170 10 20 1 175 10 40 2 170 10 20 2 175 10 40 1 Router E Router C AS 300 AS 400 AS 500 170 10 0 0 175 10 0 0 When a BGP speaker...

Pokaż mi serce nie opętane zwodniczymi marzeniami, a pokażę ci człowieka szczęśliwego.

This
behavior of IBGP is why it is necessary for BGP speakers within an AS to be fully meshed.
For example, in Figure 3-18, if there were no IBGP session between Routers B and D, Router A
would send updates from Router B to Router E but not to Router D. If you want Router D to receive updates from Router B, Router B must be configured so that Router D is a BGP peer.
Designing Large-Scale IP Internetworks 3-33
BGP Internetwork Design Guidelines
Loopback Interfaces. Loopback interfaces are often used by IBGP peers. The advantage of using loopback interfaces is that they eliminate a dependency that would otherwise occur when you use
the IP address of a physical interface to configure BGP. Figure 3-19 shows a network in which using the loopback interface is advantageous.
Figure 3-19
Use of loopback interfaces.
Loopback interface 0.150 212 1 1
E0
192 208 102
IBGP
E1
190 225 11 1
Router A
Router B
E2
E3
AS 100
In Figure 3-19, Routers A and B are running IBGP within AS 100. If Router A were to specify the
IP address of Ethernet interface 0, 1, 2, or 3 in the neighbor remote-as router configuration command, and if the specified interface were to become unavailable, Router A would not be able to establish a TCP connection with Router B. Instead, Router A specifies the IP address of the loopback interface that Router B defines. When the loopback interface is used, BGP does not have to rely on the availability of a particular interface for making TCP connections.
Note Loopback interfaces are rarely used between EBGP peers because EBGP peers are usually directly connected and, therefore, depend on a particular physical interface for connectivity.
External BGP (EBGP)
When two BGP speakers that are not in the same AS run BGP to exchange routing information, they
are said to be running EBGP.
Synchronization
When an AS provides transit service to other ASs when there are non-BGP routers in the AS, transit traffic might be dropped if the intermediate non-BGP routers have not learned routes for that traffic via an IGP. The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP. The topology shown in Figure 3-20 demonstrates this synchronization rule.
3-34
Internetwork Design Guide
BGP Operation
Figure 3-20
EBGP synchronization rule.
AS100
150.10.10. 0
Router E
IGP
IGP
Router A
Router B
IGBP
2.2.2.2
2.2.2.1
Router C
Router D
AS300
170.10.10. 0
AS400
16567
In Figure 3-20, Router C sends updates about network 170.10.0.0 to Router A. Routers A and B are
running IBGP, so Router B receives updates about network 170.10.0.0 via IBGP. If Router B wants
to reach network 170.10.0.0, it sends traffic to Router E. If Router A does not redistribute network 170.10.0.0 into an IGP, Router E has no way of knowing that network 170.10.0.0 exists and will drop the packets.
If Router B advertises to AS 400 that it can reach 170.10.0.0 before Router E learns about the
network via IGP, traffic coming from Router D to Router B with a destination of 170.10.0.0 will flow to Router E and be dropped.
This situation is handled by the synchronization rule of BGP. It states that if an AS (such as AS 100
in Figure 3-20) passes traffic from one AS to another AS, BGP does not advertise a route before all routers within the AS (in this case, AS 100) have learned about the route via an IGP. In this case, Router B waits to hear about network 170.10.0.0 via an IGP before it sends an update to Router D.
Disabling Synchronization
In some cases, you might want to disable synchronization. Disabling synchronization allows BGP
to converge more quickly, but it might result in dropped transit packets. You can disable
synchronization if one of the following conditions is true:
• Your AS does not pass traffic from one AS to another AS.
• All the transit routers in your AS run BGP.
BGP and Route Maps
Route maps are used with BGP to control and modify routing information and to define the
conditions by which routes are redistributed between routing domains. The format of a route map is as follows:
route-map map-tag [[permit | deny] | [sequence-number]]
Designing Large-Scale IP Internetworks 3-35
BGP Internetwork Design Guidelines
The map-tag is a name that identifies the route map, and the sequence-number indicates the position that an instance of the route map is to have in relation to other instances of the same route map.
(Instances are ordered sequentially.) For example, you might use the following commands to define a route map named MYMAP:
route-map MYMAP permit 10
! First set of conditions goes here.
route-map MYMAP permit 20
! Second set of conditions goes here.
When BGP applies MYMAP to routing updates, it applies the lowest instance first (in this case,
instance 10). If the first set of conditions is not met, the second instance is applied, and so on, until either a set of conditions has been met, or there are no more sets of conditions to apply.