BGP troubleshooting – Prepanding AS, but not affected?

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Here is handy BGP troubleshooting tip you might interesting.

I spoke to one of my customer…. He said he had changed BGP configuration last night. He wanted to make other ISP link as the best path, so he prepanding his AS 7 times(wow! that’s a lot) on his secondary link. But traffic is still going thru secondary link. why?

He thought BGP will update new changes in sometime soon. He didn’t reset BGP session, b/c he was scare to see BGP would bouncing.

I guess he didn’t know the command "Clear IP BGP [Neighbor IP] Soft out".

Also, he was not aware that existing route which is being announced won’t be trigged BGP update. See example below.

 

 

 

If you are currently announcing 10.0.0.0/24 thru ISP link.

 

Customer_Router#sh ip bgp nei 190.10.18.1 ad
BGP table version is 2182698, local router ID is 17.3.8.5
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete

Originating default network 0.0.0.0

Network Next Hop Metric LocPrf Weight Path
10.0.0.0/24 190.10.18.1 0 0 i

 

Total number of prefixes 0
Customer_Router#

 

Not prepanding AS 1004 three times on 10.0.0.0/24.

Without reset BGP session, your change won’t be affected

Only time when you reset(hard / soft) BGP, then your changes will be affected.

However, if you are announcing new route which was not being propagated, then you don’t need to reset BGP session to start BGP update.