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New Member

need help with this BGP

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

  • LAN Switching and Routing
1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: need help with this BGP

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

19 REPLIES
Hall of Fame Super Silver

Re: need help with this BGP

Nelson

Your attempts to hide sensitive data give us very little to work with. You say that there should be 16 class Cs advertised but there are only 3 network statements and 2 aggregate addresses (which might duplicate the address space of the network statements). So where are the 16 class Cs?

The other obvious comment is that the routes to be advertised are controlled by the distribute list 101. But you have not given us any information about what is in that list. That may be the critical item in determining what is advertised.

If you want better help you need to give us better information to work with.

HTH

Rick

Hall of Fame Super Silver

Re: need help with this BGP

Nelson

To answer your question about the aggregate address it is used as a way to summarize advertisements. When you configure an aggregate address then BGP looks for longer prefixes that are present within the address space referenced by the aggregate address and if at least one longer prefix is present in the routing table then BGP will advertise the aggregate (summary) address.

HTH

Rick

Hall of Fame Super Silver

Re: need help with this BGP

Hello,

the aggregate-address will create two /24 prefixes if at list a subnet component is in the BGP table (one for each prefix)

two checks to be done :

a) what is permitted by ACL 101 ? only what is permitted will be advertised

b) Are the 16 class C prefixes in the router BGP tables as connected or static

Verify with the show ip bgp if the 16 prefixes are in the local BGP table

Hope to help

Giuseppe

New Member

Re: need help with this BGP

that's where im confused, the sh ip bgp does not list all 16 classe c's...

Note I'm taking over from someone that was fired... so I'm trying my best to get the information and put the puzzle together...

List 101 says:

Extended IP access list 101

10 permit ip host 216.24.239.0 host 255.255.255.0

20 permit ip host 216.24.238.0 host 255.255.255.0 (4 matches)

30 permit ip host 216.24.224.0 host 255.255.255.0 (4 matches)

Hall of Fame Super Silver

Re: need help with this BGP

Nelson

If the entire content of access list 101 is these three statements then your router can advertise only the 3 class C networks to that neighbor. (and based on the matches I would guess that only 2 are actually being advertised).

HTH

Rick

New Member

Re: need help with this BGP

i still am confused with that aggregate-address portion...

Re: need help with this BGP

As Rick and the others already stated; the three networks in list 101 are the only ones that are gonna be advertised.

Also, the aggregates do not seem to do very much, I would have expected another mask, 255.255.254.0 for example. The aggregate could be intended to advertise a route like 216.24.224.0 /20 (224 - 239).

Most important question: which adress range is supposed to be advertised?

regards,

Leo

New Member

Re: need help with this BGP

216.24.224.0 /20 which is the 16 classes they own, but looks like there not advertising them all...

Bronze

Re: need help with this BGP

have you added them to your distribute list? anything not in there now will not be advertised out to the neighbor because of the implicit deny at the end of the ACL

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