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In case of broken AS, you can configure BGP with allow-as-loop to advertise internal routes

Publication Date:  2012-07-27 Views:  37 Downloads:  1
Issue Description
We get one AS 31205 from ISP(20485) for two sites. In this situation 
this AS becomes "broken". Also we have no GRE support in our border
 routers. In this case we can not establish independent iBGP peer
 between two sites of broken AS. In attachments you can find description.
Alarm Information
Null
Handling Process
To solve this problem we configure allow-as-loop feature in eBGP peer.
1) [nsk-NE40-4-1-bgp]peer 217.150.36.72 allow-as-loop //peer to ISP
   
VERIFICATION
<nsk-NE40-4-1>ping 83.149.48.249
  PING 83.149.48.249: 56  data bytes, press CTRL_C to break
    Reply from 83.149.48.249: bytes=56 Sequence=1 ttl=253 time = 18 ms
    Reply from 83.149.48.249: bytes=56 Sequence=2 ttl=253 time = 18 ms
    Reply from 83.149.48.249: bytes=56 Sequence=3 ttl=253 time = 17 ms
    Reply from 83.149.48.249: bytes=56 Sequence=4 ttl=253 time = 17 ms
    Reply from 83.149.48.249: bytes=56 Sequence=5 ttl=253 time = 17 ms
  --- 83.149.48.249 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 17/17/18 ms
<nsk-NE40-4-1>dis bgp ro 83.149.48.0
BGP route 83.149.48.0/24
From         : 217.150.36.72(10.54.0.11)
State        : valid, external, best,
Nexthop      : 217.150.36.72
Origin       : IGP
As-path      : 20485 31205
It's OK.
2) [krk-NE40-4-2-bgp]peer 217.150.37.65 allow-as-loop // peer to ISP
   
[krk-NE40-4-2]ping 83.149.49.249
  PING 83.149.49.249: 56  data bytes, press CTRL_C to break
    Reply from 83.149.49.249: bytes=56 Sequence=1 ttl=253 time = 19 ms
    Reply from 83.149.49.249: bytes=56 Sequence=2 ttl=253 time = 17 ms
    Reply from 83.149.49.249: bytes=56 Sequence=3 ttl=253 time = 18 ms
    Reply from 83.149.49.249: bytes=56 Sequence=4 ttl=253 time = 17 ms
    Reply from 83.149.49.249: bytes=56 Sequence=5 ttl=253 time = 31 ms
  --- 83.149.49.249 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 17/20/31 m
[krk-NE40-4-2]dis bgp ro 83.149.49.0
BGP route 83.149.49.0/24
From         : 217.150.37.65(10.24.0.11)
State        : valid, external, best,
Nexthop      : 217.150.37.65
Origin       : IGP
As-path      : 20485 31205
Root Cause
According to AS_PATH attribute, BGP process did not accept routes via eBGP from same AS.
Suggestions
BGP configuration on NSK
<nsk-NE40-4-1>dis c c b
#
bgp 31205
 network 83.149.49.0 255.255.255.0
 undo synchronization
 peer 217.150.36.72 as-number 20485
 peer 217.150.36.72 allow-as-loop
#
=============================================
BGP configuration on KRK
<krk-NE40-4-2>dis c c b
#
bgp 31205
 network 83.149.48.0 255.255.255.0
 undo synchronization
 peer 217.150.37.65 as-number 20485
 peer 217.150.37.65 allow-as-loop 
#
return

END