DD2491 p2 2009/2010
Load balancing and traffic control
in BGP
Olof Hagsand KTH /CSC
1
Issues in load balancing
• Load balancing: spread traffic on several paths instead of
a single.
•Why?
•Use resources better
Can postpone upgrade of infra-structure
•Geography
Example: Dont use trans-oceanic links twice
•Cost reasons
Balance traffic for optimal price
2
Redundancy
•Redundancy and load balancing are not always aligned
•Example: if you use load balancing over several links, and
one link goes down: can you still forward all traffic on the
single link?
•For redundancy it is may sometimes be better to
send/announce all traffic on a single link and then have
redundant links for backup
•At the edge, it may be easier to get symmetrical routing
which is better for filtering
3
Sub-link-layer load-balancing
10Gb/s
20 Gb/s
LAG
10Gb/s
• Two physical links is aggregated into a single LinkAggregate Group.
• Single (20Mb/s) link
• Load balancing normally on flows
4
Link-layer load-balancing
• In (metro) Ethernet, load balancing can not be made:
Spanning tree computes a single link
•Example: only one link between 222 and 333 can be used
for forwarding.
444
1
2
A
1
333
3
4
Ethernet
switch
2
2
1
3
222
4
B
5
IGP load balancing in IBGP
•IS-IS / OSPF equal-cost multipath provides load balancing
in IBGP
Since next-hop self uses peering between loop-backs, and the
IBGP neighbor may be reachable via more than one nexthop
12.0.0.1
13.0.0.1
Route
130.2.3.0/24
10.0.0.1/32
10.0.0.1/32
10.0.0.1
Nexthop Metric
10.0.0.1
12.0.0.1 10
13.0.0.1 10
Protocol
IBGP
IGP
Equal-cost
IGP
multipath
130.2.3.0/24
6
Loopback peering in EBGP
•Loopback peering can also be used in EBGP, but routing of
loopback is then set-up using static routes
Uncommon to use IGP between AS
AS1
EBGP
RTA
RTB
AS2
7
BGP Multipath
•By default, the...