Basic BGP configuration

The basic configuration flow for BGP is as follows

  1. Enabling the BGP routing process
  2. Configuring the Neighbor
  3. Generating BGP routes

Enabling the BGP routing process

In BGP configuration, first enable the BGP routing process. In global configuration mode, enter the following command.

Enabling the BGP routing process

(config)#router bgp <AS>
(config-router)#

<AS> : AS　number

Only one BGP routing process can be enabled.

Configuring the Neighbor

Next, configure BGP neighbors to exchange route information with BGP. unlike OSPF and other IGPs, BGP explicitly configures the neighbors manually. enter the following command in BGP’s configuration mode.

Configuring the Neighbor

(config)#router bgp <AS1>
(config-router)#neighbor <ip-address> remote-as <AS2>

<AS1> : Own AS number
<ip-address> : neighbor IP address
<AS2> : neighbor AS number

EBGP and IBGP neighbors are determined by the AS number of the router bgp command and the AS number of the neighbor remote-as command.

  • EBGP neighbor
    • router bgp AS number ≠ neighbor remote-as AS number
  • IBGP neighbor
    • router bgp AS number = neighbor remote-as AS number

The BGP neighbor must be configured with matching recognition on both routers that establish the neighbor. In order to establish a neighbor, the source IP address and AS number of the received BGP OPEN message must match the neighbor’s IP address and AS number specified in the neighbor remote-as command. The following figure illustrates this.

Figure BGP Neighbor Conditions
Figure BGP Neighbor Conditions

The destination IP address of the BGP message is the IP address specified by the neighbor remote-as command. The source IP address is the IP address of the output interface. if you are an IBGP neighbor, you may need to change the interface used as the source IP address for BGP messages. To change the interface used as the source IP address, use the neighbor update-source command.

Generating BGP routes

If you configure a BGP neighbor, you can receive BGP routes from that neighbor. You can then further advertise BGP routes to other BGP neighbors.

This behavior is only possible for EBGP neighbors; for IBGP neighbors, the behavior is different depending on the split horizon rules.

In addition to advertising routes received from other ASes, there are three ways to generate the route information of the network in your AS as a BGP route.

  • network command
  • redistribute from IGPs within AS
  • aggregate-address command

You can use any of these, or a combination of them, to advertise a route in your AS to another AS in BGP. A route can only be advertised to the best path on the BGP table. In other words, advertising a route in your AS by BGP can be rephrased as putting the route information you want to advertise in the BGP table and making it the best path.

network command

The network command is a command to generate route information to be advertised in BGP. enter it in BGP configuration mode as follows.

Generating BGP routes network command

(config)#router bgp <AS>
(config-router)#network <address> [mask <subnetmask>]

<address> : Network address
<subnetmask> : subnet mask

The network command places the specified route information on the BGP table as the best path. the mask option is required for subnetting or aggregation. The mask option is not required for classful network addresses.

For the network command of BGP described above to work, there is a condition. That is, the route information must exist in the routing table that exactly matches the route information specified by the network command. The source of the route information can be anything: it can be an IGP such as OSPF or EIGRP, a static route, or a directly connected. Note that you need the route information to be an “exact” match.

As long as it’s in the routing table, you can generate a BGP route, even if it’s a network that’s not directly connected to that router. In the figure below, R1 is not directly connected to the 100.0.0.0/8 network. However, if 100.0.0.0.0/8 is registered in the routing table of R1, the network command can generate 100.0.0.0/8 route information as a BGP route.

Fig. Example of BGP route generation by the network command
Fig. Example of BGP route generation by the network command

IGP such as OSPF, RIP, and EIGRP also use the network command, but note that the network command is completely different from the BGP network command, which is used to specify the interface on which the routing protocol works. is. Therefore, it specifies the network address that is directly connected to the router. On the other hand, BGP is a command for specifying the route information you want to advertise with BGP. You do not need to specify the network address directly connected to the router.

The path attribute of the Well known Mandatory is added to the BGP route information generated by the network command. The default path attribute of the Well known Mandatory is as follows.

  • ORIGIN attribute: “i (IGP)”
  • NEXT_HOP attribute: “0.0.0.0”or the next hop of an IGP route in the routing table.
  • AS_PATH attribute : blanks
In addition to the Well known Mandatory attribute, the MED is also added, which is the metric of the corresponding route in the routing table.

redistribute from IGPs within AS

The network command must be configured to generate BGP routes one by one. If there are 100 BGP routes to be created, then the network command must also be configured for 100 routes. This is a lot of work. To generate BGP routes in batches, redistribute from the IGP within AS is done by using the redistribute command in the BGP configuration mode.

Generating BGP routes redistribute from IGPs

(config)#router bgp <AS>
(config-router)#redistribute <protocol> route-map <route-map-name>

<AS> : AS number
<protocol> : redistribute source
<route-map-name> : Name of the route map to apply

R1 in the figure below is an example of 10 OSPF routes (100.1.1.0/24 to 100.1.10.0/24) that have been registered as a batch of BGP routes in the BGP table.

Figure redistribute from IGPs within AS
Figure redistribute from IGPs within AS
The figure is a simplified version of the OSPF LSDB. the LSDB is not necessarily registered in a network address/subnet mask format.

The following is the path attribute of the Well known Mandatory that is added to the BGP route generated by the redistribute command.

  • ORIGIN attribute : “? (Incomplete)”
  • NEXT_HOP attribute : “0.0.0.0”or the next hop of an IGP route in the routing table.
  • AS_PATH attribute : Blanks

Note that ORIGIN is “? (Incomplete)”, it is not desirable to advertise a BGP route to other ASes with the ORIGIN of “?”. It is common to apply a route map on redistribute to change the ORIGIN of the BGP route from “?” to “i”. Also, not all IGP routes are redistributed to BGP . When redistributing, filter the routes with a route map and use only the routes you want to generate as BGP routes.

aggregate-address command

The aggregate-address command is used to generate a BGP aggregate route. You can generate a BGP route that aggregates multiple BGP routes. use the following command in BGP configuration mode.

Generating BGP routes aggregate-address command

(config)#router bgp <AS>
(config-router)#aggregate-address <network-address> <subnet-mask>

<AS> : AS number
<network-address> : network address of the aggregate route
<subnet-mask> : subnet mask

The route specified by the aggregate-address command is placed in the BGP table and becomes the best path and is advertised to other BGP neighbors. However, the condition for using the aggregate-address command is that at least one BGP route in the BGP table that is in the range of the aggregate route.

Verify BGP configuration

To verify BGP configurations, use the show command as follows

  • show ip protocols
  • show ip bgp summary
  • show ip bgp
  • show ip route bgp

show ip protocols

The show ip protocols command is a command that allows you to see a general overview of the routing protocols configured on the router. you can see the following information as an overview of BGP.

  • AS number
  • Configured neighbors
  • Filters applied to the neighbors
Router#show ip protocols
Routing Protocol is "bgp 8127"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  IGP synchronization is disabled
  Automatic route summarization is disabled
  Neighbor(s):
    Address          FiltIn FiltOut DistIn DistOut Weight RouteMap
    161.8.17.2                                           
    192.10.32.254         1                               WEIGHT
    212.12.8.254                                         
  Maximum path: 1
  Routing Information Sources:
    Gateway         Distance      Last Update
    192.10.32.254         20      01:46:22
    212.12.8.254          20      00:26:17
    161.8.17.2           200      00:18:58
  Distance: external 20 internal 200 local 200

show ip bgp summary

The show ip bgp summary command displays a summary of the BGP table. You can verify the status of the BGP neighbor at the end of that display.

R1#show ip bgp summary
BGP router identifier 203.8.1.1, local AS number 8127
BGP table version is 87, main routing table version 87
41 network entries using 4141 bytes of memory
70 path entries using 3360 bytes of memory
10 BGP path attribute entries using 600 bytes of memory
1 BGP rrinfo entries using 24 bytes of memory
5 BGP AS-PATH entries using 120 bytes of memory
1 BGP community entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
4 BGP filter-list cache entries using 48 bytes of memory
BGP using 8317 total bytes of memory
BGP activity 77/36 prefixes, 107/37 paths, scan interval 60 secs

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
161.8.17.2      4  8127     188     180       87    0    0 01:49:03       10
192.10.32.254   4     1     179     161       87    0    0 01:48:46       29
212.12.8.254    4     1      49      50       87    0    0 00:26:19       30

To see if the BGP neighbor is operating normally, look at the “State/PfxRcd” on the right side of the display. If this part is a number, the BGP neighbor is in an established state. The number is the number of BGP routes received from the corresponding neighbor.

Note that show ip bgp summary only gives you an overview of the BGP neighbor. use the show ip bgp neighbor command if you want to see detailed information about the BGP neighbor.

show ip bgp

The show ip bgp command is used to verify the BGP table. all route information received from and advertised to BGP neighbors is stored in the BGP table.

R1#show ip bgp
BGP table version is 87, local router ID is 203.8.1.1
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
*  2.0.0.0          212.12.8.254             0             0 1 200 i
*>                  192.10.32.254            0           250 1 200 i
*> 3.0.0.0          212.12.8.254             0             0 1 500 400 300 i
*  4.0.0.0          212.12.8.254             0             0 1 i
*>                  192.10.32.254            0           250 1 i
*  5.0.0.0          212.12.8.254             0             0 1 i
*>                  192.10.32.254            0           250 1 i
*  6.0.0.0/23       212.12.8.254             0             0 1 i
*>                  192.10.32.254            0           250 1 i

A path attribute is appended to the BGP route information. you can use the show ip bgp command to see the following path attributes for each BGP route information.

  • NEXT_HOP attribute(Next hop column)
  • MED attribute (Metric column)
  • LOCAL_PREFERENCE attribute (LocPrf column)
  • WEIGHT attribute (Weight column)
  • AS_PATH attribute (Path column)
  • ORIGIN attribute (right-most character in Path)

Also, the best path is represented by “>”. For example, for the network address 2.0.0.0/8 in the example output above, there are two route information

AttributesRoute information 1Route information 1
NEXT_HOP212.12.8.254192.10.32.254
MED00
LOCAL_PREFERENCENoneNone
WEIGHT0250
AS_PATH1 2001 200
ORIGINii
Table 2.0.0.0/8

Of the two route information for 2.0.0.0/8, route information 2 was chosen as the best path. In this example, the best path is determined by the WEIGHT attribute, which is a Cisco proprietary attribute.

If you want to see all the path attributes attached to each route information, use the show ip bgp command to specify the individual route information.

The show ip bgp command displays the entire BGP table. To extract only the BGP routes that have been sent and received to and from a specific neighbor from here, use the show command: show ip bgp

  • show ip bgp neighbor <ip-address> advertised-routes
    • Display only BGP route information advertised to a specified BGP neighbor.
  • show ip bgp neighbor <ip-address> routes
    • Display only the BGP route information received from the specified BGP neighbor.

show ip route bgp

The show ip route bgp command allows you to display only route information in the routing table that has been learned by BGP. route information in BGP is represented by the code “B”. The administrative distance for BGP is 20 for EBGP routes and 200 for IBGP routes.

How the BGP works