BGP Allow-as

Dans cette configuration, R1 R2 et R3 forment l’AS 200 et n’annoncent aucun prefix :

R4 et R5 forment l’AS 100 et annoncent respectivement les prefix 4.0.0.0 et 5.0.0.0

R1#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*>i4.0.0.0          10.1.24.4                0    100      0 100 i
*>i5.0.0.0          10.1.35.5                0    100      0 100 i

R2#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 4.0.0.0          10.1.24.4                0             0 100 i
*>i5.0.0.0          10.1.35.5                0    100      0 100 i

R3#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*>i4.0.0.0          10.1.24.4                0    100      0 100 i
*> 5.0.0.0          10.1.35.5                0             0 100 i

Problème :

R4 n’apprend pas le prefix 5.0.0.0 de R5 :

R4#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 4.0.0.0          0.0.0.0                  0         32768 i

R5 n’apprend pas le prefix 4.0.0.0 de R4 :

R5#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 5.0.0.0          0.0.0.0                  0         32768 i

Vérification des annonces :

R2 annonce correctement le prefix 5.0.0.0 à R4

R2#show ip bgp neighbors 10.1.24.4 advertised-routes
BGP table version is 5, local router ID is 10.1.123.2
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
*>i5.0.0.0          10.1.35.5                0    100      0 100 i

Total number of prefixes 1

R3 annonce correctement le prefix 4.0.0.0 à R5

R3#show ip bgp neighbors 10.1.35.5 advertised-routes
BGP table version is 5, local router ID is 10.1.123.3
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
*>i4.0.0.0          10.1.24.4                0    100      0 100 i

Total number of prefixes 1

Solution: « allawas-in »

R4#conf t
R4(config)#router bgp 100
R4(config-router)#neighbor 10.1.24.2 allowas-in

   
R5#conf t
R5(config)#router bgp 100
R5(config-router)#neighbor 10.1.35.3 allowas-in

Vérifications:

R4#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 4.0.0.0          0.0.0.0                  0         32768 i
*> 5.0.0.0          10.1.24.2                              0 200 100 i

R5#show ip bgp | be Net
   Network          Next Hop            Metric LocPrf Weight Path
*> 4.0.0.0          10.1.35.3                              0 200 100 i
*> 5.0.0.0          0.0.0.0                  0         32768 i