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Border Gateway Protocol (BGP4)
Border Gateway Protocol (BGP)
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Routing/Forwarding basics
Building blocks
Exercises
BGP protocol basics
Exercises
BGP path attributes
Best path computation
Exercises
Border Gateway Protocol (BGP)...
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Typical BGP topologies
Routing Policy
Exercises
Redundancy/Load sharing
Best current practices
Routing/Forwarding
Basics
IP route lookup:Longest match
routing
R3
Packet: Destination
IP address: 10.1.1.1
R1
R2
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
30/8 -> R6
…..
R2’s IP routing table
All 10/8 except
10.1/16
R4
10.1/16
IP route lookup: Longest match
routing
R3
Packet: Destination
IP address: 10.1.1.1
R1
R4
R2
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
All 10/8 except
10.1/16
10.1/16
10.1.1.1 & FF.0.0.0
is equal to
10.0.0.0 & FF.0.0.0
…..
R2’s IP routing table
Match!
IP route lookup: Longest match
routing
R3
Packet: Destination
IP address: 10.1.1.1
R1
R4
R2
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
All 10/8 except
10.1/16
10.1/16
10.1.1.1 & FF.FF.0.0
is equal to
10.1.0.0 & FF.FF.0.0
…..
R2’s IP routing table
Match as well!
IP route lookup: Longest match
routing
R3
Packet: Destination
IP address: 10.1.1.1
R1
R4
R2
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
All 10/8 except
10.1/16
10.1/16
10.1.1.1 & FF.0.0.0
is equal to
Does not match!
20.0.0.0 & FF.0.0.0
R2’s IP routing table
IP route lookup: Longest match
routing
R3
Packet: Destination
IP address: 10.1.1.1
R1
R2
All 10/8 except
10.1/16
R4
10.1/16
10/8 -> R3
10.1/16 -> R4
20/8 -> R5
…..
R2’s IP routing table
Longest match, 16 bit netmask
IP route lookup: Longest match
routing
• default is 0.0.0.0/0
• can handle it using the normal longest
match algorithm
• matches everything. Always the shortest
match.
Forwarding
• Uses the routing table built by routing
protocols
• Performs the lookup to find next-hop and
outgoing interface
• Switches the packet with new encapsulation
as per the outgoing interface
Building Blocks
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Autonomous System (AS)
Types of Routes
IGP/EGP
DMZ
Policy
Egress
Ingress
Autonomous System (AS)
AS 100
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Collection of networks with same policy
Single routing protocol
Usually under single administrative control
IGP to provide internal connectivity
Autonomous System(AS)...
• Identified by ‘AS number’
• Public & Private AS numbers
• Examples:
– Service provider
– Multi-homed customers
– Anyone needing policy discrimination
Routing flow and packet flow
packet flow
egress
AS 1
accept
announce
announce
Routing flow
accept
AS2
ingress
packet
flow
For networks in AS1 and AS2 to communicate:
AS1 must announce routes to AS2
AS2 must accept routes from AS1
AS2 must announce routes to AS1
AS1 must accept routes from AS2
Egress Traffic
• Packets exiting the network
• Based on
– Route availability (what others send you)
– Route acceptance (what you accept from others)
– Policy and tuning (what you do with routes from
others)
– Peering and transit agreements
Ingress Traffic
• Packets entering your network
• Ingress traffic depends on:
– What information you send and to who
– Based on your addressing and ASes
– Based on others’ policy (what they accept from
you and what they do with it)
Types of Routes
• Static Routes
– configured manually
• Connected Routes
– created automatically when an interface is ‘up’
• Interior Routes
– Routes within an AS
• Exterior Routes
– Routes exterior to AS
What Is an IGP?
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Interior Gateway Protocol
Within an Autonomous System
Carries information about internal prefixes
Examples—OSPF, ISIS, EIGRP…
What Is an EGP?
• Exterior Gateway Protocol
• Used to convey routing information between
ASes
• De-coupled from the IGP
• Current EGP is BGP4
Why Do We Need an EGP?
• Scaling to large network
– Hierarchy
– Limit scope of failure
• Define administrative boundary
• Policy
– Control reachability to prefixes
Interior vs. Exterior
Routing Protocols
• Interior
– Automatic
discovery
– Generally trust
your IGP routers
– Routes go to all
IGP routers
• Exterior
Specifically configured
peers
Connecting with outside
networks
Set administrative
boundaries
Hierarchy of Routing Protocols
Other ISP’s
BGP4
BGP4 / OSPF
BGP4
Local NAP
FDDI
BGP4/Static
Customers
Demilitarized Zone (DMZ)
A
C
DMZ
Network
AS 100
B
AS 101
D
E
AS 102
• Shared network between ASes
Addressing - ISP
• Need to reserve address space for its
network.
• Need to allocate address blocks to its
customers.
• Need to take “growth” into consideration
• Upstream link address is allocated by
upstream provider
BGP Basics
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Terminology
Protocol Basics
Messages
General Operation
Peering relationships (EBGP/IBGP)
Originating routes
Terminology
• Neighbor
– Configured BGP peer
• NLRI/Prefix
– NLRI - network layer reachability information
– Reachability information for a IP address &
mask
• Router-ID
– Highest IP address configured on the router
• Route/Path
– NLRI advertised by a neighbor
Protocol Basics
Peering
A
C
AS 100
AS 101
B
• Routing protocol used
between ASes
–if you aren’t connected
to multiple ASes, you
don’t need BGP :)
• Runs over TCP
• Path vector protocol
D
E
AS 102
BGP Basics ...
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Each AS originates a set of NLRI
NLRI is exchanged between BGP peers
Can have multiple paths for a given prefix
Picks the best path and installs in the IP
forwarding table
• Policies applied (through attributes)
influences BGP path selection
BGP Peers
A
C
AS 101
AS 100
220.220.16.0/24
220.220.8.0/24
B
BGP speakers
are called peers
Peers in different AS’s
are called External Peers
D
E
AS 102
220.220.32.0/24
eBGP TCP/IP
Peer Connection
Note: eBGP Peers normally should be directly connected.
BGP Peers
A
C
AS 101
AS 100
220.220.16.0/24
220.220.8.0/24
B
BGP speakers are
called peers
Peers in the same AS
are called Internal Peers
iBGP TCP/IP
Peer Connection
D
E
AS 102
220.220.32.0/24
Note: iBGP Peers don’t have to be directly connected.
BGP Peers
A
C
AS 101
AS 100
220.220.16.0/24
220.220.8.0/24
B
BGP Peers exchange
Update messages
containing Network
Layer Reachability
Information (NLRI)
BGP Update
Messages
D
E
AS 102
220.220.32.0/24
Configuring BGP Peers
AS 100
AS 101
eBGP TCP Connection
222.222.10.0/30
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220.220.8.0/24
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B
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C
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220.220.16.0/24
.1
D
interface Serial 0
ip address 222.222.10.2 255.255.255.252
interface Serial 0
ip address 222.222.10.1 255.255.255.252
router bgp 100
network 220.220.8.0 mask 255.255.255.0
neighbor 222.222.10.1 remote-as 101
router bgp 101
network 220.220.16.0 mask 255.255.255.0
neighbor 222.222.10.2 remote-as 100
• BGP Peering sessions are established using the BGP
“neighbor” configuration command
– External (eBGP) is configured when AS numbers are different
Configuring BGP Peers
AS 101
AS 100
iBGP TCP Connection
222.222.10.0/30
A
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220.220.8.0/24
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220.220.16.0/24
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D
interface Serial 1
ip address 220.220.16.2 255.255.255.252
interface Serial 1
ip address 222.220.16.1 255.255.255.252
router bgp 101
network 220.220.16.0 mask 255.255.255.0
neighbor 220.220.16.1 remote-as 101
router bgp 101
network 220.220.16.0 mask 255.255.255.0
neighbor 220.220.16.2 remote-as 101
• BGP Peering sessions are established using the BGP
“neighbor” configuration command
– External (eBGP) is configured when AS numbers are different
– Internal (iBGP) is configured when AS numbers are same
Configuring BGP Peers
AS 100
B
A
iBGP TCP/IP
Peer Connection
C
• Each iBGP speaker must peer with every other
iBGP speaker in the AS
Configuring BGP Peers
215.10.7.1
AS 100
B
A
215.10.7.3
iBGP TCP/IP
Peer Connection
215.10.7.2
C
• Loopback interface are normally used as
peer connection end-points
Configuring BGP Peers
215.10.7.1
AS 100
B
A
215.10.7.3
iBGP TCP/IP
interface
loopback 0
ip
address
215.10.7.1 255.255.255.255
Peer
Connection
router bgp 100
network 220.220.1.0
neighbor 215.10.7.2
neighbor 215.10.7.2
neighbor 215.10.7.3
neighbor 215.10.7.3
remote-as 100
update-source loopback0
remote-as 100
update-source loopback0
215.10.7.2
C
Configuring BGP Peers
215.10.7.1
AS 100
215.10.7.2
B
A
215.10.7.3
iBGP TCP/IP
Peer Connection
interface loopback 0
ip address 215.10.7.2 255.255.255.255
C
router bgp 100
network 220.220.5.0
neighbor 215.10.7.1
neighbor 215.10.7.1
neighbor 215.10.7.3
neighbor 215.10.7.3
remote-as 100
update-source loopback0
remote-as 100
update-source loopback0
Configuring BGP Peers
215.10.7.1
AS 100
B
A
215.10.7.3
iBGP TCP/IP
Peer Connection
C
interface loopback 0
ip address 215.10.7.3 255.255.255.255
router bgp 100
network 220.220.1.0
neighbor 215.10.7.1
neighbor 215.10.7.1
neighbor 215.10.7.2
neighbor 215.10.7.2
remote-as 100
update-source loopback0
remote-as 100
update-source loopback0
215.10.7.2
BGP Updates — NLRI
• Network Layer Reachability Information
• Used to advertise feasible routes
• Composed of:
– Network Prefix
– Mask Length
BGP Updates — Attributes
• Used to convey information associated with
NLRI
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AS path
Next hop
Local preference
Multi-Exit Discriminator (MED)
Community
Origin
Aggregator
AS-Path Attribute
• Sequence of ASes a route
has traversed
• Loop detection
• Apply policy
AS 300
AS 200
AS 100
170.10.0.0/16
180.10.0.0/16
Network
Path
180.10.0.0/16 300 200 100
170.10.0.0/16 300 200
AS 400
150.10.0.0/16
AS 500
Network
180.10.0.0/16
170.10.0.0/16
150.10.0.0/16
Path
300 200 100
300 200
300 400
Next Hop Attribute
AS 300
AS 200
150.10.0.0/16
140.10.0.0/16
192.10.1.0/30
C
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.2
D
E
B
.2
.1
A
AS 100
160.10.0.0/16
BGP Update
Messages
Network
Next-Hop
160.10.0.0/16 192.20.2.1
Path
100
• Next hop to reach a network
• Usually a local network is the next
hop in eBGP session
Next Hop Attribute
AS 300
AS 200
150.10.0.0/16
140.10.0.0/16
192.10.1.0/30
C
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.2
D
E
B
.2
.1
A
Network
Next-Hop
150.10.0.0/16 192.10.1.1
160.10.0.0/16 192.10.1.1
Path
200
200 100
• Next hop to reach a network
• Usually a local network is the next
hop in eBGP session
AS 100
160.10.0.0/16
BGP Update
Messages
• Next Hop updated between
eBGP Peers
Next Hop Attribute
AS 300
AS 200
150.10.0.0/16
140.10.0.0/16
192.10.1.0/30
C
.1
.2
D
E
B
.2
.1
A
AS 100
160.10.0.0/16
BGP Update
Messages
Network
Next-Hop
150.10.0.0/16 192.10.1.1
160.10.0.0/16 192.10.1.1
• Next hop not changed
between iBGP peers
Path
200
200 100
Next Hop Attribute (more)
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IGP should carry route to next hops
Recursive route look-up
Unlinks BGP from actual physical topology
Allows IGP to make intelligent forwarding
decision
BGP Updates —
Withdrawn Routes
• Used to “withdraw” network reachability
• Each Withdrawn Route is composed of:
– Network Prefix
– Mask Length
BGP Updates —
Withdrawn Routes
AS 321
AS 123
.1
192.168.10.0/24
.2
BGP Update
Message
Withdraw Routes
192.192.25.0/24
x
Connectivity lost
Network
Next-Hop
Path
150.10.0.0/16
192.168.10.2 321 200
192.192.25.0/24 192.168.10.2 321
192.192.25.0/24
BGP Routing Information Base
BGP RIB
Network
*>i160.10.1.0/24
*>i160.10.3.0/24
Next-Hop
192.20.2.2
192.20.2.2
Path
i
i
router bgp 100
network 160.10.0.0 255.255.0.0
no auto-summary
D
D
D
R
S
10.1.2.0/24
160.10.1.0/24
160.10.3.0/24
153.22.0.0/16
192.1.1.0/24
Route Table
BGP ‘network’ commands are normally
used to populate the BGP RIB with
routes from the Route Table
BGP Routing Information Base
BGP RIB
Network
*> 160.10.0.0/16
* i
s> 160.10.1.0/24
s> 160.10.3.0/24
Next-Hop
0.0.0.0
192.20.2.2
192.20.2.2
192.20.2.2
Path
i
i
i
i
router bgp 100
network 160.10.0.0 255.255.0.0
aggregate-address 160.10.0.0 255.255.0.0 summary-only
no auto-summary
D
D
D
R
S
10.1.2.0/24
160.10.1.0/24
160.10.3.0/24
153.22.0.0/16
192.1.1.0/24
Route Table
BGP ‘aggregate-address’ commands
may be used to install summary routes
in the BGP RIB
BGP Routing Information Base
BGP RIB
Network
*> 160.10.0.0/16
* i
s> 160.10.1.0/24
s> 160.10.3.0/24
*> 192.1.1.0/24
Next-Hop
0.0.0.0
192.20.2.2
192.20.2.2
192.20.2.2
192.20.2.2
Path
i
i
i
i
?
router bgp 100
network 160.10.0.0 255.255.0.0
redistribute static route-map foo
no auto-summary
D
D
D
R
S
10.1.2.0/24
160.10.1.0/24
160.10.3.0/24
153.22.0.0/16
192.1.1.0/24
Route Table
access-list 1 permit 192.1.0.0 0.0.255.255
route-map foo permit 10
match ip address 1
BGP ‘redistribute’ commands can also
be used to populate the BGP RIB with
routes from the Route Table
BGP Routing Information Base
IN Process
Update
Update
Network
Next-Hop
173.21.0.0/16 192.20.2.1
OUT Process
BGP RIB
Network
*>i160.10.1.0/24
*>i160.10.3.0/24
* > 173.21.0.0/16
Next-Hop
192.20.2.2
192.20.2.2
192.20.2.1
Path
i
i
100
Path
100
• BGP “in” process
• receives path information from peers
• results of BGP path selection placed in the BGP table
• “best path” flagged (denoted by “>”)
BGP Routing Information Base
IN Process
OUT Process
BGP RIB
Network
*>i160.10.1.0/24
*>i160.10.3.0/24
*> 173.21.0.0/16
Next-Hop
192.20.2.2
192.20.2.2
192.20.2.1
Path
i
i
100
Update
Network
160.10.1.0/24
160.10.3.0/24
173.21.0.0/16
Next-Hop
192.20.2.2
192.20.2.2
192.20.2.1
192.20.2.2
Update
Path
200
200
200 100
• BGP “out” process
• builds update using info from RIB
• may modify update based on config
• Sends update to peers
Next-Hop changed
BGP Routing Information Base
BGP RIB
Network
*>i160.10.1.0/24
*>i160.10.3.0/24
*> 173.21.0.0/16
D
D
D
R
S
B
10.1.2.0/24
160.10.1.0/24
160.10.3.0/24
153.22.0.0/16
192.1.1.0/24
173.21.0.0/16
Route Table
Next-Hop
192.20.2.2
192.20.2.2
192.20.2.1
Path
i
i
100
• Best paths installed in routing table if:
• prefix and prefix length are unique
• lowest “protocol distance”
The ‘Bible’ & other resources
• Route-views.oregon-ix.net
• Internet Routing Architectures
– Bassam Halabi
– pg. 168 BGP Decision Process Summary
Types of BGP Messages
• OPEN
– To negotiate and establish peering
• UPDATE
– To exchange routing information
• KEEPALIVE
– To maintain peering session
• NOTIFICATION
– To report errors (results in session reset)
Internal BGP Peering (IBGP)
AS 100
D
A
B
E
• BGP peer within the same AS
• Not required to be directly connected
• Maintain full IBGP mesh or use Route Reflection
External BGP Peering (EBGP)
A
AS 100
C
AS 101
B
• Between BGP speakers in different AS
• Directly connected or peering address is reachable
An Example…
35.0.0.0/8
AS3561
A
AS200
F
B
AS21
C
D
AS101
E
AS675
Learns about 35.0.0.0/8 from F & D
Basic BGP commands
Configuration commands
router bgp <AS-number>
neighbor <ip address> remote-as <as-number>
Show commands
show ip bgp summary
show ip bgp neighbors
Originating routes...
• Using network command or redistribution
network <ipaddress>
redistribute <protocol name>
• Requires the route to be present in the
routing table
•
•
•
•
Originating routes/Inserting
prefixes into BGP
network command
network 198.10.4.0 mask 255.255.254.0
ip route 198.10.0.0 255.255.254.0 serial 0
matching route must exist in the routing
table before network is announced!
• Origin: IGP
Update message
• Withdrawn routes
• Path Attributes
• Advertised routes
Stable IBGP peering
• Unlinks IBGP peering from physical
topology.
• Carry loopback address in IGP
router ospf <ID>
passive-interface loopback0
• Unlink peering from physical topology
router bgp <AS1>
neighbor <x.x.x.x> remote-as <AS1>
neighbor <x.x.x.x> update-source loopback0
BGP4 continued...
BGP Path Attributes: Why ?
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•
•
•
•
Encoded as Type, Length & Value (TLV)
Transitive/Non-Transitive attributes
Some are mandatory
Used in path selection
To apply policy for steering traffic
BGP Path Attributes...
•
•
•
•
•
•
•
Origin
AS-path
Next-hop
Multi-Exit Discriminator (MED)
Local preference
BGP Community
Others...
AS-PATH
• Updated by the sending router with its AS
number
• Contains the list of AS numbers the update
traverses.
• Used to detect routing loops
– Each time the router receives an update, if it
finds its AS number, it discards the update
AS-Path
AS 200
AS 100
170.10.0.0/16
180.10.0.0/16
• Sequence of ASes a route has
traversed
AS 300
• Loop detection
180.10.0.0/16
dropped
AS 400
150.10.0.0/16
AS 500
180.10.0.0/16
170.10.0.0/16
150.10.0.0/16
300 200 100
300 200
300 400
Next-Hop
150.10.1.1
150.10.1.2
AS 200
150.10.0.0/16
A
B
AS 300
150.10.0.0/16 150.10.1.1
160.10.0.0/16 150.10.1.1
AS 100
160.10.0.0/16
• Next hop router to reach a network
• Advertising router/Third party in EBGP
• Unmodified in IBGP
0799_04F7_c2
Cisco Systems Confidential
20
Third Party Next Hop
AS 200
192.68.1.0/24
C
150.1.1.1
peering
150.1.1.3
150.1.1.2
A
B
192.68.1.0/24
AS 201
• More efficient, but
bad idea!
150.1.1.3
Next Hop...
•
•
•
•
IGP should carry route to next hops
Recursive route look-up
Unlinks BGP from actual physical topology
Allows IGP to make intelligent forwarding
decision
Local Preference
• Not for EBGP, mandatory for IBGP
• Default value is 100 on Ciscos
• Local to an AS
• Used to prefer one exit over another
• Path with highest local preference wins
Local Preference
AS 100
160.10.0.0/16
AS 200
AS 300
D
500
800
A
160.10.0.0/16
> 160.10.0.0/16
500
800
B
AS 400
C
E
Multi-Exit Discriminator
• Non-transitive
• Represented as a numeric value (0-0xffffffff)
• Used to convey the relative preference of entry points
• Comparable if paths are from the same AS
• Path with lower MED wins
• IGP metric can be conveyed as MED
Multi-Exit Discriminator (MED)
AS 200
C
preferred
192.68.1.0/24
2000
192.68.1.0/24
A
B
192.68.1.0/24
AS 201
1000
Origin
• Conveys the origin of the prefix
• Three values:
– IGP - Generated using “network” statement
• ex: network 35.0.0.0
– EGP - Redistributed from EGP
– Incomplete - Redistribute IGP
• ex: redistribute ospf
• IGP < EGP < INCOMPLETE
Communities
•
•
•
•
Transitive, Non-mandatory
Represented as a numeric value (0-0xffffffff)
Used to group destinations
Each destination could be member of multiple
communities
• Flexibility to scope a set of prefixes within or
across AS for applying policy
Community...
Service Provider AS 200
C
Community
201:110
201:120
D
Community:201:110
Community:201:120
A
B
192.68.1.0/24
Customer AS 201
Local Preference
110
120
Synchronization
1880
C
A
D
690
OSPF
35/8
• C not running BGP (non-pervasive BGP)
B is in sync
• A won’t advertise 35/8 to D until the IGP
• Turn synchronization off!
– Run pervasive BGP
router bgp 1880
no sync
209
BGP Route Selection (bestpath)
Only one path as the bestpath !
• Route has to be synchronized
Prefix in forwarding table
• Next-hop has to be accessible
Next-hop in forwarding table
• Largest weight
Local to the router
• Largest local preference
Spread within AS
• Locally sourced
Via redistribute or network statement
BGP Route Selection ...
• Shortest AS-path length
number of ASes in the AS-path attribute
• Lowest origin
IGP < EGP < INCOMPLETE
• Lowest MED
between paths from same AS
• External over internal
closest exit from a router
• Closest next-hop
Lower IGP metric, closer exit from as AS
• Lowest router-id
• Lowest IP address of neighbor
BGP Route Selection...
AS 100
AS 200
AS 300
D
Increase AS path attribute
length by at least 1
A
B
AS 400
AS 400’s Policy to reach AS100
AS 200 preferred path
AS 300 backup
Stub AS
• Typically no need for BGP
• Point default towards the ISP
• ISP advertises the stub network to
Internet
• Policy confined within ISP policy
Stub AS
B
A
AS 100
Customer
AS 101
Provider
Multi-homed AS
• Only border routers speak BGP
• IBGP only between border routers
• Exterior routes must be redistributed in
a controlled fashion into IGP or use
defaults
Multi-homed AS
AS 100
provider
AS 300
D
A
C
B
AS 200
customer
provider
Service Provider Network
• IBGP used to carry exterior routes
• IGP keeps track of topology
• Full IBGP mesh is required
Common Service Provider
Network
AS 100
A
H
B
C
AS 300
D
provider
E
G
AS 400
F
AS 200
Routing Policy
• Why?
– To steer traffic through preferred paths
– Inbound/Outbound prefix filtering
– To enforce Customer-ISP agreements
• How ?
– AS based route filtering - filter list
– Prefix based route filtering - distribute list
– BGP attribute modification - route maps
Distribute list - using IP access lists
access-list 1 deny 10.0.0.0
access-list 1 permit any
access-list 2 permit 20.0.0.0
… more access-lists as prefixes are added ...
router bgp 100
neighbor 171.69.233.33 remote-as 33
neighbor 171.69.233.33 distribute-list 1 in
neighbor 171.69.233.33 distribute-list 2 out
Filter list rules
Regular Expressions
• RE is a pattern to match against an input
string
• Used to match against AS-path attribute
• ex: ^3561.*100.*1$
• Flexible enough to generate complex filter
list rules
Filter list - using as-path access list
ip as-path access-list 1 permit 3561
ip as-path access-list 2 deny 35
ip as-path access-list 2 permit .*
router bgp 100
neighbor 171.69.233.33 remote-as 33
neighbor 171.69.233.33 filter-list 1 in
neighbor 171.69.233.33 filter-list 2 out
Route Maps
router bgp 300
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 route-map SETCOMMUNITY out
!
route-map SETCOMMUNITY permit 10
match ip address 1
match community 1
set community 300:100
!
access-list 1 permit 35.0.0.0
ip community-list 1 permit 100:200
Route-map match & set clauses
Match Clauses
• AS-path
• Community
• IP address
Set Clauses
•
•
•
•
•
•
•
AS-path prepend
Community
Local-Preference
MED
Origin
Weight
Others...
Route-map Configuration Example
ISP2
C21
H
ethH
C22
H
eth H
ISP3
Inbound route-map
to set community
H
C31
eth
H
C32
H
eth
H
neighbor <y.y.y.y> route-map AS200_IN in
!
route-map AS200_IN permit 10
match community 1
set local-preference 200
!
ip community-list 1 permit 100:200
neighbor <x.x.x.x> route-map AS100_IN in
!
route-map AS100_IN permit 10
set community 100:200
Load Sharing & Redundancy
using BGP
Load-sharing - single path
Router A:
interface loopback 0
ip address 20.200.0.1 255.255.255.255
!
router bgp 100
neighbor 10.200.0.2 remote-as 200
neighbor 10.200.0.2 update-source loopback0
neighbor 10.200.0.2 ebgp-multi-hop 2
!
ip route 10.200.0.2 255.255.255.255 <DMZ-link1, link2>
A
AS100
Loopback 0
10.200.0.2
AS200
Loopback 0
20.200.0.1
Load Sharing - Multiple paths
from the same AS
Router A:
router bgp 100
neighbor 10.200.0.1 remote-as 200
neighbor 10.300.0.1 remote-as 200
maximum-paths 2
A
100
Note:A still only advertises one “best” path to ibgp peers
200
Redundancy - Multi-homing
• Reliable connection to Internet
• 3 common cases of multi-homing:
- default from all providers
- customer + default routes from all
- full routes from all
Default from all providers
• Low memory/CPU solution
• Provider sends BGP default
– provider is selected based on IGP metric
• Inbound traffic decided by providers’ policy
– Can influence using outbound policy, example:
AS-path prepend
Default from all providers
Provider
Provider
AS 200
AS 300
D
E
A
B
AS 400
C
Customer + default from all
providers
• Medium memory and CPU solution
• Granular routing for customer routes and
default for the rest
• Inbound traffic decided by providers’ policy
– Can influence using outbound policy
Customer routes from all
providers
Customer
AS 100
160.10.0.0/16
Provider
Provider
AS 200
AS 300
D
C chooses shortest AS
path
E
A
B
AS 400
C
Full routes from all providers
• More memory/CPU
• Full granular routing
• Usually transit ASes take full routes
• Usually pervasive BGP
Full routes from all providers
AS 100
AS 500
AS 200
AS 300
D
C chooses shortest AS
path
E
A
B
AS 400
C
Best Practices
IGP in Backbone
• IGP connects your backbone together, not
your client’s routes
• IGP must converge quickly
• IGP should carry netmask information OSPF, IS-IS, EIGRP
Best Practices...
Connecting to a customer
• Static routes
– You control directly
– No route flaps
• Shared routing protocol or leaking
– You must filter your customers info
– Route flaps
• BGP for multi-homed customers
Best Practices...
Connecting to other ISPs
•
•
•
•
Use BGP4
Advertise only what you serve
Take back as little as you can
Take the shortest exit
Best Practices...
The Internet Exchange
• Long distance connectivity is expensive
• Connect to several providers at a single
point
Q &A