BGP: Communities: No-Advertise

BGP: Communities:  No-Advertise

·         In BGP, communities are basically used to tag the routes so that we can filter them where ever we want.

·         NO-ADVERTISE is a Special Community value, simply it instructs the router, not to forward this route to any neighbor (not even to IBGP neighbor).

·         Let me take a small topology to configure this

               R1-------------------R2

·         AS 100 : R1, R2

·         R1 is advertising a prefix 1.1.1.1/32 into BGP with “no-advertise” community

·         R1 will not forward this prefix to R2.

Let me configure it and see…

R1#show run

router bgp 100

 no synchronization

 bgp router-id 1.1.1.1

 bgp log-neighbor-changes

 network 1.1.1.1 mask 255.255.255.255 route-map NO-ADV

 network 11.11.11.11 mask 255.255.255.255

 neighbor 100.1.12.2 remote-as 100

 no auto-summary

!

route-map NO-ADV permit 10

 set community no-advertise

!

R2(config-router)#do show run

router bgp 100

 no synchronization

 bgp router-id 2.2.2.2

 bgp log-neighbor-changes

 neighbor 100.1.12.1 remote-as 100

 no auto-summary

!

R2(config-router)#

R1#show ip bgp summary

BGP router identifier 1.1.1.1, local AS number 100

BGP table version is 3, main routing table version 3

2 network entries using 234 bytes of memory

2 path entries using 104 bytes of memory

3/2 BGP path/bestpath attribute entries using 372 bytes of memory

1 BGP community entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 734 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

100.1.12.2      4   100     253     254        3    0    0 04:10:00        0

R1#

R2#show ip bgp summary

BGP router identifier 2.2.2.2, local AS number 100

BGP table version is 2, main routing table version 2

1 network entries using 117 bytes of memory

1 path entries using 52 bytes of memory

2/1 BGP path/bestpath attribute entries using 248 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 417 total bytes of memory

BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

100.1.12.1      4   100     254     253        2    0    0 04:10:32        1

R2#

·         Neighbor relationship is up

R1#show ip bgp

BGP table version is 3, local router ID is 1.1.1.1

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

*> 1.1.1.1/32       0.0.0.0                  0         32768 i

*> 11.11.11.11/32   0.0.0.0                  0         32768 i

R1#

R1#show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 2

Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to any peer)

  Not advertised to any peer

  Local

    0.0.0.0 from 0.0.0.0 (1.1.1.1)

      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best

      Community: no-advertise

R1#

R2#show ip bgp

BGP table version is 2, local router ID is 2.2.2.2

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

*>i11.11.11.11/32   100.1.12.1               0    100      0 i

R2#

Observations:

·         1.1.1.1/32 prefix is there only in R1’s BGP table.

·         R1 didn’t advertise this prefix to R2