R6#sh
ip bgp
BGP
table version is 4, local router ID is 150.6.6.6
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
*>
6.1.0.0/16 0.0.0.0 0 32768 i
*>
6.2.0.0/16 0.0.0.0 0 32768 i
*>
6.3.0.0/16 0.0.0.0 0 32768 i
R6 will send the update to its eBGP peer R1
lets check on R1.
R1#sh
ip bgp
BGP
table version is 4, local router ID is 150.1.1.1
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
*>
6.1.0.0/16 155.1.16.6 0 0 600 i
*>
6.2.0.0/16 155.1.16.6 0 0 600 i
*>
6.3.0.0/16 155.1.16.6 0 0 600 i
Here
we got all the update form R6 note it says best path and valid route.
Then
R1 is fully meshed with R2 ad R3 these updates will be updated to R2 and R3.
R2#sh
ip bgp
BGP
table version is 1, local router ID is 150.2.2.2
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
*
i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*
i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*
i6.3.0.0/16 155.1.16.6 0 100
0 600 i
R3#sh
ip bgp
BGP
table version is 1, local router ID is 150.3.3.3
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
*
i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*
i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*
i6.3.0.0/16 155.1.16.6 0 100
0 600 i
Note
we are getting the update but its not marked as best route. But why ?? Huh the
next hop.
What
we read in iBGP peer?? BGP rule says the next hop value can only be changed by eBGP
by default iBGP peer wont modify the next hop address.
We
can solve this using 3 method.
Method
1: Advertise the transit link into the ospf.
R6(config)#router
ospf 1
R6(config-router)#network
155.1.16.0 0.0.0.255 area 1
R1(config)#router
ospf 1
R1(config-router)#network
155.1.16.0 0.0.0.255 area 1
Now
lets check on R2 and R3 is the received routes are marked as best route.
R2#sh
ip bgp
BGP
table version is 4, local router ID is 150.2.2.2
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
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 i
R3#sh
ip bgp
BGP
table version is 4, local router ID is 150.3.3.3
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
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 i
R2#sh
ip route 6.1.0.1
Routing
entry for 6.1.0.0/16
Known via "bgp 100", distance 200,
metric 0
Tag 600, type internal
Last update from 155.1.16.6 00:03:54 ago
Routing Descriptor Blocks:
* 155.1.16.6, from 150.1.1.1, 00:03:54 ago
Route metric is 0, traffic share count is
1
AS Hops 1
Route tag 600
Its
learned via BGP next hop is 155.1.16.6
R2#sh
ip route 155.1.16.6
Routing
entry for 155.1.16.0/24
Known via "ospf 1", distance 110,
metric 65, type inter area
Last update from 155.1.123.1 on
FastEthernet1/0, 00:04:08 ago
Routing Descriptor Blocks:
* 155.1.123.1, from 150.1.1.1, 00:04:08 ago,
via FastEthernet1/0
Route metric is 65, traffic share count
is 1
Its
learned via OSPF next hop is 155.1.123.1
R2#sh
ip route 155.1.123.1
Routing
entry for 155.1.123.0/24
Known via "connected", distance 0,
metric 0 (connected, via interface)
Routing Descriptor Blocks:
* directly connected, via FastEthernet1/0
Route metric is 0, traffic share count is
1
R2#sh
ip cef 6.1.0.1
6.1.0.0/16,
version 26, epoch 0, cached adjacency 155.1.123.1
0
packets, 0 bytes
via 155.1.16.6, 0 dependencies, recursive
next hop 155.1.123.1, FastEthernet1/0
via 155.1.16.0/24
valid cached adjacency
R5#sh
ip bgp | in 0.0.0.0
*>
5.1.0.0/24 0.0.0.0 0 32768 i
*>
5.2.0.0/24 0.0.0.0 0 32768 i
*>
5.3.0.0/24 0.0.0.0 0 32768 i
R3#sh
ip bgp neighbors 150.2.2.2 advertised-routes
BGP
table version is 7, local router ID is 150.3.3.3
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
*>
5.1.0.0/24 155.1.35.5 0 0 500 i
*>
5.2.0.0/24 155.1.35.5 0 0 500 i
*>
5.3.0.0/24 155.1.35.5 0 0 500 i
R2#sh
ip bgp
BGP
table version is 4, local router ID is 150.2.2.2
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
*
i5.1.0.0/24 155.1.35.5 0 100
0 500 i
*
i5.2.0.0/24 155.1.35.5 0 100
0 500 i
*
i5.3.0.0/24 155.1.35.5 0 100
0 500 i
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 I
Note
the route advertised via R3 is not chosen as the best path because the nest hop
is not reachable.
Lets
modify the next hop self.
R3#sh
run | sec router bgp
router
bgp 100
no synchronization
bgp log-neighbor-changes
neighbor iBGP_Peer peer-group
neighbor iBGP_Peer remote-as 100
neighbor iBGP_Peer update-source Loopback0
neighbor iBGP_Peer next-hop-self ==> [Since R1 and R2 having peer with R3 using update source lo
0 the next will be changed to the lo 0 address]
neighbor 150.1.1.1 peer-group iBGP_Peer
neighbor 150.2.2.2 peer-group iBGP_Peer
neighbor 155.1.34.4 remote-as 400
neighbor 155.1.35.5 remote-as 500
no auto-summary
R2#sh
ip bgp
BGP
table version is 7, local router ID is 150.2.2.2
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
*>i5.1.0.0/24 150.3.3.3 0 100
0 500 i
*>i5.2.0.0/24 150.3.3.3 0 100
0 500 i
*>i5.3.0.0/24 150.3.3.3 0 100
0 500 i
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 I
R1#sh
ip bgp
BGP
table version is 10, local router ID is 150.1.1.1
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
*>i5.1.0.0/24 150.3.3.3 0 100
0 500 i
*>i5.2.0.0/24 150.3.3.3 0 100
0 500 i
*>i5.3.0.0/24 150.3.3.3 0 100
0 500 i
*>
6.1.0.0/16 155.1.16.6 0 0 600 i
*>
6.2.0.0/16 155.1.16.6 0 0 600 i
*>
6.3.0.0/16 155.1.16.6 0 0 600 I
R1#sh
ip cef 5.1.0.1
5.1.0.0/24,
version 38, epoch 0, cached adjacency 155.1.123.3
0
packets, 0 bytes
via 150.3.3.3, 0 dependencies,
recursive
next hop 155.1.123.3, FastEthernet2/0
via 150.3.3.3/32
valid cached adjacency
Method
3: Now
lets have a look on R4 advertised network.
R4#sh
ip bgp | in 0.0.0.0
*>
4.1.0.0/24 0.0.0.0 0 32768 i
*>
4.2.0.0/24 0.0.0.0 0 32768 i
*>
4.3.0.0/24 0.0.0.0 0 32768 i
R3#sh
ip bgp
BGP
table version is 10, local router ID is 150.3.3.3
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
*>
4.1.0.0/24 155.1.34.4 0 0 400 i
*>
4.2.0.0/24 155.1.34.4 0 0 400 i
*>
4.3.0.0/24 155.1.34.4 0 0 400 i
*>
5.1.0.0/24 155.1.35.5 0 0 500 i
*>
5.2.0.0/24 155.1.35.5 0 0 500 i
*>
5.3.0.0/24 155.1.35.5 0 0 500 i
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 i
R2#sh
ip bgp
BGP
table version is 10, local router ID is 150.2.2.2
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
*>i4.1.0.0/24 150.3.3.3 0 100
0 400 i
*>i4.2.0.0/24 150.3.3.3 0 100
0 400 i
*>i4.3.0.0/24 150.3.3.3 0 100
0 400 i
*>i5.1.0.0/24 150.3.3.3 0
100 0 500 i
*>i5.2.0.0/24 150.3.3.3 0 100
0 500 i
*>i5.3.0.0/24 150.3.3.3 0 100
0 500 i
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 i
Since
we have used the 2nd method i.e next-hop-self command
We
can change the next hop manually.
R3#sh
route-map
route-map
nexthop, permit, sequence 10
Match clauses:
Set clauses:
ip next-hop 155.1.123.3
Policy routing matches: 0 packets, 0 bytes
R3#sh
run | s router bgp
router
bgp 100
no synchronization
bgp log-neighbor-changes
neighbor iBGP_Peer peer-group
neighbor iBGP_Peer remote-as 100
neighbor iBGP_Peer update-source Loopback0
neighbor iBGP_Peer next-hop-self
neighbor iBGP_Peer route-map nexthop out
neighbor 150.1.1.1 peer-group iBGP_Peer
neighbor 150.2.2.2 peer-group iBGP_Peer
neighbor 155.1.34.4 remote-as 400
neighbor 155.1.35.5 remote-as 500
no auto-summary
R2#sh
ip bgp
BGP
table version is 16, local router ID is 150.2.2.2
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
*>i4.1.0.0/24 155.1.123.3 0 100
0 400 i
*>i4.2.0.0/24 155.1.123.3 0 100
0 400 i
*>i4.3.0.0/24 155.1.123.3 0 100
0 400 i
*>i5.1.0.0/24 155.1.123.3 0 100
0 500 i
*>i5.2.0.0/24 155.1.123.3 0 100
0 500 i
*>i5.3.0.0/24 155.1.123.3 0 100
0 500 i
*>i6.1.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.2.0.0/16 155.1.16.6 0 100
0 600 i
*>i6.3.0.0/16 155.1.16.6 0 100
0 600 I
R1#sh
ip bgp
BGP
table version is 19, local router ID is 150.1.1.1
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
*>i4.1.0.0/24 155.1.123.3 0 100
0 400 i
*>i4.2.0.0/24 155.1.123.3 0 100
0 400 i
*>i4.3.0.0/24 155.1.123.3 0 100
0 400 i
*>i5.1.0.0/24 155.1.123.3 0 100
0 500 i
*>i5.2.0.0/24 155.1.123.3 0 100
0 500 i
*>i5.3.0.0/24 155.1.123.3 0 100
0 500 i
*>
6.1.0.0/16 155.1.16.6 0 0 600 i
*>
6.2.0.0/16 155.1.16.6 0 0 600 i
*>
6.3.0.0/16 155.1.16.6 0 0 600 i
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