BGP Neighbors and Configuration

BGP Neighbors

BGP neighbors are routers forming TCP connection for exchanging BGP updates it’s also called as BGP peers or BGP speakers.

There are two types of BGP neighbor relationship

IBGP (internal BGP)

EBGP (external BGP)

BGP first forms a neighbor relationship with BGP speakers (peer). Than its learns information from its peer, BGP place that information in its BGP table and analyze the table to choose the best working route for each subnet in the BGP table, placing those routes into the IP routing table.

There are several requirements for forming BGP neighbors:-

• A local router’s autonomous system number must match the neighboring router’s ASN.
• The BGP router ID of the two routers must not be the same.
• Each router must be part of a TCP connection.
• Authentication must be passing if it’s configured

BGP message types

• Open- it’s used to established a neighbor relationship and exchange its parameter, its include ASN and authentication values.
• Keepalive it’s used to maintain the neighbor relationship and it’s sent periodically
• Update- its used to exchange path attribute.
• Notification- it’s used error signals. It does occur in neighbor relationship reset.

BGP neighbor states

• Idle — it’s a down state by administratively down or waiting for the next retry attempt.

R1# show ip bgp summary

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

1.1.1.2 4 501 11 9 8 0 0 00:04:28 4

4.1.1.1 4 501 0 0 1 0 0 never Idle

• Connect- this state means its waiting for the TCP connection to be completed.
• Active- this state means the TCP connection has been completed but there is no BGP messages sent yet to its peer.
• Opensent- this state means the TCP connection has been established and BGP open message is sent to its BGP speakers but the matching Open message is not received yet from the peer router.
• Openconfirm- this state means open message has been sent and received from BGP speaker router. But it’s waiting for keepalive message.
• Established- this state means the entire neighbor parameters match and the neighbor relationship has been established. Now BGP speakers (peers) can exchange update messages.

Configuration:-

Topology:-

www.internetworks.in

GOAL:

• configure the topology as per the diagram
• configure IBGP peering AS 501 as per our topology using directly connected interfaces.
• ensure the connectivity and make sure all bgp routers should be able exchange the routes through BGP.

R3(config-if)#do sh ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 30.1.1.1 YES manual up up
Serial3/1 2.1.1.2 YES manual up up
Serial3/2 3.1.1.1 YES manual up up
Loopback0 13.0.0.1 YES manual up up
Loopback1 13.0.1.1 YES manual up up
Loopback2 13.0.2.1 YES manual up up
Loopback3 13.0.3.1 YES manual up up
R4#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 40.1.1.1 YES manual up up
Serial3/2 3.1.1.2 YES manual up up
Serial3/3 4.1.1.1 YES manual up up
Loopback0 14.0.0.1 YES manual up up
Loopback1 14.0.1.1 YES manual up up
Loopback2 14.0.2.1 YES manual up up
Loopback3 14.0.3.1 YES manual up up

R1(config)#router bgp 501
R1(config-router)#neighbor 1.1.1.2 remote-as 501
R1(config-router)#neighbor 4.1.1.1 remote-as 501
R1(config-router)#network 11.0.0.0 mask 255.255.255.0
R1(config-router)#network 10.0.0.0
R1(config-router)#network 1.0.0.0
R1(config-router)#network 4.0.0.0
R1(config-router)#no auto-summary
R1(config-router)#no synchronization
R1(config-router)#end

R2(config)#router bgp 501
R2(config-router)#neighbor 1.1.1.1 remote-as 501
R2(config-router)#neighbor 2.1.1.2 remote-as 501
R2(config-router)#network 12.0.0.0 mask 255.255.255.0
R2(config-router)#network 20.0.0.0
R2(config-router)#network 2.0.0.0
R2(config-router)#network 1.0.0.0
R2(config-router)#no auto-summary
R2(config-router)#no synchronization
R2(config-router)#end

R3(config)#router bgp 501

R3(config-router)#neighbor 2.1.1.1 remote-as 501

R3(config-router)#neighbor 3.1.1.2 remote-as 501

R3(config-router)#network 13.0.0.0 mask 255.255.255.0

R3(config-router)#network 30.0.0.0

R3(config-router)#network 3.0.0.0

R3(config-router)#network 2.0.0.0

R3(config-router)#no auto-summary

R3(config-router)#no synchronization

R3(config-router)#end

R4(config)#router bgp 501

R4(config-router)#neighbor 4.1.1.2 remote-as 501

R4(config-router)#neighbor 3.1.1.1 remote-as 501

R4(config-router)#network 14.0.0.0 mask 255.255.255.0

R4(config-router)#network 3.0.0.0

R4(config-router)#network 4.0.0.0

R4(config-router)#no auto-summary

R4(config-router)#no synchronization

R4(config-router)#end

R1#show ip bgp summary

BGP router identifier 11.0.3.1, local AS number 501

BGP table version is 10, main routing table version 10

9 network entries using 1296 bytes of memory

11 path entries using 880 bytes of memory

2/2 BGP path/bestpath attribute entries using 272 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 2448 total bytes of memory

BGP activity 9/0 prefixes, 12/1 paths, scan interval 60 secs

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

1.1.1.2 4 501 25 22 10 0 0 00:15:30 4

4.1.1.1 4 501 12 12 10 0 0 00:05:01 3

R3#show ip bgp summary

BGP router identifier 13.0.3.1, local AS number 501

BGP table version is 13, main routing table version 13

9 network entries using 1296 bytes of memory

11 path entries using 880 bytes of memory

2/2 BGP path/bestpath attribute entries using 272 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 2448 total bytes of memory

BGP activity 10/1 prefixes, 12/1 paths, scan interval 60 secs

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

2.1.1.1 4 501 21 17 13 0 0 00:09:25 4

3.1.1.2 4 501 14 12 13 0 0 00:05:25 3

R1#show ip route bgp

Codes: L — local, C — connected, S — static, R — RIP, M — mobile, B — BGP

D — EIGRP, EX — EIGRP external, O — OSPF, IA — OSPF inter area

N1 — OSPF NSSA external type 1, N2 — OSPF NSSA external type 2

E1 — OSPF external type 1, E2 — OSPF external type 2

i — IS-IS, su — IS-IS summary, L1 — IS-IS level-1, L2 — IS-IS level-2

ia — IS-IS inter area, * — candidate default, U — per-user static route

o — ODR, P — periodic downloaded static route, H — NHRP, l — LISP

+ — replicated route, % — next hop override

Gateway of last resort is not set

B 2.0.0.0/8 [200/0] via 1.1.1.2, 00:05:09

B 3.0.0.0/8 [200/0] via 4.1.1.1, 00:05:09

12.0.0.0/24 is subnetted, 1 subnets

B 12.0.0.0 [200/0] via 1.1.1.2, 00:05:09

14.0.0.0/24 is subnetted, 1 subnets

B 14.0.0.0 [200/0] via 4.1.1.1, 00:05:09

B 20.0.0.0/8 [200/0] via 1.1.1.2, 00:05:09

(because of the split horizon rules router 2 not get 30.0.0.0/8 and 13.0.0.0/24networks)

R3#show ip route bgp

Codes: L — local, C — connected, S — static, R — RIP, M — mobile, B — BGP

D — EIGRP, EX — EIGRP external, O — OSPF, IA — OSPF inter area

N1 — OSPF NSSA external type 1, N2 — OSPF NSSA external type 2

E1 — OSPF external type 1, E2 — OSPF external type 2

i — IS-IS, su — IS-IS summary, L1 — IS-IS level-1, L2 — IS-IS level-2

ia — IS-IS inter area, * — candidate default, U — per-user static route

o — ODR, P — periodic downloaded static route, H — NHRP, l — LISP

+ — replicated route, % — next hop override

Gateway of last resort is not set

B 1.0.0.0/8 [200/0] via 2.1.1.1, 00:06:43

B 4.0.0.0/8 [200/0] via 3.1.1.2, 00:10:41

12.0.0.0/24 is subnetted, 1 subnets

B 12.0.0.0 [200/0] via 2.1.1.1, 00:15:08

14.0.0.0/24 is subnetted, 1 subnets

B 14.0.0.0 [200/0] via 3.1.1.2, 00:10:53

B 20.0.0.0/8 [200/0] via 2.1.1.1, 00:15:08

(here also router 3 is not getting 10.0.0.0/8 and 11.0.0.0/24 networks because of split horizon rules. the solution is full mesh neighbor-ship or we can use route reflector)