Configuration procedure

Procedure

Configure IP addresses for interfaces (omitted.)
Configure iBGP.
1. To prevent route flapping caused by port state changes, this example uses loopback interfaces to establish iBGP connections.
2. Because loopback interfaces are virtual interfaces, you need to use the
  peer connect-interface
  command to specify the loopback interface as the source interface for establishing BGP connections.
3. Enable OSPF in AS 65009 to ensure that Switch B can communicate with Switch C through loopback interfaces.

# Configure Switch B

switch(config)# router bgp 65009
switch(bgp)# bgp router-id 2.2.2.2
switch(bgp)# neighbor 3.3.3.3 remote-as 65009
switch(bgp)# exit
switch(config)# router ospf
switch(ospf)# enable
switch(ospf)# area 0
switch(ospf)# network 2.2.2.2/32
switch(ospf)# network 9.1.1.1/24
switch(ospf)# exit
switch(config)# vlan 300
switch(vlan-300)# ip ospf

# Configure Switch C

switch(config)# router bgp 65009
switch(bgp)# bgp router-id 3.3.3.3
switch(bgp)# neighbor 2.2.2.2 remote-as 65009
switch(bgp)# neighbor 2.2.2.2 connect-interface loopback 0
switch(bgp)# exit
switch(config)# router ospf
switch(ospf)# enable
switch(ospf)# area 0
switch(ospf)# network 3.3.3.3/32
switch(ospf)# network 9.1.1.0/24
switch(ospf)# exit
switch(config)# vlan 300
switch(vlan-300)# ip ospf
switch(vlan-300)# show ip bgp summary

Peer Information

Remote Address Remote-AS Local-AS State Admin  Status
-------------- --------- -------- ------------ -----
2.2.2.2        65009     65009    Established  Start

The output information shows that Switch C has established an iBGP peer relationship with Switch B.
Configure eBGP.
1. The eBGP peers, Switch A and Switch B (usually belonging to different carriers), are located in different ASs. Their loopback interfaces are not reachable to each other, so directly connected interfaces are used for establishing BGP sessions.
2. To enable Switch C to access the network 8.1.1.0/24 that is connected directly to Switch A, add network 8.1.1.0/24 to the BGP routing table of Switch A.

# Configure Switch A.

switch(config)# router bgp 65008
switch(bgp)# bgp router-id 1.1.1.1
switch(bgp)# neighbor 3.1.1.1 remote-as 65009
switch(bgp)# network 8.1.1.1/24
switch(bgp)# exit

# Configure Switch B.

switch(config)# router bgp 65009
switch(bgp)# neighbor 3.1.1.2 remote-as 65008
switch(bgp)# exit

# Show IP bgp peer information on Switch B.

switch(config)# show ip bgp summary

Peer Information

Remote Address Remote-AS Local-AS State        Admin Status
-------------- --------- -------- --------     ------------
2.2.2.2        65009     65009    Established  Start
3.1.1.2        65008     65009    Established  Start

The output shows that Switch B has established an iBGP peer relationship with Switch C and an eBGP peer relationship with Switch A.

# Display the BGP routing table on Switch A.

switch(bgp)# show ip bgp

Local AS : 100
Local Router-id : 20.0.0.1
BGP Table Version : 0
Status codes: * - valid, > - best, i - internal, e -
external, s - stale
Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Nexthop  Metric LocalPref  Weight AsPath
----------------------------------------------------------
*> 8.1.1.0/24                   0            32768    I
*> 8.1.1.0/24     0.0.0.0       0               0     I

# Display the BGP routing table on Switch B.

switch# show ip bgp

Local AS : 100
Local Router-id : 20.0.0.1

BGP Table Version  : 0
Status codes: * - valid, > - best, i - internal, e - external, s - stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network        Nexthop  Metric LocalPref   Weight AsPath
---------------------------------------------------------
*>e 8.1.1.0/24                0              0    65008i

# Display the BGP routing table on Switch C.

switch(bgp)# show ip bgp

Local AS : 100
Local Router-id : 20.0.0.1

BGP Table Version  : 0
Status codes: * - valid, > - best, i - internal, e - external, s - stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network         Nexthop    Metric LocalPref Weight AsPath
---------------------------------------------------------
*>i 8.1.1.0/24                  0            0     65008i

NOTE:

From the above outputs, you see that Switch A has not learned a route to AS 65009, and Switch C has learned network 8.1.1.0 but the next hop 3.1.1.2 is unreachable, so the route is invalid.

Redistribute connected routes.
Configure BGP to redistribute direct routes on Switch B, so that Switch A can obtain the route to 9.1.1.0/24 and Switch C can obtain the route to 3.1.1.0/24.

# Configure Switch B.

switch(config)# router bgp 65009
switch(bgp)# redistribute connected

# Display the BGP routing table on Switch A.

switch# show ip bgp

Local AS : 65009
Local Router-id : 1.1.1.1

BGP Table Version  : 0
Status codes: * - valid, > - best, i - internal, e - external, s - stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network         Nexthop    Metric LocalPref Weight AsPath
---------------------------------------------------------
*>e 2.2.2.2/32  3.1.1.1        0             0     65009?
*>e 3.1.1.0/24  3.1.1.1        0             0     65009?
*>e 8.1.1.0/24                 0             0     65008i
*>e 8.1.1.0/24                 0             0     65008i

Two routes 2.2.2.2/32 and 9.1.1.0/24 have been added in Switch A’s routing table.

# Display the BGP routing table on Switch C.

switch(config)# show ip bgp

Local AS : 65009
Local Router-id : 3.3.3.3

BGP Table Version  : 1

Status codes: * - valid, > - best, i - internal, e - external, s - stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network         Nexthop Metric LocalPref Weight AsPath
-------------------------------------------------------
*>e 2.2.2.2/32   9.1.1.1           0      100     I
*>e 3.1.1.0/24   9.1.1.1           0      100     I
*>e 8.1.1.0/24                     0      0       65008i
*e 9.1.1.0/24                      0      0       65008i
*>e 9.1.1.0/24                     0      0       65008i

Route 8.1.1.0 becomes valid with the next hop as Switch A.
Verification.