Changing BGP Local Preference: How can I analyze changes to ip routing table and other appropriate indicators?

[John12321]

Hi All

I hope you can help. We have an AS with 3 routers with 2 or 3 BGP peers on each router.

I'm planning to change the Local Preference on one of the BGP peers so that all of the possible traffic is sent via that peer (it is partial transit so not all traffic will be sent via that link).

We have Icinga set up to monitor traffic on the appropriate virtual interfaces so I will be able to see over time any changes in traffic flow.

However, I wanted to be able to analyze changes to appropriate indicators before and after the change. What I thought of being able to do was to:

• summarise no of entries in the routing table by next hop ip address (i.e. the BGP peer address) - hopefully seeing a rise in those to the BGP peer with the increased local preference
• summarise no of entries in the routing table by route-map - hopefully seeing a rise in those associated with the route-map assigned to the BGP peer with the increased local preference

However, I can't see anyway of doing this. Does anybody have any suggestions about the validity of what I'm trying to do and/or better ways of analyzing appropriate indicators to be able to determine the effects of changing BGP Local preference.

Many thanks in advance

John

[Marius Gunnerud]

• summarise no of entries in the routing table by next hop ip address (i.e. the BGP peer address) - hopefully seeing a rise in those to the BGP peer with the increased local preference

For this you could use the command show ip bgp.  This will show the next hop and the configured local preference.

• summarise no of entries in the routing table by route-map - hopefully seeing a rise in those associated with the route-map assigned to the BGP peer with the increased local preference

Not entirely sure what you are looking for with this.

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[John12321]

Hi Marius

#show ip bgp justs shows the entire bgp table entry by entry. I was wanting to summarise the routing table (show ip route) to see how many entries for each next hop.

I could then see how the numbers change once the "local preference" is changed for a particular peer.

I can't see how to do this using Cisco IOS command line but was hoping somebody could help.

Also, somebody might have an idea on different indicators to analyse to see how the routing table changes when local preferences are changed.

John