Configuring BGP

Table 6-1 lists the basic BGP configuration commands and their functions.


The BGP Network Command
In most IGPs, the network command starts the routing process on an interface. In BGP, the command tells the router to originate an advertisement for that network. The network does not have to be connected to the router; it just has to be in the routing table. In theory, it could even be a network in a different autonomous system (not usually recommended).

When advertising a network, BGP assumes you are using the default classful subnet mask. If you want to advertise a subnet, you must use the optional keyword mask and specify the subnet mask to use. Note that this is a subnet mask, not the inverse mask used by OSPF and EIGRP network statements. The routing table must contain an exact match (prefix and subnet mask) to the network listed in the network statement before BGP will advertise the route.


BGP Peering
BGP assumes that external neighbors are directly connected and that they are peering with the IP address of the directly connected interface of their neighbor. If not, you must tell BGP to look more than one hop away for its neighbor, with the neighbor ip-address ebgp-multihop number-of-hops command. You might use this command if you are peering with loopback interface IP addresses, for instance. BGP assumes that internal neighbors might not be directly connected, so this command is not needed with IBGP.

BGP Peering States
The command show ip bgp neighbors shows a list of peers, and the status of their peering session. This status can include the following states:

• Idle—No peering; router is looking for neighbor. Idle (admin) means that the neighbor relationship has been administratively shut down.
• Connect—TCP handshake completed.
• OpenSent, or Active—An open message was sent to try to establish the peering.
• OpenConfirm—Router has received a reply to the open message.
• Established—Routers have a BGP peering session. This is the desired state.

You can troubleshoot session establishment with debug commands. Use debug ip bgp events or debug ip bgp ipv4 unicast (in IOS versions 12.4 and up) to see where the process fails. Some common failure causes include AS number misconfiguration, neighbor IP address misconfiguration, neighbor with no neighbor statement for your router, and neighbor with no route to the source address of your router’s BGP messages.


BGP Path Selection
IGP, such as EIGRP or OSPF, choose routes based on lowest metric. They attempt to find the shortest, fastest way to get traffic to its destination. BGP, however, has a very different way of route selection. It assigns various attributes to each path; these attributes can be administratively manipulated in order to control the path that is selected. It then examines the value of these attributes in an ordered fashion until it is able to narrow all the possible routes down to one path.


BGP Attributes
BGP chooses a route to network based on the attributes of its path. Four categories of attributes exist:

• Well-known mandatory—Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers. For example, AS path, origin, and next hop.
  

• Well-known discretionary—Must be recognized by all BGP routers and passed on to other BGP routers, but need not be present in an update. For example, local preference.
  

• Optional transitive—Might or might not be recognized by a BGP router, but is passed on to other BGP routers. If not recognized, it is marked as partial. For example, aggregator, community.
  

• Optional nontransitive—Might or might not be recognized by a BGP router and is not passed on to other routers. For example, Multi-Exit Discriminator (MED), originator ID.

Table 6-2 lists common BGP attributes, their meanings, and their category.


Influencing BGP Path Selection
BGP was not created to be a fast protocol; it was created to allow as much administrative control over route path selection as possible. Path selection is controlled by manipulating BGP attributes, usually using route maps. You can set a default local preference by using the command bgp default local-preference and a default MED for redistributed routes with the default-metric command under the BGP routing process. But by using route maps, you can change attributes for certain neighbors only or for certain routes only. This is higher than the default local preference of 120, so routers within the AS are more likely to prefer that path than others.

Route maps can also be applied to routes sent to or received from a neighbor. The following example shows a simple route map that sets MED on all routes advertised out to an EBGP neighbor:

route-map MED permit 10
set metric 50
!
router bgp 65001
neighbor 10.1.1.1 route-map MED out

When attributes are changed, you must tell BGP to apply the changes. Either clear the BGP session (clear ip bgp * ) or do a soft reset (clear ip bgp * soft in | out). Routers using recent IOS versions will do a route refresh when the session in cleared inbound.


BGP Path Selection Criteria
BGP tries to narrow its path selection down to one best path; it does not load balance by default. To do so, it examines the path attributes of any loop-free, synchronized (if synchronization is enabled) routes with a reachable next-hop in the following order:

1. Choose the route with the highest weight.
2. If weight is not set, choose the route with the highest local preference.
3. Choose routes that you advertise.
4. Choose the path with the shortest autonomous system path.
5. Choose the path with the lowest origin code (i is lowest, e is next, ? is last).
6. Choose the route with the lowest MED, if the same autonomous system advertises the possible routes.
7. Choose an eBGP route over an iBGP route.
8. Choose the route through the nearest IGP neighbor.
9. Choose the oldest route.
10. Choose a path through the neighbor with the lowest router ID.
11. Choose a path through the neighbor with the lowest IP address.

To enable BGP to load balance over more than one path, you must enter the command maximum-paths number-of-paths. BGP can load balance over a maximum of six paths.