From: Ron Royston (ccie6824@xxxxxxxxxxx)
Date: Tue Nov 06 2001 - 20:48:33 GMT-3
Your software version is fine. I haven't seen the 'bgp conditional
advertisement' used too much in the field. The real question, I think, is
when to use this feature. About a year ago, Some other people on this list
and I went over this feature. I believe I was trying to find an alternate
way to bring up a DDR circuit, It was:
router bgp 100
***text omitted***
neighbor 17.255.255.6 remote-as 300
neighbor 17.255.255.6 advertise-map ToL7 non-exist-map If_No_47
!
ip classless
access-list 10 permit 47.0.0.0 0.255.255.255
access-list 17 permit 17.0.0.0 0.255.255.255
access-list 101 permit ip any any
route-map If_No_47 permit 10
match ip address 10
!
route-map ToL7 permit 10
match ip address 101
*** With frame link administratively down ***
L7#sho ip bgp
BGP table version is 55, local router ID is 77.255.255.17
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 11.0.0.0 17.255.255.5 0 0 100 ?
*> 14.0.0.0 17.255.255.5 0 0 100 ?
*> 17.0.0.0 0.0.0.0 0 32768 ?
*> 23.0.0.0 17.255.255.5 0 100 ?
*> 33.0.0.0 17.255.255.5 0 100 ?
*> 34.0.0.0 17.255.255.5 0 100 ?
*> 44.0.0.0 17.255.255.5 0 100 ?
*> 77.0.0.0 0.0.0.0 0 32768 ?
L7#conf t
Enter configuration commands, one per line. End with CNTL/Z.
L7(config)#int se0
L7(config-if)#no shut
****after about 60 seconds for reconvergence***
L7#sho ip bgp
BGP table version is 63, local router ID is 77.255.255.17
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 11.0.0.0 47.255.255.5 0 100 ?
*> 14.0.0.0 47.255.255.5 0 0 100 ?
*> 17.0.0.0 0.0.0.0 0 32768 ?
*> 22.0.0.0 47.255.255.5 0 100 200 ?
*> 23.0.0.0 47.255.255.5 0 100 ?
*> 33.0.0.0 47.255.255.5 0 100 ?
*> 34.0.0.0 47.255.255.5 0 0 100 ?
*> 44.0.0.0 47.255.255.5 0 0 100 ?
*> 47.0.0.0 0.0.0.0 0 32768 ?
*> 77.0.0.0 0.0.0.0 0 32768 ?
So, I was advertising everything via one gateway if a particular link went
down. The topology was:
My senario looks like,
AS100 <---> AS100
R3--------R4---------R1
| \Frame |ISDN
| \ |
| \__ R7(AS300)
R2(AS200)
R7 is running eBGP w/ R4 and R1. R1 is running the conditional
advertisement feature. There is no dynamic IGP between AS100 and AS300
(all loopbacks are router# twice, e.g. 11, 22, 77, etc. and all links are R#
to R#, e.g., 34 is the link between R3 and R4)
Without the 'non-exist-map' statement, the remote router gets routes from
both peers in his bgp database. See R7's bgp database without the
conditional feautre on R1:
Network Next Hop Metric LocPrf Weight Path
* 11.0.0.0 17.255.255.5 0 0 100 ?
*> 47.255.255.5 0 100 ?
* 14.0.0.0 17.255.255.5 0 0 100 ?
*> 47.255.255.5 0 0 100 ?
*> 17.0.0.0 0.0.0.0 0 32768 ?
***text ommitted***
L7 chooses to send his packets to only one peer in this case. Convergence
is faster, however, after the frame link is brought up, L7 does not return
to L4 as his gateway of choice. See below:
L7#sho ip bgp 11.0.0.0/8
***text ommitted***
47.255.255.5 from 47.255.255.5 (44.255.255.17)
Origin incomplete, localpref 100, valid, external, ref 2
100
17.255.255.5 from 17.255.255.5 (11.255.255.17)
Origin incomplete, metric 0, localpref 100, valid, external, best, ref
2
After a 'clear ip bgp *':
L7#sho ip bgp 11.0.0.0/8
***text ommitted***
17.255.255.5 from 17.255.255.5 (11.255.255.17)
Origin incomplete, metric 0, localpref 100, valid, external, ref 2
100
47.255.255.5 from 47.255.255.5 (44.255.255.17)
Origin incomplete, localpref 100, valid, external, best, ref 2
So, the traffic won't take the frame link after the link is broght back up
until you restart bgp. To fix this, I set the weight of all routes heard
from R4 to 20000 on R7. Now, after a frame link up/down/up L4 preempted
without a 'clear ip bgp'. Basically, I have observed this conditional
advertisement feature to work as the name says, 'conditional advertisement'.
In my case, when link 47.0.0.0 goes down, L1 tells L7 of his routes, when
link 47.0.0.0 is up, L1 stops advertising those routes.
As mentioned above, I also got the backup senario working by adding weight
to routes. Without adding wieght, the backup link continues to be used
until a 'clear ip bgp'.
<><><><><><><><><><><><><>
Ron Royston
Avnet Enterprise Solutions
http://www.nsd.avnet.com/
>From: "Jason Whelan" <Jasonw@logic.bm>
>Reply-To: "Jason Whelan" <Jasonw@logic.bm>
>To: <ccielab@groupstudy.com>
>Subject: BGP conditional advert
>Date: Tue, 6 Nov 2001 12:15:59 -0400
>
>I am having problems with getting cond adv to work.
>
>If I use neighbor xxx.xxx.xxx.xxx advmap-map1 non-existmap map2 ,
>and map 1 permits acl1 which is a route in my igp
>and map2 permits acl2 which is a route from a another rtr loopback,
>when I shut down the source rtr loopback, the route disappears from the
>routing table, but the advmap route does not adveertise out to the
>specified neighbor.
>The sh ip bgp neigh shows the conditional map as advertising (even if
>the nonexist route is there or not)
>IOS 12.07T, and also tried IOS 12.1 and 12.05
>??????
>Cheers
>Jason
>
>
>Jason Whelan, MCSE,MCNE,CCNP
>Network Ops
>Logic Communications
>www.logic.bm
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