This tutorial describes how to configure bgp with two routers , check bgp peering status and understand different bpg show commands.
An eBGP peering can be established by configuring peers in two different autonomous systems. Your task is to configure egbp peering between R1-R2 and advertise loopback networks from both routers.You can do the following steps to make an eBGP peering in the above topology.
Step1 : Configure IP address in the physical interface connecting R1 and R2
Step2: Add loopback interfaces in both routers and configure IP address as in the topology
Step3: Ping between R1 to R2 to check connectivity
Step4: Configure bgp in R1 and R2, add the neighbor and advertise loopback network.
Step5: Verify the peering and check the entries in bgp table.
R1#
R1#configure terminal
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#no shutdown
R1(config-if)#int loopback 1
R1(config-if)#ip address 10.0.0.1 255.0.0.0
R1(config-if)#exit
R1(config)#router bgp 1000
R1(config-router)#neighbor 192.168.1.2 remote-as 2000
R1(config-router)#network 10.0.0.0 mask 255.0.0.0
R1(config-router)#exit
R1(config)#exit
R1#
R2#
R2#configure terminal
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip address 192.168.1.2 255.255.255.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface loopback 1
R2(config-if)#ip address 20.0.0.1 255.0.0.0
R2(config-if)#exit
R2(config)#router bgp 2000
R2(config-router)#neighbor 192.168.1.1 remote-as 1000
R2(config-router)#network 20.0.0.0 mask 255.0.0.0
R2(config-router)#exit
R2(config)#exit
R2#
Show ip bgp can be used to check all routes learned and advertised by bgp.
In the above output you can see two networks (10.0.0.0 and 20.0.0.0) in bgp table.
Status code: *>
The status code will show “*” if a route is a valid one and the NOH is reachable. “>” shows that this path is the best path to reachable destination. Status code will be “*>i” if it is a route learned from ibgp peer.
Next Hop:
If the Next Hop column is set 0.0.0.0 , it shows that this network is directly connected one. In the above output R1 has set NOH as 0.0.0.0 for the connected network 10.0.0.0
Metric:
This column shows the value of MED attribute, by default MED is set to 0
LocPrf:
Local preference attribute is not applicable for ebgp peering, so this column is not populated in this scenario. Local preference will be by default 100 for routes learned from ibgp peers.
Weight:
Weight is set to 32768 by default for all local networks and will be 0 for ibgp and ebgp routes
Path and Origin code:
This column shows the value of AS-PATH attribute.
The symbol at the end of line shows the origin code. In the above output origin code is “i” meaning that the network is originally advertised by a bgp speaking router connected to that network
Show ip bgp summary can be used check how many neighbors are added , status of the peering and the number of routes received from each neighbor.
In the above output, the last column State/PfxRcd is used to understand the status of BGP peering, if it set to a number (0 or more) , it means peering has been established and that value shows how many networks has been learned from that peer. If that colum is showing “Active” or “Idle” , it means bgp peering has not been completed.
You can get more details about a path by using the show ip bgp network-address command.
Note: Configuring ibgp peering is similar to ebgp , one thing to remember is that you need to disable “synchronization” if it is not disabled by default. You can do this by using the command “no synchronization” after getting in to bgp configuration mode.
Remember the following important points when loopback interfaces are used for ebgp peering
Peer’s loopback IP should be reachable:
Both routers should have a route to reach the neigbor’s loopback network, you may use a static route or RIP/OSPF to advertise this network before BGP is configured.
Update the source IP address:
By default the source IP address of all bgp packets will be IP address of the interface that is used to send the packet to that neighbor, but in this case, peer is expecting packets with source IP set to the peer’s loopback interface. To change the source IP address , use “update-source” with neighbor command.
Change the default TTL
TTL is set to 1 when ebgp peerings are formed, but this will not work when loopback interfaces are used . You have to set the TTL value to 2 or higher using the “ebgp-multihop” option.
Configure IP address and loopback interface similar to exercise 1. In R1, add one more loopback interface with IP address 21.0.0.1/8.
R1#
R1#configure terminal
R1(config)#ip route 20.0.0.0 255.0.0.0 192.168.1.2
R1(config)#router bgp 1000
R1(config-router)#neighbor 20.0.0.1 remote-as 2000
R1(config-router)#neighbor 20.0.0.1 update-source loopback 1
R1(config-router)#neighbor 20.0.0.1 ebgp-multihop 2
R1(config-router)#network 21.0.0.0 mask 255.0.0.0
R1(config-router)#exit
R1(config)#exit
R1#
R2#
R2#configure terminal
R2(config)#ip route 10.0.0.0 255.0.0.0 192.168.1.1
R2(config)#router bgp 2000
R2(config-router)#neighbor 10.0.0.1 remote-as 1000
R2(config-router)#neighbor 10.0.0.1 update-source loopback 1
R2(config-router)#neighbor 10.0.0.1 ebgp-multihop 2
R2(config-router)#exit
R2(config)#exit
R2#
Checking peering and networks learned:
show ip bgp neighbors can be used to get detailed information about all neighbors
Related:
Cisco OSPF configuration commands
Cisco RIP configuration commands
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