What is BGP?
The Border Gateway Protocol (BGP) is an interautonomous system routing protocol. An autonomous system is a network or group of networks under a common administration and with common routing policies. BGP is used to exchange routing information for the Internet and is the protocol used between Internet service providers (ISP). Customer networks, such as universities and corporations, usually employ an Interior Gateway Protocol (IGP) such as RIP or OSPF for the exchange of routing information within their networks. Customers connect to ISPs, and ISPs use BGP to exchange customer and ISP routes. When BGP is used between autonomous systems (AS), the protocol is referred to as External BGP (EBGP). If a service provider is using BGP to exchange routes within an AS, then the protocol is referred to as Interior BGP (IBGP).
First we need to configure some interfaces on two routers as follows:
R1(config)#interface fastethernet0/0 R1(config-if)#ip address 126.96.36.199 255.255.255.0 R1(config-if)#no shutdown R1(config-if)#interface loopback 0 R1(config-if)#ip address 188.8.131.52 255.255.255.0R2(config)#interface fastethernet0/0 R2(config-if)#ip address 184.108.40.206 255.255.255.0 R2(config-if)#no shutdown R2(config-if)#interface loopback 0 R2(config-if)#ip address 220.127.116.11 255.255.255.0
So we have just configured interface fa0/0 and loopback0 on both routers. Next we will configure the BGP configuration part on R1:
R1(config)#router bgp 1 R1(config-router)#neighbor 18.104.22.168 remote-as 2
The configuration is very simple with only two lines on R1. In the first line, BGP configuration begins with a familiar type of command: the router bgp command, where AS number is the BGP AS number used by that router (same as EIGRP, OSPF configuration).
The next command defines the IP address of the neighbor. Unlike OSPF or EIGRP, BGP cannot discover its neighbors automatically so we have to explicitly declare them. We also have to know and declare the neighbor’s BGP AS number as well. In this case R1 wants to establish BGP neighbor relationship with R2 (in BGP AS 2) so it choose an interface on R2 (Fa0/0: 22.214.171.124) and specify R2 is in BGP AS 2 via the command “neighbor 126.96.36.199 remote-as 2“. At the other end R2 will do the same thing for R1 to set up BGP neighbor relationship.
R2(config-router)#neighbor 188.8.131.52 remote-as 1 R2(config-router)#neighbor 184.108.40.206 remote-as 1
After a moment we should see a message (on each router) similar to the following, letting us know that an adjacency has been formed:
*Aug 17 00:09:38.453: %BGP-5-ADJCHANGE: neighbor 220.127.116.11 Up
*Aug 17 00:09:38.453: %BGP-5-ADJCHANGE: neighbor 18.104.22.168 Up
So after forming BGP neighbor relationship we can verify by using the “show ip bgp summary” command on both routers:
R1#show ip bgp summary BGP router identifier 22.214.171.124, local AS number 1 BGP table version is 1, main routing table version 1 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 126.96.36.199 4 2 19 19 1 0 0 00:16:21 0
R2#show ip bgp summary BGP router identifier 188.8.131.52, local AS number 2 BGP table version is 1, main routing table version 1 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 184.108.40.206 4 1 20 20 1 0 0 00:17:13 0
Please pay attention to the “State/PfxRcd” column of the output. It indicates the number of prefixes that have been received from a neighbor. If this value is a number (including “0”, which means BGP neighbor does not advertise any route) then the BGP neighbor relationship is good. If this value is a word (including “Idle”, “Connect”, “Active”, “OpenSent”, “OpenConfirm”) then the BGP neighbor relationship is not good.
In the outputs above we see the BGP neighbor relationship between R1 & R2 is good with zero Prefix Received (PfxRcd) because they have not advertised any routes yet.
How about the BGP routing table? We can check with the “show ip bgp” command but currently this table is empty! This is because although they formed BGP neighbor relationship but they have not exchanged any routes. Let’s try advertising the loopback 0 interface on R1 to R2:
R1(config-router)#network 220.127.116.11 mask 255.255.255.0
As you see, unlike other routing protocols like OSPF or EIGRP, we have to use subnet mask (255.255.255.0 in this case), not wildcard mask, to advertise the routes in the “network” command.
Note: With BGP, you must advertise the correct network and subnet mask in the “network” command ( in this case network 18.104.22.168/24). BGP is very strict in the routing advertisements. In other words, BGP only advertises the network which exists exactly in the routing table (in this case network 22.214.171.124/24 exists in the routing table as the loopback 0 interface). If you put the command “network 126.96.36.199 mask 255.255.0.0” or “network 188.8.131.52 mask 255.0.0.0” or “network 184.108.40.206 mask 255.255.255.255” then BGP will not advertise anything.
Now the BGP routing tables on these two routers contain this route:
R1#sh ip bgp BGP table version is 4, local router ID is 220.127.116.11 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 *> 18.104.22.168/24 0.0.0.0 0 32768 i
R2#sh ip bgp BGP table version is 2, local router ID is 22.214.171.124 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 *> 126.96.36.199/24 188.8.131.52 0 0 1 i
An asterisk (*) in the first column means that the route has a valid next hop. A greater-than sign (>) indicates the route has been selected as the best path to that network.
The “Metric” column here is not the usual metric like in OSPF or EIGRP. It is the Multi Exit Discriminator (MED) attribute of BGP. “Weight” is another BGP attribute. The default values of both MED and Weight are 0 (as you see at the outputs above).
The “Path” column shows the AS paths that prefix were sent to reach us. It would better to read the “Path” from right to left to understand which path this prefix travel to reach our router. Letter “i” is considered the starting point of the prefix and the next number is the originating AS where this prefix originated. Next numbers are the recorded paths it traveled. For example if a prefix had to travel from AS 1 -> 2 -> 3 -> 4 -> 5 (our AS) then we will see the path “4 3 2 1 i” on our router.
Note: A blank AS path (only letter “i” is shown) means that the route was originated in the local AS. In the R1 output above, network 184.108.40.206/24 is originated from R1 so we see the path only has one letter “i”.
One notice is on R1 the “Next Hop” is 0.0.0.0 which means this prefix is originated from the local router. On R2 the Next Hop is pointing toward the interface Fa0/0 of R1 (220.127.116.11) to which R2 will send traffic for the destination 18.104.22.168/24.
Now R1 advertised prefix 22.214.171.124/24 to R2 so we can re-check R2 with the “show ip bgp summary” command to see the “Prefix received” increased to 1:
R2#sh ip bgp summary BGP router identifier 126.96.36.199, local AS number 2 BGP table version is 2, main routing table version 2 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 1/0 prefixes, 1/0 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 188.8.131.52 4 1 5 4 2 0 0 00:01:36 1
Also in the routing table of R2 we will see this prefix, which is advertised with BGP from R1:
R2#show ip route Codes: 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 Gateway of last resort is not set 184.108.40.206/24 is subnetted, 1 subnets B 220.127.116.11 [20/0] via 18.104.22.168, 00:00:20 22.214.171.124/24 is subnetted, 1 subnets C 126.96.36.199 is directly connected, Loopback0 188.8.131.52/24 is subnetted, 1 subnets C 184.108.40.206 is directly connected, FastEthernet0/0
This lab only mentioned about the most basic configuration of BGP and introduced two most important commands of BGP, which are “show ip bgp summary” and “show ip bgp”. In practical BGP is often much more complex with many attributes, routing policies, redistribution,… used.