BGP For Beginners (i.e for CCNA Students)
First time in the CCNA Cisco added this protocol in CCNA R and S exam ( and ICND2 Exam)
In previous CCNA syllabus, it was just mentioned as IGP i.e Interior Gateway Protocol, but working was not discussed in that old syllabus. Current new syllabus discusses this protocol with its working & its there in the upcoming syllabus as well, which will be effective from the year 2020.
What is the Role of Routing Protocol,
- Learn routes: Learn about subnets/masks, and calculate the local router’s route for that subnet, with outgoing interface and next-hop router.
- Choose the best route: If one routing protocol learns of multiple routes exist for a
single subnet, pick the best route, using some kind of metric
- Converge: When network changes happen, converge—either remove routes, add new ones, or change a failed route with a working route
What is BGP?
BGP stands for Border Gateway Protocol.
We need to understand connection types first in order to understand BGP
These are following internet connection types,
- Dual Homed
- Single Multihomed
- Dual Multihomed
Diagrammatically it is represented as follows,
In an enterprise or company means within an autonomous system we use IGP i.e Interior Gateway Protocol like RIP, OSPF & EIGRP.
BGP means Border Gateway Protocol is an only protocol which works between multiple AS i.e autonomous system, means between multiple companies. So whole INTERNET is basically working on BGP
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Traffic from your computer to a website like Google or Facebook has traveled over the internet to reach there via BGP.
BGP has to advertise routes on the internet as well as learn routes from the internet to provide connectivity.
BGP needs to choose the best route among numbers of available routes based on metric i.e selection criteria.
BGP needs to converge i.e remove, update or replace routes.
BGPs internet routing table is massive as compared to the routing table of IGP like OSPF or EIGRP.
ACTIVE BGP Entries from 1989 to 2019
We can access bgp routes reviews i.e publically accessible routers of ISP,
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Oregon Exchange BGP Route Viewer
route-views.oregon-ix.net / route-views.routeviews.org
route views data is archived on http://archive.routeviews.org
This hardware is part of a grant by the NSF.
Please contact help@routeviews.org if you have questions, or
if you wish to contribute your view.
This router has views of full routing tables from several ASes.
The list of peers is located at http://www.routeviews.org/peers
in route-views.oregon-ix.net.txt
NOTE: The hardware was upgraded in August 2014.
To login, use the username “rviews”.
**********************************************************************
User Access Verification
Username: rviews
route-views>
route-views>show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 1.0.0.0/24 194.85.40.15 0 0 3267 13335 i
* 193.0.0.56 0 3333 1257 13335 i
* 91.218.184.60 0 49788 13335 i
* 137.39.3.55 0 701 3356 13335 i
* 207.172.6.20 0 0 6079 13335 i
* 207.172.6.1 0 0 6079 13335 i
* 195.208.112.161 0 3277 3267 13335 i
* 94.142.247.3 0 0 8283 13335 i
* 212.66.96.126 0 20912 13335 i
* 134.222.87.1 700 0 286 13335 i
* 202.93.8.242 0 24441 13335 i
* 162.251.163.2 0 53767 13335 i
* 12.0.1.63 0 7018 6453 13335 i
* 198.58.198.255 0 1403 13335 i
* 198.58.198.254 0 1403 13335 i
* 162.250.137.254 0 4901 6079 13335 i
* 37.139.139.17 0 0 57866 13335 i
* 89.149.178.10 10 0 3257 13335 i
* 173.205.57.234 0 53364 3257 13335 i
* 209.124.176.223 0 101 101 13335 i
* 144.228.241.130 80 0 1239 2914 13335 i
* 140.192.8.16 0 54728 20130 6939 13335 i
* 203.62.252.83 0 1221 13335 i
* 208.51.134.254 0 0 3549 3356 13335 i
* 114.31.199.1 0 4826 13335 i
* 154.11.12.212 0 0 852 13335 i
* 206.24.210.80 0 3561 209 3356 13335 i
*> 202.232.0.2 0 2497 13335 i
* 132.198.255.253 0 1351 13335 i
* 64.71.137.241 0 6939 13335 i
* 217.192.89.50 0 3303 13335 i
* 1.0.4.0/24 193.0.0.56 0 3333 6939 4826 38803 56203 i
* 91.218.184.60 0 0 49788 6939 4826 38803 56203 i
* 137.39.3.55 0 701 174 4826 38803 56203 i
–More–
route-views>sh ip bgp summary
BGP router identifier 128.223.51.103, local AS number 6447
BGP table version is 285650008, main routing table version 285650008
814776 network entries using 202064448 bytes of memory
24723576 path entries using 2966829120 bytes of memory
3824642/138368 BGP path/bestpath attribute entries using 948511216 bytes of memory
3514524 BGP AS-PATH entries using 180352888 bytes of memory
5 BGP ATTR_SET entries using 200 bytes of memory
126987 BGP community entries using 15799630 bytes of memory
1147 BGP extended community entries using 72912 bytes of memory
BGP using 4313630214 total bytes of memory
BGP activity 8090382/7197301 prefixes, 833184086/805021444 paths, scan interval 60 secs
Peer V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State /PfxRcd
4.69.184.193 4 3356 0 0 1 0 0 35w4d Active
5.101.110.2 4 14061 0 0 1 0 0 never Active
12.0.1.63 4 7018 11315115 45715 285649834 0 0 4w0d 757803
–More–
BGP Neighbor 12.0.1.63
AS 14061
Routes Learned 757803
BGP Router ID 128.223.51.103
BGP Table Version 285641878
202064448 Bytes of Memory Consumed
Memory Consumption is as follows
route-views>sh ip route summary
IP routing table name is default (0x0)
IP routing table maximum-paths is 32
Route Source Networks Subnets Replicates Overhead Memory (bytes)
connected 0 2 0 192 576
static 1 86 0 8352 25056
application 0 0 0 0 0
bgp 6447 204774 609952 0 78213696 234641088
External: 814726 Internal: 0 Local: 0
internal 9469 63390712
Total 214244 610040 0 78222240 298057432
route-views>
Types of BGP
iBGP = Internal BGP
eBGP = External BGP
In our studies of CCNA we have to deal with eBGP only means scenario as follows,
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Config
ENT(config)#int s0/0/0
ENT(config-if)#ip add 100.1.1.2 255.255.255.0
ENT(config-if)#no shutdown
ENT(config)#int f0/0
ENT(config-if)#ip add 192.0.1.1 255.255.255.0
ENT(config-if)#no shutdown
ENT#sh ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.0.1.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 100.1.1.2 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
Vlan1 unassigned YES unset administratively down down
ENT#
ENT#show ip route
Gateway of last resort is not set
100.0.0.0/24 is subnetted, 1 subnets
C 100.1.1.0 is directly connected, Serial0/0/0
C 192.0.1.0/24 is directly connected, FastEthernet0/0
ISP End Config
ISP>en
ISP#conf t
Enter configuration commands, one per line. End with CNTL/Z.
ISP(config)#int s0/0/0
ISP(config-if)#ip add 100.1.1.1 255.255.255.0
ISP(config-if)#no sh
ISP(config-if)#exit
ISP(config)#int f0/0
ISP(config-if)#ip add 9.1.1.1 255.0.0.0
ISP(config-if)#no sh
ISP(config-if)#exit
ISP(config)#exit
ISP#
%SYS-5-CONFIG_I: Configured from console by console
ISP#sh ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 9.1.1.1 YES manual up up
FastEthernet0/1 unassigned YES unset administratively down down
Serial0/0/0 100.1.1.1 YES manual up up
Serial0/0/1 unassigned YES unset administratively down down
Vlan1 unassigned YES unset administratively down down
ISP#
ISP#
ISP#conf t
Enter configuration commands, one per line. End with CNTL/Z.
ISP(config)#ip route 0.0.0.0 0.0.0.0 9.1.1.2
ISP(config)#
ISP#
%SYS-5-CONFIG_I: Configured from console by console
sh
ISP#show ip route
Gateway of last resort is 9.1.1.2 to network 0.0.0.0
C 9.0.0.0/8 is directly connected, FastEthernet0/0
100.0.0.0/24 is subnetted, 1 subnets
C 100.1.1.0 is directly connected, Serial0/0/0
S* 0.0.0.0/0 [1/0] via 9.1.1.2
ISP#
Now we will look at BGP config,
ENT#conf t
Enter configuration commands, one per line. End with CNTL/Z.
ENT(config)#router bg
ENT(config)#router bgp 1
ENT(config-router)#neigh
ENT(config-router)#neighbor 100.1.1.1 re
ENT(config-router)#neighbor 100.1.1.1 remote-as 2
ENT(config-router)#exit
ENT(config)#exit
ENT#
%SYS-5-CONFIG_I: Configured from console by console
ISP(config)#router bgp 2
ISP(config-router)#neighbor 100.1.1.2 remote-as 1
ISP(config-router)#
%BGP-5-ADJCHANGE: neighbor 100.1.1.2 Up
Now we will look at BGP verification commands,
ENT#sh ip bgp summary
BGP router identifier 192.0.1.1, local AS number 1
0 network entries using 0 bytes of memory
0 path entries using 0 bytes of memory
0/0 BGP path/bestpath attribute entries using 0 bytes of memory
0 BGP AS-PATH entries using 0 bytes of memory
BGP using 32 total bytes of memory
Peer V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
100.1.1.1 4 2 4 4 1 0 0 00:02:36 4
Only one neighbor i.e ISP end IP address 100.1.1.1
Now we will add 192.0.1.0/24 and start advertising to ISP
ENT(config)#router bgp 1
ENT(config-router)#network 192.0.1.0 mask 255.255.255.0
ENT(config-router)#
ENT#sh ip bgp
BGP table version is 2, local router ID is 192.0.1.1
Network Next Hop Metric LocPrf Weight Path
*> 192.0.1.0/24 0.0.0.0 0 0 32768 i
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ENT#
This is local route so next hop is 0.0.0.0 means this ENT router itself generating this routes.
But if you run this command on ISP, see the difference
ISP#sh ip bgp
BGP table version is 2, local router ID is 100.1.1.1
Network Next Hop Metric LocPrf Weight Path
*> 192.0.1.0/24 100.1.1.2 0 0 0 1 i
ISP#
Observe the next hop 100.1.1.2 i.e ENT router ip address, but we are not receiving from ISP end to ENT end, so now we will give default route from ISP to ENT
ISP#sh ip route
Gateway of last resort is 9.1.1.2 to network 0.0.0.0
C 9.0.0.0/8 is directly connected, FastEthernet0/0
100.0.0.0/24 is subnetted, 1 subnets
C 100.1.1.0 is directly connected, Serial0/0/0
B 192.0.1.0/24 [20/0] via 100.1.1.2, 01:15:47
S* 0.0.0.0/0 [1/0] via 9.1.1.2
ISP#conf t
Enter configuration commands, one per line. End with CNTL/Z.
ISP(config)#router bgp 2
ISP(config-router)#network 0.0.0.0 mask 0.0.0.0
ISP(config-router)#
ISP#
This will get reflected on Enterprise end router ENT,
ENT>sh ip bgp
BGP table version is 4, local router ID is 192.0.1.1
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0/0 100.1.1.1 0 0 0 2 i
*> 192.0.1.0/24 0.0.0.0 0 0 32768 i
ENT>
That’s how we can learn & advertise routes in BGP.
Now we will see how BGP forms neighbor-ship
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| BGP Neighbor State | Details |
| IDLE | Peer administratively disabled (neighbor shutdown), or
the router is waiting before the next retry. |
| CONNECT | TCP connection attempted but has not completed. |
| ACTIVE | TCP connection completed, but no BGP messages have been sent yet. |
| OPENSENT | TCP connection is there, router has sent the OPEN message to
establish the BGP neighborship. |
| OPENCONNECT | TCP connection exists, local router has received an Open
message from the other router. The neighbor relationship may still be rejected. |
| ESTABLISHED | The routers are now peers and can exchange update messages. |
It means TCP used in layer 4 and manually we need to configure BGP peer its not possible to dynamic neighbor discovery.
BGP is vast topic but still I tried to explain it with one scenario, explore it in more detail as the whole internet is working on this protocol.
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