Adding a network to eBGP configuration - practical considerations.

Are there any things to consider when adding another subnet to eBGP router?

Existing:
router bgp 43447
 bgp router-id 192.168.0.111
 address-family ipv4 unicast
  bgp attribute-download
network 16.17.236.0/22

Proposed:
config t
router bgp 43447
network 17.18.236.0/22
exit
wri mem

Do I need to notify the ISP? Any other gotchas? This site has had the one /22 for ages and
want to add this new network as unobtrusively as possible. Any tips appreciated!
LVL 2
amigan_99Network EngineerAsked:
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JustInCaseCommented:
router bgp 43447
 address-family ipv4 unicast
  network 17.18.236.0/22
end 
wr

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Generally, that should be it except if ISP is filtering incoming routes.
Sure, you need to have exact matching route in routing table (both prefix and subnet mask, otherwise prefix will not be advertised).
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amigan_99Network EngineerAuthor Commented:
By route matching in routing table - you mean at least a null route like

ip route 17.18.236.0/22 null0

?

At least as a first step.
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atlas_shudderedSr. Network EngineerCommented:
amigan - no, the network will need to be known to the bpg router.  In other words, you can input the network statement to bgp as noted and affirmed by Pre but, if the router doesn't actually have it's own actual route for that network statement pointing downstream (i.e. static route, connected, redistribution from OSPF/EIGRP/blah, blah), it won't actual advertise out.  It has to have an actual (other than the bgp statement itself) match to the route in the router's route table to be installed and advertised by bgp.
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amigan_99Network EngineerAuthor Commented:
So if I add "ip route 17.18.236.0/22 null0 " to a neighboring router which redistributes statics into iBGP and the edge router learns it that way - THEN all would be good. Is that right?
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atlas_shudderedSr. Network EngineerCommented:
yes or you could just redistribute static on the bgp router and then create the static locally.  The route just has to be active in the route table and cannot be learned from the bgp process.

So -
Scenario 1
Local BGP router
ip route 17.18.236.0/22 null0
router bgp 43447
 bgp router-id 192.168.0.111
 address-family ipv4 unicast
  bgp attribute-download
network 17.18.236.0/22

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Route advertises

Scenario 2
Remote router:
ip route 17.18.236.0/22 null0
router ospf 1
redistribute static subnets
network 0.0.0.0 255.255.255.255 area 0

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Local BGP router
router ospf 1
network 0.0.0.0 255.255.255.255 area 0

router bgp 43447
 bgp router-id 192.168.0.111
 address-family ipv4 unicast
  bgp attribute-download
network 17.18.236.0/22

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Route advertises

Scenario 3
Local BGP router
show ip bgp 17.18.236.0

BGP routing table entry for 17.18.236.0/22, version 57531
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     13        
  Local
    0.0.0.0 from 0.0.0.0 (172.21.22.23.)
      Origin incomplete, metric 1, localpref 100, weight 32768, valid, sourced, best

ip route 17.18.236.0/22 null0
router bgp 43447
 bgp router-id 192.168.0.111
 address-family ipv4 unicast
  bgp attribute-download
network 17.18.236.0/22

Open in new window

Route fails to advertise
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amigan_99Network EngineerAuthor Commented:
Thank you!
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JustInCaseCommented:
There is a little confusion here. Scenario 3 may be misleading without more details.
:)

In scenario 3 route maybe will not be readvertised because of rib failure (if route was not also learned through IGP), however route ip route 17.18.236.0/22 null0 will still be advertised if router that got route from BGP neighbor has any more specific contributing route in its routing table (learned statically or dynamically). Generally, advertisement should fail only if route was not learned also through IGP (earned only through BGP which will mark next hop as invalid).
For example there is configured:
ip route 17.18.237.0 255.255.255.0 1.1.1.1
(and if next-hop to 1.1.1.1 is valid next hop).

Null 0 is typically configured in cases where there are more specific routes and then only summary route should be advertised into BGP. If exact match is learned through some other process there is no need to configure null0 route.
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amigan_99Network EngineerAuthor Commented:
Thanks Predrag.
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