need help with this BGP

Answered Question
Jul 22nd, 2008

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

I have this problem too.
0 votes
Correct Answer by Richard Burts about 8 years 4 months ago

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

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Richard Burts Tue, 07/22/2008 - 09:48

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

Richard Burts Tue, 07/22/2008 - 09:50

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

Giuseppe Larosa Tue, 07/22/2008 - 09:50

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

pipsadmin Tue, 07/22/2008 - 10:03

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)

Richard Burts Tue, 07/22/2008 - 10:15

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

lgijssel Tue, 07/22/2008 - 10:28

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

pipsadmin Tue, 07/22/2008 - 10:36

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

gojericho0 Tue, 07/22/2008 - 10:43

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

lgijssel Tue, 07/22/2008 - 10:57

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

pipsadmin Tue, 07/22/2008 - 11:27

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?

pipsadmin Tue, 07/22/2008 - 11:50

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.

Richard Burts Tue, 07/22/2008 - 12:00

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

Giuseppe Larosa Tue, 07/22/2008 - 11:28

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

Correct Answer
Richard Burts Tue, 07/22/2008 - 11:43

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 Wed, 07/23/2008 - 04:56

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 Wed, 07/23/2008 - 09:35

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

Actions

This Discussion