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Prefix list Logic ---Pl help

Unanswered Question
Jul 9th, 2010
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Dear All,

I have following prefix list on my internet router for which need to understand the  Prefix list logic.

router bgp xxxx (public AS)

nei x.x.x.x remote-as 4755

neighbor x.x.x.x prefix-list VSNL-ANNOUNCE out

nei y.y.y.y remote-as 1236

neighbor y.y.y.y prefix-list Bharti-ANNOUNCE out

network 203.99.216.0 mask 255.255.254.0
network 203.99.216.0
network 203.99.217.0

ip prefix-list Bharti-ANNOUNCE seq 5 permit 203.99.216.0/24
ip prefix-list Bharti-ANNOUNCE seq 15 permit 203.99.216.0/23
!
ip prefix-list DENY-ALL seq 5 deny 0.0.0.0/0 le 32
!
ip prefix-list  VSNL-ANNOUNCE seq 10 permit 203.99.217.0/24
ip prefix-list VSNL-ANNOUNCE seq 15 permit 203.99.216.0/23

Please help me to understand the logic of the prefix list.

Because Bharti SP is getting 203.99.216.0/24 networks only & VSNL is getting  203.99.216.0/23 networks.


Please help

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netops044 Fri, 07/09/2010 - 02:41
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Hi Yogesh,


The BGP config looks ok. Pls check the below points:


1. Networks are originated only if there is an exact match entry in the IGP. So please check in your local BGP table that you have all three entries that you mentioned. "show ip bgp". Since you said VSNL is not receiving x.x.17.0/24 check whether it exists in your local BGP table. One way of injecting would be to have a route pointing towards null0 with higher AD but care must be taken.


2. Also check what are the prefixes that you are advertising to the neighbors "show ip bgp neighbor x.x.x.x advertised-routes". If this looks ok, then mostly chances could be the neighbor is filtering the prefixes.


3. In that case you can ask them to check for any filters applied. If they have enabled soft-reconfiguration inbound then neighbors can check the routes receiving from you with "show ip bgp neighbor x.x.x.x received-routes".


HTH..


thanks,

Arun

yogesh.suryawanshi Fri, 07/09/2010 - 03:15
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Hi,

Thanks for your reply.

Point 1 - It is already taken care. All routes are injecting in to BGP

SP's have confirmed that they dont have any filters.


Point2 -

Let me repeate the prefixes received by other SP.

VSNL :  203.99.216.0/23 (traffice sources from  203.99.216.0/24 & 203.99.217.0/24  moves over VSNL)

Bharti : 203.99.216.0/24 ( Traffice soruced only from 203.99.216.0/24 moves not from 203.99.217.0/24

Below output looks intresting

VSNL Output...(

RTR##sh ip bgp neighbors 121.244.78.230 advertised-routes
BGP table version is 26, local router ID is 203.99.217.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 203.99.216.0/23  0.0.0.0                  0         32768 i
*> 203.99.217.0     203.99.216.3             0         32768 i

Total number of prefixes 2
CTS-MUM-Internet#

Bharti Output

RTR#sh ip bgp neighbors 125.18.0.209 advertised-routes
BGP table version is 26, local router ID is 203.99.217.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric      LocPrf Weight Path
*> 203.99.216.0     0.0.0.0                  0         32768 i
*> 203.99.216.0/23  0.0.0.0                  0         32768 i

Total number of prefixes 2

Output marked in RED saying the same which pattern we are seeing...


I think logic is working as under (Please correct me if i am wrong) Let's recall prefix-list


ip prefix-list Bharti-ANNOUNCE seq 5 permit 203.99.216.0/24
ip prefix-list Bharti-ANNOUNCE seq 15 permit 203.99.216.0/23
!
ip prefix-list VSNL-ANNOUNCE seq 10 permit 203.99.217.0/24
ip prefix-list VSNL-ANNOUNCE seq 15 permit 203.99.216.0/23


Bharti : If seq no.s 5 is satisfied no need to come on seq 15 or seq 5 is satisfied & seq 15 already contains 203.99.216.0/24 hence it is not considered.


VSNL : If seq no.s 10 is satisfied no need to come on seq 15 or it may like that seq 10 is satisfied & seq 15 already contains 203.99.217.0/24 hence it is not considered.



Please help to understand

Regards

Yogesh

mlund Tue, 07/20/2010 - 02:29
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  • Silver, 250 points or more

Hi Yogesh


As I see it, it's exactly how You have defined it.


for Bharti you hae these two lines

ip prefix-list Bharti-ANNOUNCE seq 5 permit 203.99.216.0/24 << this line matches redmarked line
ip prefix-list Bharti-ANNOUNCE seq 15 permit 203.99.216.0/23 << this line matches the unmarked line

when looking at what Bharti receives

*> 203.99.216.0     0.0.0.0                  0         32768 i
*> 203.99.216.0/23  0.0.0.0                  0         32768 i


for VSNL

ip prefix-list  VSNL-ANNOUNCE seq 10 permit 203.99.217.0/24 << this line matches the unmarked line
ip prefix-list VSNL-ANNOUNCE seq 15 permit 203.99.216.0/23 << this line matches the redmarked line


this is what VSNL receives

*> 203.99.216.0/23  0.0.0.0                  0         32768 i
*> 203.99.217.0     203.99.216.3             0         32768 i



Mikael

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