Information Technology Reference
In-Depth Information
Use AS Override in conjunction with SOO to prevent routing information loops in a multi-
homed site. Figure 10-11 shows such a scenario. Site 1 is connected as shown in Figure 10-9,
but Site 2 is multihomed. To allow connectivity between Site 1 and Site 2, AS Override is
configured on all three PE devices.
Figure 10-11
Routing Loop Scenario with AS Override in a Multihomed Site
172.16.0.0/16
AS_PATH:
65000
10.1.0.0/16
AS_PATH:
65000
AS 100
AS 65000
AS 65000
CE1
PE1
PE2
CE2
VPNa
Site 1
PE3
CE3
10.1.0.0/16
AS_PATH:
100 100
10.1.0.0/16
AS_PATH:
100 100
VPNa
Site 2
The prefix 10.1.0.0/16 is generated from Site 2 and is advertised from CE2 to PE2. When
PE3 advertises the prefix back to CE3, the AS_PATH is changed to 100 100 because of AS
Override. CE3 accepts this advertisement because there appears to be no AS_PATH loop.
Thus, a routing information loop forms.
To break the loop, use the extended community SOO. On an inbound route map to Site 2
on both PE2 and PE3, you can configure a value of SOO representing Site 2 using the
command
set extcommunity soo
. When the prefix 10.1.0.0 is advertised from PE2 to PE3,
the SOO is attached. PE3 doesn't send the prefix back to Site 2, because PE3 detects the
same SOO. Note that SOO loop detection is automatic (because of the configured inbound
route map), and no outbound route map is needed. The prefix advertisement to Site 1 is
unaffected.
Example 10-16 shows the
debug
output, where 192.168.47.7 is CE3. The prefix 10.1.0.0/16
is not advertised to CE3 because of the SOO loop. Note that the prefix 172.16.0.0/16 from
CE1 is advertised as before.
Example 10-16
Output of
debug ip bgp update out
on PE3
*Dec 16 23:18:21.267: BGP(2): 192.168.47.7 soo loop detected for 10.1.0.0/16 -
sending unreachable
*Dec 16 23:18:21.267: BGP(0): 192.168.47.7 send UPDATE (format) 172.16.0.0/16,
next 192.168.47.4, metric 0, path 65000, extended community RT:100:100
