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need help with this BGP

pipsadmin
Level 1
Level 1

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

1 Accepted Solution

Accepted Solutions

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

HTH

Rick

View solution in original post

19 Replies 19

Richard Burts
Hall of Fame
Hall of Fame

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

HTH

Rick

Richard Burts
Hall of Fame
Hall of Fame

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

HTH

Rick

Giuseppe Larosa
Hall of Fame
Hall of Fame

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

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)

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

HTH

Rick

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

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

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

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

As you inherited this config it is hard to figure out why it looks like it does. Perhaps the other subnets were never needed until now? To perform what is desired, the aggregate-address should be: 216.24.224.0 255.255.240.0

also, acl 101 line 30 must be changed: permit ip host 216.24.224.0 host 255.255.240.0

regards,

Leo

I'm not going to change anything on here due to the fact that all the c classe are going to be moved to another carrier, and a new 3845...

I was just trying to figure out what we being advertised...

Also, I called the carrier and due to some BGP issue that happened back in december, they where requested to add a static route on there edge network for the 16 class C's to point to a completly different router (we call it R3 as this one is called R1)...

My question now is since R1 is advertising 3 class c's (224.0 , 238.0 , 239.0) what happens to this if the carrier adds a static route of 216.24.224.0/20 to point to R3 ?

Does BGP have precedence to Static routes? I know static has priority over RIP, but what about BGP?

It has nothing to do with BGP specifically.

Route selection dictates that more specific routes (/24 against /21) are more preferable.

Please check this url:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml

Leo

so I don't understand why the carrier had to put a Static route in order to fix this BGP...?!?

I'll investigate more on that end.

Nelson

Without knowing more about the previous problem we certainly can not say why the carrier had to put in static routes. But my guess is that some or all of the routes were not being properly advertised with BGP and the carrier put in the static routes as a quick way to get the problem solved. (does this issue perhaps relate in some way to the reason why the previous person was fired?)

If you get the BGP issues straightened out and are properly advertising the networks then the carrier could probably remove their static routes.

One thing to be careful about in trying to understand what and why the carrier did is to be very careful about what static routes the carrier put in. Your discussion seems to assume that it would have been 216.24.224.0/20. I wonder if they did not put in 16 separate static routes, one for each of the 16 class C networks.

HTH

Rick

HTH

Rick
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