BGP route advertisments

Answered Question
Oct 10th, 2008
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My company have an eBGP peering to one of our two ISPs to whom we have historically always advertised only a 21x.xx.0.0/20 summarized address.


For the most part this ISP is a secondary "backup" internet connection in case the primary fails and as such we are prepending the AS number 3 times in order to make this link the least preferred path for return traffic into our AS for the entire summarized network range.


However due to a recent failure of one of our major WAN links which cut us off totally from the 21x.xx.2.0/24 network range for a considerable amount of time I have decided that we need to advertise this route out specifically into BGP over the backup link so in the case of any other WAN failures over to this site, our customers internet connectivity will remain up, since all external traffic to the subnet would take the more direct path to this subnet by entering our AS via the backup internet link.



In order to acheive this I have configured as follows


created a static route pointing to null0 for 21x.xx.2.0/24, then advertised this route into BGP


as follows

network 21x.xx.2.0 mask 255.255.255.0

in addition to the already advertised

network 21x.xx.0.0 mask 255.255.240.0

which as stated we have always had configured.


Now in doing this I have setup a distribute list so as not to advertise this subnet to the other IBGP peer because I obviously don;t want to advertise this subnet out of the primary link as well.


The problem is when I check the config it appears as follows


router bgp 1xxxx

network 21x.xx.0.0 mask 255.255.240.0

network 21x.xx.2.0

etc etc


and when I check the advertised routes it appears as follows, which as you can see does not contain the network mask.


dr-72k-bgp-01#sh ip bgp neigh 2xx.2xx.38.233 advertised-routes

BGP table version is 401110, local router ID is 2xx.xx.1.246

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete


Network Next Hop Metric LocPrf Weight Path

*> 2xx.xx.0.0/20 2xx.xx.0.2 20 32768 i

*> 2xx.xx.2.0 0.0.0.0 0 32768 i



This seems wrong to me and it probably something simple, however right now I don't understand why the mask is not being accepted in my BGP network statements?


Any help would be appreciated.

Correct Answer by Giuseppe Larosa about 8 years 7 months ago

Hello Chris,

BGP hides the mask when it is the default one:

a net 21x is a Class C major network so BGP removes the /24 mask in the command and in the output of sh ip bgp.


I think everything is fine in your setup at least you see the specific /24 is advertised as you want.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Fri, 10/10/2008 - 08:54
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Hello Chris,

BGP hides the mask when it is the default one:

a net 21x is a Class C major network so BGP removes the /24 mask in the command and in the output of sh ip bgp.


I think everything is fine in your setup at least you see the specific /24 is advertised as you want.


Hope to help

Giuseppe


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