1. BGP multihoming with uneven links - sample config and Qs
I was hoping for some input from some of the BGP gurus here, in regards
to a suggested configuration of a dual ISP + 1 router, with uneven links
(DS3 and T1). I have asked earlier about this, and I have been advised to
upgrade the T1 to DS3, but we do not have that financial capability, and
would rather eliminate some Internet services, if the DS3 goes down ...
but this is just logistics. The issue I would like to raise is a
configuration suggested to us by the ISP with the T1 (I'll call them
ISP-T1), involving something along these lines:
router bgp <my-ASN>
no bgp log-neighbor-changes
network <my-public-net-class-C> --> this one comes from the ISP-T1
neighbor ISP-T1 peer-group
neighbor ISP-T1 remote-as XXXX --> ISP-T1 AS number
neighbor ISP-T1 version 4
neighbor ISP-T1 send-community
neighbor ISP-DS3 peer-group
neighbor ISP-DS3 remote-as YYYY --> ISP-T1 AS number
neighbor ISP-DS3 version 4
neighbor ISP-DS3 route-map localpref in
neighbor ISP-DS3 route-map ISP-DS3 out
.... various ip route ...
ip bgp-community new-format
ip as-path access-list 10 deny _XXXX_
ip as-path access-list 10 permit .*
ip as-path access-list 20 deny _YYYY_
ip as-path access-list 20 permit .*
ip flow-export version 5
ip flow-export destination <my-flow-collector>
.... other access lists ...
route-map ISP-T1 permit 10
match as-path 10
set community ZZZZ:WW
route-map localpref permit 10
set local preference 300
route-map ISP-DS3 permit 10
match as-path 20
So - looking at the above: ISP-T1 implies that he could help us control
the majority of the traffic through ISP-DS3, because of a community
ZZZZ:WW used at his end, also, and the local preference. My questions are:
1. I do not understand (yet) all this, so I have no idea what the
community woullld do (how does it get defined at the ISP end, top allow -
presumably - more routes to come the other way (through ISP-DS3)?
2. What would happen in case of ISP-T1 failure (under two scenarios: my
link to them, or their end)?
NOTE: all of the above has been typed just now, as we scribbled the stuff
over the phone, on a notepad, so if there are typos, please do not
consider them fundamental mistakes.
I would appreciate any comments,
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