prov ider #4
*IX
P
BG
Internet
serv ice
prov ider #2
BG
P
Internet
serv ice
prov ider #3
Network Architecture and Design
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Configuration #4
Transit AS (carrying traffic from other AS’es)
Leaf AS
BG
P
BGP
Transit AS
BG
Internet
P
Leaf AS
Network Architecture and Design
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BGP Message Types
OPEN
UPDATE
Advertise or withdraw routes
NOTIFICATION
Initialize communication
Response to an incorrect message
KEEPALIVE
Actively test peer connectivity
Network Architecture and Design
12
BGP Path Attributes
BGP metrics are called path attributes
Part of the BGP Update Packet
Implemented as TLVs (Type-LengthValue)
Used by BGP peers as route selection
criteria
Well-known vs Optional attributes
Network Architecture and Design
13
Well-known Attributes
Well-known attributes
Well-known mandatory attributes
must be present in all update messages
Well-known discretionary attributes
must be recognized by all compliant
implementations
could be present in update messages
All well-known attributes are propagated to
other neighbors
Network Architecture and Design
14
Optional Attributes
Optional attributes
recognized by some implementations (could be private),
expected not to be recognized by everyone
Optional transitive attributes
propagated to other neighbors if not recognized
Optional non-transitive attributes
discarded if not recognized
Recognized optional attributes are propagated to other
neighbors based on their meaning (not constrained by
transitive bit)
Network Architecture and Design
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Well-known Mandatory
Attributes
Origin Code
Specifies the origin of a BGP route
IGP = route originated in an IGP (network
command)
EGP = route originated in an EGP (and
redistributed)
Incomplete (?) = route redistributed into BGP from
IGP/static
AS_Path
sequence of AS numbers through which the network is
accessible
Next_Hop
IP address of the next-hop router
Network Architecture and Design
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Well-known Discretionary
Attributes
Local preference
Used for consistent routing policy within AS
Atomic aggregate
informs the neighbor AS that the originating router
aggregated routes
Network Architecture and Design
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AS-Path Attribute
The AS-path attribute is empty when a local route is
inserted in the BGP table
The sender’s AS number is prepended to the ASpath attribute when the routing update crosses AS
boundary
The receiver of BGP routing information can use
the AS-path to determine through which AS the
information has passed
An AS that receives routing information with its own
AS number in the AS-path silently ignores the
information
Network Architecture and Design
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AS-Path Attribute
Network Architecture and Design
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Next Hop Attribute
Next-hop attribute indicates the next-hop IP
address used for packet forwarding
Usually set to the IP address of the sending BGP
router
Can be set to a third-party IP address to optimize
routing
Network Architecture and Design
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Next Hop Processing in Shared
Media
If the receiving BGP router is in the same subnet as the
current next-hop, the next-hop address is not changed to
optimize packet forwarding
Network Architecture and Design
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Next Hop Processing in NBMA
BGP next-hop processing can break connectivity with
improper network designs over partially-meshed WAN
networks
Correct use of subinterfaces and subnets alleviates the
problem
Network Architecture and Design
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BGP Session Establishment
BGP does not auto-discover Neighbors - they must
be configured manually
Configuration must be done on both sides of the
connection
Both routers will attempt to connect to the other with
a TCP session on port number 179
Only one session will remain if both connection
attempts succeed
Source IP address of incoming connection attempts
is verified against a list of configured neighbors
Network Architecture and Design
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Example Network
2.3.4.5
2.3.4.6
AS 21
4.5.6.8
21.0.0.1
3.4.5.7
AS 123
4.5.6.7
1.2.3.4
3.4.5.6
AS 37
37.0.0.1
Network Architecture and Design
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BGP Neighbors – Idle State
Initially all BGP sessions to the neighbors are idle
as123>show ip bgp sum
BGP table version is 1, main routing table version 1
Neighbor
V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State
2.3.4.5
4 21
0
0
0 0 0 never Idle
3.4.5.6
4 37
0
0
0 0 0 never Idle
Network Architecture and Design
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BGP Neighbors – Steady State
All neighbors shall be up (no state info)
Neighbor
V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State
2.3.4.5
4 21
17
22
10 0 0 0:01:47
3.4.5.6
4 37
11
17
10 0 0 0:07:07
Network Architecture and Design
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External BGP - EBGP
BGP neighbors in different AS
Router B:
router bgp 110
network 150.10.0.0
neighbor 131.108.10.1
remote-as 109
AS 109
131.108.0.0
Router A:
router bgp 109
network 131.108.0.0
neighbor 131.108.10.2
remote-as 110
Network Architecture and Design
A
.1
131.108.10.0
.2
B
150.10.0.0
AS 110
27
External BGP - EBGP
Network Architecture and Design
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Internal BGP - IBGP
AS 109
BGP Neighbors in same AS
Router B:
router bgp 109
A
131.108.30.1
network 131.108.0.0
neighbor 131.108.30.1 remote-as 109
B
131.108.30.2
neighbor 131.108.30.1 update-source lo0
Router A:
router bgp 109
neighbor 131.108.10.2 remote-as 109
neighbor 131.108.30.2 update-source lo0
Update source is loopback0 to make sessions insensitive to
topology changes within the AS
Network Architecture and Design
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Internal BGP - IBGP
Network Architecture and Design
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BGP Attributes – AS Path
AS-Path
Contains the list of AS’s traversed by
the update
Sending router updates AS with its own
AS
Used for loop detection:
if a router receives an update containing its
own AS, then it discards it
Network Architecture and Design
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BGP Attributes – AS Path
1983
192.2.0.0/24
AS-Path
Ordered list of
AS’s traversed by
route update
AS-Set
Route update
traversed one or
more members of
a set
1981
192.2.1.0/24
1980
192.2.2.0/24
1982
192.2.3.0/24
192.2.0.0/24, 1980 1983
192.2.1.0/24, 1980 1981
192.2.2.0/24, 1980
192.2.3.0/24, 1980 1982
192.2.0.0/22 {1980, 1981, 1982, 1983}
Network Architecture and Design
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BGP Attributes – AS Path
AS-Path – Loop detection
B
1. Router A sends update for
146.124.54.0/24 with AS_PATH: 1560
670
2. Router B sends update
for 146.124.54.0/24 with
AS_PATH: 670 1560 (prepends its AS path)
1560
A
146.124.54.0/24
4. Router A detects its own AS in the
update’s AS_PATH and discards it
210
C
3. Router C sends update for
146.124.54.0/24 with AS_PATH: 210
670 1560 (pre-pends its AS path)
Network Architecture and Design
33
BGP Attributes – Local
Preference
Local Preference
Is sent only to routers in the same AS
Path with highest local-preference is preferred
Default is 100
65500
65002
65001
A Local Pref 120
65003
Need to go to 65500
B 65000
Network Architecture and Design
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End of Fifth Lecture
Network Architecture and Design
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