Once the connection has been established over the external BGP peer session, BGP routers send incremental updates that include summarized address ranges and AS numbers. They also send
“keepalives” to maintain the session. All BGP messages are sent to TCP port 179.
Network Layer Reachability Information
A “route” is not a network or subnet as in the previous route exchange protocols, but is a unit of information that pairs a destination with path attributes.
62
AS 100
150.10.0.0/16 AS_PATH: 400
AS 400 150.10.0.0/16 AS
200
AS 300 198.155.70.0/24 198.155.70.0/24
AS_PATH: 300
AS_PATH: 400 200 198.155.70.0/24 AS_PATH: 300 200 150.10.0.0/16
Figure 22: Network Layer Reachability Information
A “destination” is a range of IP addresses that are reported using prefix/length notation, where the “prefix” is the starting address of the range and “length” defines the size or boundaries of the range. For example, the destination 150.10.0.0/16 is a range consisting of the 65,536 (216) IP addresses between 150.10.0.0 and 150.10.255.255.
Path attributes, known in the BGP specification as AS_PATH, is a list of the autonomous systems through which a route passes. BGP4 routers can use this list of traversed autonomous systems to detect and eliminate routing loops.
Internal BGP
The ProCurve Secure Router 7000dl series supports both Internal iBGP and External eBGP.
While there are many similarities between external and internal BGP, the most important difference is that the BGP speakers in an Internal BGP peer session are in the same AS.
Internal BGP is used within a transit AS, as is shown in the diagram below. Please check http://www.procurve.com for free software updates due soon that will include further enhancements to BGP.
63
iBGP iBGP
AS 100 AS 200
AS 500
eBGP eBGP
Figure 23: Internal BGP
If an AS has multiple BGP speakers, it could be used as a transit service for other ASs. As you can see in the iBGP diagram, AS 500 is a transit AS for AS 100 and AS 200.
It is necessary to ensure reachability within an AS before sending the information to an external AS. This is done by a combination of internal BGP peering between router inside an AS and by redistributing BGP information to Internal Gateway Protocols (IGPs) running in the AS.
When BGP is running between routers belonging to the same AS, it is call iBGP. When BGP is running between routers that are in different ASs, it is called eBGP.
ProCurve 7203dl Hostname: Central
T1 3/1, T1 3/2 192.168.2.x/30 MLPPP
ProCurve 7102dl Hostname: Remote T1 1/1, T1 1/2
192.168.2.x/30 MLPPP
AS 65300
ISP
Figure 24: BGP Example
ProCurve Secure Router 7203dl
!
hostname "Central"
no enable password
!
ip subnet-zero ip classless
64
ip routing
!
event-history on no logging forwarding no logging email
logging email priority-level info
!
!
!
interface eth 0/1
ip address 192.168.3.254 255.255.255.0 no shutdown
!
interface eth 0/2 no ip address shutdown
!
!
!
interface t1 3/1
tdm-group 1 timeslots 1-24 speed 64 no shutdown
!
interface t1 3/2
tdm-group 2 timeslots 1-24 speed 64 no shutdown
!
interface t1 3/3 shutdown
!
interface t1 3/4 shutdown
!
interface t1 3/5 shutdown
!
interface t1 3/6 shutdown
!
interface t1 3/7 shutdown
!
interface t1 3/8
shutdown
!
interface ppp 1
ip address 192.168.2.2 255.255.255.252 ppp multilink
router BGP 65300 no auto-summary no synchronization
network 192.168.2.0 mask 255.255.255.252
no ip http secure-server no ip snmp agent
ProCurve Secure Router 7102dl Remote#sh run
Building configuration...
!
!
hostname "Remote"
no enable password
!
event-history on no logging forwarding no logging email
logging email priority-level info
!
!
!
!
!
interface eth 0/1
ip address 192.168.1.254 255.255.255.0 no shutdown
!
interface eth 0/2 no ip address shutdown
!
!
!
interface t1 1/1 clock source internal
tdm-group 1 timeslots 1-24 speed 64 no shutdown
!
interface t1 1/2 clock source internal
tdm-group 2 timeslots 1-24 speed 64 no shutdown
!
interface adsl 2/1
training-mode multi-mode shutdown
!
interface ppp 1
ip address 192.168.2.1 255.255.255.252 ppp multilink
router BGP 65300 no auto-summary no synchronization
network 192.168.2.0 mask 255.255.255.252
neighbor 192.168.2.2 no default-originate soft-reconfiguration inbound remote-as 65300
no ip http secure-server no ip snmp agent
Classless InterDomain Routing (CIDR)
It is within the BGP environment that the benefits of Classless InterDomain Routing (CIDR) become apparent. CIDR was suggested in the late 1980s and then later mandated by the Internet Engineering Task Force in the early 1990s when the number of networks attached to the Internet started to increase rapidly.
The diagram below illustrates the addressing relationship between an ISP, its subscribers, and other ISPs. Recall that in classless addressing, a starting address followed by a mask defines a range address whose size is some power of 2, regardless of the value in the first octet of the address.
In the example, ISP A owns the range of addresses 202.0.0.0/14, provides Internet service to 1,024 businesses. This means that 262,144 (218) IP addresses between 202.0.0.0 and 202.3.255.255 are all reachable through ISP A. This ISP allocates a range of 256 addresses to each of its subscribers. Each of these address ranges is equivalent to a Class C network.
66
When ISP A advertises its address range to another service provider, ISP C, Classless
InterDomain Routing allows the service provider to summarize the address range using a mask that breaks the rules of classful addressing because it is shorter than the natural mask.
If classless addressing were not allowed, this service provider would have to individually advertise 1,024 Class C network numbers (each with a classful 24-bit mask), beginning with 202.0.0.0, 202.0.1.0, 202.0.2.0, and continuing on through 202.3.255.0.
Given the fact that the total 32-bit IP address space allows over 2 million individual IP addresses, the ability to aggregate address space beyond classful boundaries minimizes the number of IP address ranges in the “core” of the Internet.
In the diagram below, ISP B advertises another address range –- 202.4.0.0/14.
subscribers
ISP A
ISP C
ISP B
subscribers
advertise:
202.0.0.0/14
advertise:
202.4.0.0/14 202.0.0.0/24
202.0.1.0/24 202.0.2.0/24 . . .
202.3.255.0/24
202.4.0.0/23 202.4.2.0/26 202.4.2.64/26 202.4.2.128/26 . . .
202.7.255.0/24