You have two EBGP peers connected via two parallel serial lines. What should you do to be able
to load-balance between two EBGP speakers over the parallel serial lines in both directions?
A.
nothing, BGP automatically load-balances the traffic between different autonomous systems on
all available links
B.
peer between the eBGP speaker’s loopbacks, configuring eBGP multihop as required, and use
an IGP to load-share between the two equal-cost paths between the loopback addresses
C.
configure a loopback as update source for both EBGP peers and have on each AS an IGP to
introduce two equal-cost paths to reach the EBGP peer loopback address; it is also necessary to
use the next-hop-self command
D.
use the ebgp-load-balance command on the neighbor statement on both sides
E.
configure a loopback as update source for both EBGP peers and have on each AS an IGP to
introduce two equal-cost paths to reach the peer loopback address; it is also necessary to use the
ebgp-multihop and next-hop-self commands
Explanation:
This example illustrates the use of loopback interfaces, update-source, and ebgp-multihop. The
example is a workaround in order to achieve load balancing between two eBGP speakers over
parallel serial lines. In normal situations, BGP picks one of the lines on which to send packets, and
load balancing does not happen. With the introduction of loopback interfaces, the next hop for
eBGP is the loopback interface. You use static routes, or an IGP, to introduce two equal-cost
paths to reach the destination. RTA has two choices to reach next hop 160.10.1.1: one path via
1.1.1.2 and the other path via 2.2.2.2. RTB has the same choices.
Load balancing with parallel EBGP sessions
Establishing parallel EBGP sessions across parallel links between two edge routers (EBGP
peers), as displayed in Figure 1, is the most versatile form of EBGP load balancing. It does not
require static routing or extra routing protocol (like the design running EBGP between routers’
loopback interfaces), IOS- specific tricks (configuring the same IP address on multiple interfaces)
or specific layer-2 encapsulation (like Multilink PPP). It even allows proportional load-balancing
across unequal-bandwidth links and combinations of various layer-2 technologies (for example,
load-balancing between a serial line and an Ethernet interface). The only drawback
of this design is the increased size of the BGP table, as every BGP prefix is received from the
EBGP neighbor twice.
Figure 1: Parallel EBGP sessions
Basic configuration
To implement parallel EBGP sessions, configure multiple neighbors on both EBGP routers, one for
each IP subnet (parallel link between the EBGP peers) and enable EBGP multipath load balancing
with the maximum paths router configuration command. A sample configuration is shown in the
following table:
http://wiki.nil.com/Load_balancing_with_parallel_EBGP_sessions
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.
shtml#ebgpmulithoploadbal
