BGP: Route Reflector

BGP: Route Reflector

R4-------iBGP-------R1------iBGP-------R2------eBGP-------R3

·         AS 100: R4, R1 and R2

·         AS200: R3

·         eBGP between R2 & R3

·         iBGP between R2 & R1

·         iBGP between R1 & R4

·         Advertised 3.3.3.3/32 in R3

Let me take a scenario without configuring Route Reflector and see what problems I get:

BGP Configuration:

R3#show run | begin router bgp

router bgp 200

 no synchronization

 bgp log-neighbor-changes

 network 3.3.3.3 mask 255.255.255.255

 neighbor 20.1.1.2 remote-as 100

 no auto-summary

!

R2#show run | begin router bgp

router bgp 100

 no synchronization

 bgp log-neighbor-changes

 neighbor 1.1.1.1 remote-as 100

 neighbor 1.1.1.1 update-source Loopback0

 neighbor 1.1.1.1 next-hop-self

 neighbor 20.1.1.3 remote-as 200

 no auto-summary

!

R1#show run | begin router bgp

router bgp 100

 no synchronization

 bgp log-neighbor-changes

 neighbor 2.2.2.2 remote-as 100

 neighbor 2.2.2.2 update-source Loopback0

 neighbor 4.4.4.4 remote-as 100

 neighbor 4.4.4.4 update-source Loopback0

 no auto-summary

!

R4#show run | begin router bgp

router bgp 100

 no synchronization

 bgp log-neighbor-changes

 neighbor 1.1.1.1 remote-as 100

 neighbor 1.1.1.1 update-source Loopback0

 no auto-summary

!

R3#show ip bgp sum

BGP router identifier 133.133.133.133, local AS number 200

BGP table version is 8, main routing table version 8

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 4/3 prefixes, 4/3 paths, scan interval 60 secs

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

20.1.1.2        4   100     321     314        8    0    0 05:10:58        0

R3#

R2#show ip bgp sum

BGP router identifier 22.22.22.22, local AS number 100

BGP table version is 12, main routing table version 12

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

1 BGP AS-PATH 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 441 total bytes of memory

BGP activity 4/3 prefixes, 4/3 paths, scan interval 60 secs

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

1.1.1.1         4   100      21      20       12    0    0 00:15:49        0

20.1.1.3        4   200     315     322       12    0    0 05:11:18        1

R2#

R1#show ip bgp sum

BGP router identifier 111.111.111.111, local AS number 100

BGP table version is 10, main routing table version 10

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

1 BGP AS-PATH 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 441 total bytes of memory

BGP activity 4/3 prefixes, 4/3 paths, scan interval 60 secs

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

2.2.2.2         4   100      21      22       10    0    0 00:16:04        1

4.4.4.4         4   100      19      21       10    0    0 00:15:29        0

R1#

R4#show ip bgp sum

BGP router identifier 4.4.4.4, local AS number 100

BGP table version is 3, main routing table version 3

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

1.1.1.1         4   100      21      19        3    0    0 00:15:46        0

R4#

R3#show ip bgp

BGP table version is 8, local router ID is 133.133.133.133

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

*> 3.3.3.3/32       0.0.0.0                  0         32768 i

R3#

R2#show ip bgp

BGP table version is 12, local router ID is 22.22.22.22

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

*> 3.3.3.3/32       20.1.1.3                 0             0 200 i

R2#

R1#show ip bgp

BGP table version is 10, local router ID is 111.111.111.111

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

*>i3.3.3.3/32       2.2.2.2                  0    100      0 200 i

R1#

R4#show ip bgp

R4#

Observation:

·         R3 advertised 3.3.3.3/32 into BGP

·         R2 got it as an eBGP update from R3 and has it in its BGP table

·         R1 got it as an iBGP update from R2 and has it in its BGP table

·         R4 doesn’t have the prefix???

Let’s see the routes each router advertising:

R3#show ip bgp neighbor 20.1.1.2 advertised-routes

BGP table version is 8, local router ID is 133.133.133.133

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

*> 3.3.3.3/32       0.0.0.0                  0         32768 i

Total number of prefixes 1

R3#

R2#show ip bgp neighbor 1.1.1.1 advertised-routes

BGP table version is 12, local router ID is 22.22.22.22

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

*> 3.3.3.3/32       20.1.1.3                 0             0 200 i

Total number of prefixes 1

R2#

R1#show ip bgp neighbor 4.4.4.4 advertised-routes

Total number of prefixes 0

R1#

Observation:

·         R3 is sending an eBGP update to R2 about 3.3.3.3/32

·         R2 is sending an iBGP update to R1 about 3.3.3.3/32

·         But R1 is not advertising this route to R4???

·         Actually this is expected-behaviour.

·         As per the BGP RFC, any BGP Router cannot pass updates received from one iBGP neighbor to another iBGP neighbor.

·         This is what is called BGP-Split-Horizon Rule.

·         In our scenario, R1 can pass the update to R4 if it is acting as Route-Reflector for R4.

Route-Reflector:

·         Route Reflector is used to avoid the full-mesh requirement an iBGP.

·         Route Reflector can have clients, non-clients or eBGP neighbors

·         Route Reflector work on the following 3 rules:

1.  Route from a non-client peer: reflect to all the client peers with in the cluster.

2.  Route from a client peer: reflect to other clients and non-client peers.

3.  Route from an external BGP peer: reflect the update to all clients and non-client peers. 

Let’s configure R1 as the Route-Reflector for R4 and see what happens:

R1(config)#router bgp 100

R1(config-router)#neighbor 4.4.4.4 route-reflector-client

R1(config-router)#

*Jun 15 22:20:15.765: %BGP-5-ADJCHANGE: neighbor 4.4.4.4 Down RR client config change

R1(config-router)#

*Jun 15 22:20:17.897: %BGP-5-ADJCHANGE: neighbor 4.4.4.4 Up

R1(config-router)#

R1#show ip bgp neighbor 4.4.4.4 advertised-routes

BGP table version is 10, local router ID is 111.111.111.111

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

*>i3.3.3.3/32       2.2.2.2                  0    100      0 200 i

Total number of prefixes 1

R1#

** R1 is sending the update to R4 now

R4#show ip bgp

BGP table version is 4, local router ID is 4.4.4.4

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

*>i3.3.3.3/32       2.2.2.2                  0    100      0 200 i

R4#

** R4 got the update and its also the best route as Next-hop is reachable via IGP.

Lets see some more details on this newly learned route on R4:

R4#show ip bgp 3.3.3.3

BGP routing table entry for 3.3.3.3/32, version 4

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

  200

    2.2.2.2 (metric 129) from 1.1.1.1 (111.111.111.111)

      Origin IGP, metric 0, localpref 100, valid, internal, best

      Originator: 22.22.22.22, Cluster list: 111.111.111.111

R4#

Observations:

·         Originator: The router who originated this route initially

·         R2 is the one who originated this route into AS100

·         22.22.22.22 is the BGP router-ID of R2

·         Cluster-list: list of the RRs this route has passed through

·         111.111.111.111 is the Route-ID of R1

·         These attributes are used to avoid loops inside an AS