need help with this BGP

[pipsadmin]

From the BGP config bellow, what is this aggregate-address do?

Also, this is suppose to advertise 16 class C's, yet I only see 3 being advertised... ?!? Confused...

router bgp 17xxx

no synchronization

bgp log-neighbor-changes

network xxx.xxx.224.0

network xxx.xxx.238.0

network xxx.xxx.239.0

aggregate-address xxx.xxx.224.0 255.255.255.0

aggregate-address xxx.xxx.239.0 255.255.255.0

redistribute connected

redistribute static

neighbor xxx.xxx.212.241 remote-as 701

neighbor xxx.xxx.212.241 description MCI_link

neighbor xxx.xxx.212.241 version 4

neighbor xxx.xxx.212.241 soft-reconfiguration inbound

neighbor xxx.xxx.212.241 distribute-list 199 in

neighbor xxx.xxx.212.241 distribute-list 101 out

no auto-summary

[Giuseppe Larosa]

Hello,

only the last two are advsertised.

however if you need to advertise the /20 aggregate you need to add :

aggregate-address 216.24.224.0 255.255.240.0

access-list 101 permit ip host 216.24.224.0 host 255.255.240.0

Hope to help

Giuseppe

[Richard Burts · Accepted solution]

Nelson

In determining what has precedence there are 2 factors to consider: prefix length, and administrative distance.

Most people are pretty familiar with administrative distance as a way to determine precedence in which locally connected routes with AD of 0 are the best, and static routes (with default AD of 1) are next best. BGP (with default AD of 20 for EBGP) is next and has precedence over EIGRP (AD of 90) and RIP (AD 0f 120).

But prefix length is more important than AD. So if the ISP has a static route for 216.24.224.0/20 with AD of 1 pointing to router 3 and if it receives an advertisement for 216.24.224.0/24 with AD of 20 pointing to router 1, then the longer prefix (of /24) takes precedence over the AD (of 1 or 20) and packets would be routed to router 1.

HTH

Rick

[pipsadmin]

thanks for all the help...

[pipsadmin]

OK,

I got some more information this morning after getting a hold of some techs.

If the carrier setup a static route on there router (ALTER) pointing to R3 and the BGP session is with the R1.

I need to start setting up this new router on this new carrier, let's call it GT.

Out of the 216.24.224.0/20, I need to start advertising 6 of the included c classes which are :

216.24.232.0/24

216.24.233.0/24

216.24.234.0/24

216.24.235.0/24

216.24.236.0/24

216.24.237.0/24

I would rather like to advertise every block indevidualy instead of advertising a larger prefix due to the fact that I will need to cut over the 224.0/24 to 231.0/24 and 238.0/24 and 239.0/24 later on mid August when the cut-over actualy happens (scheduled for august 16th).

what I'm wondering about theses AD and Static routes the other carrier is presently doing, am I able to advertise to my new carrier the listed 6 c classes without affecting the other c classes current being used on the other carrier?

I just need to be garanteed that there wont be any impact to the other current c classes being used.

Sorry if I'm repeating myself, it's just that I dont want this to screw up. And questioning myself at this time is not something good... LOL

[Richard Burts]

Nelson

I would not assume that if you advertise some of the /24s to a new provider that it will have no impact on the old provider. If you advertise the routes to the new provider and they advertise them to the Internet (which would be the logical thing for them to do) then it certainly will impact the old provider since some parts of the Internet are likely to use the new advertised routes instead of the old advertised routes. And if the old provider is advertising your addresses in a block to the Internet and you start advertising /24s through the new provider then ALL the Internet will start using the new routes. (if the old provider is advertising a /20 (for example) and the new provider starts advertising individual /24s then the /24s will win every time.

You can certainly set up to run BGP with the new provider and check connectivity etc. But I would restrict that to not advertise any routes until you are ready to really cut over to them. (or perhaps set up a maintenance window some time when Internet traffic is minimal, advertise the routes to the new provider, check to see that the new advertisements do show up in the Internet routing table, and then remove the advertisement of routes to the new provider)

HTH

Rick