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

Publication Date:  2019-07-11 Views:  211 Downloads:  1

Issue Description

We get one AS xxx1 from ISP(xxx2) 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 xxx.xxx.36.72 allow-as-loop //peer to ISP
  
VERIFICATION
<nsk-NE40-4-1>ping xxx.xxx.48.249
  PING xxx.xxx.48.249: 56  data bytes, press CTRL_C to break
    Reply from xxx.xxx.48.249: bytes=56 Sequence=1 ttl=253 time = 18 ms
    Reply from xxx.xxx.48.249: bytes=56 Sequence=2 ttl=253 time = 18 ms
    Reply from xxx.xxx.48.249: bytes=56 Sequence=3 ttl=253 time = 17 ms
    Reply from xxx.xxx.48.249: bytes=56 Sequence=4 ttl=253 time = 17 ms
    Reply from xxx.xxx.48.249: bytes=56 Sequence=5 ttl=253 time = 17 ms
  --- xxx.xxx.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 xxx.xxx.48.0
BGP route xxx.xxx.48.0/24
From         : xxx.xxx.36.72(xxx.xxx.0.11)
State        : valid, external, best,
Nexthop      : xxx.xxx.36.72
Origin       : IGP
As-path      : xxx2 xxx1
It's OK.
2) [krk-NE40-4-2-bgp]peer xxx.xxx.37.65 allow-as-loop // peer to ISP
  
[krk-NE40-4-2]ping xxx.xxx.49.249
  PING xxx.xxx.49.249: 56  data bytes, press CTRL_C to break
    Reply from xxx.xxx.49.249: bytes=56 Sequence=1 ttl=253 time = 19 ms
    Reply from xxx.xxx.49.249: bytes=56 Sequence=2 ttl=253 time = 17 ms
    Reply from xxx.xxx.49.249: bytes=56 Sequence=3 ttl=253 time = 18 ms
    Reply from xxx.xxx.49.249: bytes=56 Sequence=4 ttl=253 time = 17 ms
    Reply from xxx.xxx.49.249: bytes=56 Sequence=5 ttl=253 time = 31 ms
  --- xxx.xxx.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 xxx.xxx.49.0
BGP route xxx.xxx.49.0/24
From         : xxx.xxx.37.65(xxx.xxx.0.11)
State        : valid, external, best,
Nexthop      : xxx.xxx.37.65
Origin       : IGP
As-path      : xxx1 xxx2

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 xxx2
 network xxx.xxx.49.0 255.255.255.0
 undo synchronization
 peer xxx.xxx.36.72 as-number xxx1
 peer xxx.xxx.36.72 allow-as-loop
#
=============================================
BGP configuration on KRK
<krk-NE40-4-2>dis c c b
#
bgp xxx2
 network xxx.xxx.48.0 255.255.255.0
 undo synchronization
 peer xxx.xxx.37.65 as-number xxx1
 peer xxx.xxx.37.65 allow-as-loop
#
return

END