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This Question is Not Answered 1 Correct Answer available (4 pts) 2 Helpful Answers available (2 pts)
645 Views 2 Replies Latest reply: Mar 18, 2012 7:42 AM by tnewshott RSS

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BGP book

Mar 18, 2012 3:39 AM

Crich 16 posts since
Dec 22, 2011

Hi! I'm reading route book of W.Odom. on page 445 (chapter 13)

 

"When a router knows routes only for subsets of the prefix that needs to be advertised, an additional step is needed when using the network command. For instance, the network 128.107.0.0 mask 255.255.224.0 command will not add this prefix to the BGP table even if routes for subsets of this range exist, such as 128.107.1.0/24. So, either configure a static route for the entire range, with outgoing interface null0, on the Internet facing router, or use IGP route summarization to create a summary route for the entire prefix with IGP."

 

i cannot understand that. can you explain why this command will not add this prefix to the BGP table?

  • Todd Riemenschneider 4 posts since
    Jun 30, 2008
    Currently Being Moderated
    1. Mar 18, 2012 5:00 AM (in response to Crich)
    Re: BGP book

    There needs to be a route for 128.107.0.0/19 in the routing table known by an IGP, a directly connected interface, or static route, before BGP will advertise it with the network command. Having a more specific prefix 128.107.1.0/24 is not the same. You could advertise the more specific prefix with the network command. So thats why he stated you need a means to get that /19 route in the route table hense the suggestion about using the route to null0. It would then be in the route table via a static route. Just remember if you cant find that exact route in the routing table known by an alternative source (IGP,directly connected interface or static route)  BGP will not advertise it using the network command.

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  • Currently Being Moderated
    2. Mar 18, 2012 7:42 AM (in response to Crich)
    Re: BGP book

    BGP needs an exact match between the network statement under the BGP process and a route in the routing table.  If there is no exact match, no advertisement. Hence the reason for summary routes. Now, you can summarize within BGP to correct this problem as well.

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