Redundancy, Symmetry and
Load Balancing
Presented by
Sagi Shporer
Terminology
Redundancy
Symmetry
Load Balancing
Terminology – Cont.
Outbound decision
NetA
Which way
should the
traffic be sent
to NetA ?
NetA
AS2
AS1
Terminology – Cont.
Inbound decision
NetC
I want to receive
inbound traffic
for NetB via
SA1 link only
NetB
AS2
AS1
Terminology – Cont.
Load Balancing – choosing ISP
ISP1
ISP2
Customer
Setting Default Routers
Gateway of last resort
Dynamic Learned Defaults
Statically Set Defaults
Setting Default Routers – Cont.
Set local pref = 100
1.1.1.1
AS1
Set local pref = 50
Primary
AS2
2.2.2.2
Gateway of last resort is 1.1.1.1
Backup
Setting Default Routers – Cont.
Set local pref = 100
1.1.1.1
Primary
AS1
AS2
2.2.2.2
Set local pref = 50
Gateway of last resort is 1.1.1.1
Backup
Setting Default Routers – Cont.
Statically Set Defaults
– IP of next-hop gateway
• Example : IP 201.1.2.3
– A network number
• Example : The route to that network number is
learned dynamically
– A specific router interface
• Example : Ethernet interface 0
Routing Tables
No Routes
Partial Routes
Full Routes
Specific Scenarios
Single-Homing
Multihoming to a Single Provider
Multihoming to Different Providers
Customers of the Same Provider with
Backup Link
Customers of Different Providers with a
Backup Link
Scenario 1 : Single-Homing
Customer
ISP
Outgoing
traffic
Default
Incoming
traffic
Static
Scenario 2 : Multihoming to a
Single Provider
Default only, one Primary, one Backup
Provider
Default-2
Default-1
Backup
Primary
Metric : 100
Mertic : 50
Customer
Scenario 2 – Cont.
Default, Primary and Backup plus partial
routing
C3
NAP
C4
C
2
C5
ISP
W: 200
Default-1
Z: 200
Rest : 250 Primary
Default-2
Backup
Y: 200
X: 200
Rest : 300
(X,Y)
C3: 300 Preference C5: 300
C2: 300
C4: 300
Rest : 200
Rest : 250
Metrics
Customer
(Z,W)
Scenario 2 – Cont.
Default, Primary and Backup plus full and
partial routing
– Full route on one link, and no route or partial
route on the other links
Scenario 2 – Cont.
Automatic load balancing
ISP
1
3
5
192.213.10.6
192.213.10.5
192.213.10.4
192.213.10.3
192.213.10.2
192.213.10.1
2
4
6
Customer
Scenario 2 – Cont.
Balancing between two routers sharing
multiple paths
BGP Session
RTA
10.10.10.1
Link1
10.10.10.2
RTB
NetX
11.11.11.1
Link2
BGP Session
11.11.11.2
BGP Session
RTA
10.10.10.1
Link1
10.10.10.2
RTB
NetX
11.11.11.1
Link2
11.11.11.2
BGP Session
RTA’s BGP Table
Destination Next Hop
NetX
10.10.10.2 (best)
NetX
11.11.11.2
BGP Session
RTA
10.10.10.1
Link1
10.10.10.2
RTB
NetX
11.11.11.1
Link2
11.11.11.2
BGP Session
RTA’s Routing Table
Destination
Next Hop
NetX
10.10.10.2
10.10.10.0/24 Link1
BGP Session
9.9.9.9
RTA
12.12.12.12
10.10.10.1
Link1
10.10.10.2
RTB
NetX
11.11.11.1
Link2
11.11.11.2
RTA’s BGP Table
Destination
Next Hop
NetX
12.12.12.12
BGP Session
9.9.9.9
RTA
12.12.12.12
10.10.10.1
Link1
10.10.10.2
RTB
NetX
11.11.11.1
Link2
11.11.11.2
RTA’s Routing Table
Destination
Next Hop
NetX
12.12.12.12
12.12.12.0/24
Link1
12.12.12.0/24
Link2
Scenario 3 : Multihoming to
Different Providers
Default only, Primary and Backup
NAP
Pref=100
192.213.0.0/16
192.213.0.0/16
Pref=200
192.213.0.0/16
Scenario 3 – Cont.
Default, Primary and Backup plus partial
routing
NAP
Default-2
backup
Default-1
primary
IBGP
Customer
Partial
Routes
Scenario 3 – Cont.
Default, Primary and Backup plus full and
partial routing
NAP
Backup
Primary
IBGP
Customer
Partial
Routes
Partial
Routes
Scenario 3 – Cont.
Customer Inbound Traffic
NAP
192.213.1.0/24 –
300 100 100
192.213.1.0/24 –
200 100
AS300
AS200
192.213.1.0/24 –
100 100
192.213.1.0/24 –
100
IBGP
Customer
192.213.1.0/24
AS100
Scenario 4 : Customers of the
Same Provider with Private Link
Private link used as pure backup
AS1
NAP
Pref=200
Backup
AS2
Pref=100
AS3
Scenario 4 – Cont.
Private link used as primary link
AS1
NAP
Pref=300 Pref=100
AS2
Pref=200
AS3
Updates that
do not contain
AS3
AS3 Routes
Scenario 5 : Customers of
Different Providers with a Private
Link
NAP
ISP2
ISP1
AS1
AS2
ISP1
Community Approach
ISP2
60
AS400
100
40
ISP routes - 60
AS1
AS2
Customer transit
routes - 40
Customer local
routes - 100
ISP1
ISP2
60
AS400
ISP routes - 60
100
Customer transit
routes - 40
40
AS1
Pattern
Customer local routes
AS2
Customer local
routes - 100
Community Local Pref.
None
100
Customer transit routes 400:40
ISP routes
400:60
40
60
ISP1
AS Path Manipulation
ISP2
AS400
AS1
AS2
AS2’s routes
via ISP2
AS_Path: 500 1
AS2’s routes
via AS1
AS_Path: 1 1 2