This example demonstrates the different
types of BGP peering sessions you will encounter. An IBGP peering
session is formed within AS3, between the loopback address of RTA
and a physical address of RTF. An EBGP session is also formed
between AS3 and AS1 by using the two directly connected IP addresses
of RTA and RTC. Another EBGP session is formed between RTF in AS3
and RTD in AS2, using IP addresses that are not on the same segment
(multihop).
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Lab
Activity |
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In this lab,
you will establish a basic BGP connection between routers and
observe differences between internal and external BGP
sessions. |
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It is important to remember that the
BGP peers will never become established unless there is IP
connectivity between the two peers. We will use OSPF as an IGP to
establish the required internal connectivity.
Note: Click on topology to view
command outputs.
RTA's configuration shows some syntax
that might be unfamiliar to you. All the syntax is explained here
generically, as well as in relation to the particular routing
scenario of the example. In subsequent examples throughout this
chapter, however, the router configuration will be reduced to
necessary commands to configure BGP or IGP; for instance, commands
that assign IP addresses to interfaces will be omitted in many cases
because of space limitations.
- ip subnet-zero
-- This
global configuration command is necessary in case you are
configuring interfaces that fall in subnet-zero subnets. With
the introduction of classless routing, using subnet-zero is very
common.
- interface type slot/port
--
This command configures an interface type and number on the
router. Any configuration that appears under the command will be
specific to that particular interface. Note that RTA has three
interface commands, one for each of its three connections. The
loopback interface is a software-only interface that is always
up.
- ip address ip-address mask
[secondary] -- This command configures an IP address and mask
on the interface. The RTA Ethernet IP address, for example, is
configured using: IP address 172.16.1.1 255.255.255.0.
- router process [process-id]
- This is a global command that defines a process such as OSPF,
RIP, or BGP, and gives the process a process ID. Some processes,
such as RIP, do not require a process ID, while others use the
process ID as an Autonomous System number.
In the RTA configuration,
"router ospf 10," for example, indicates an OSPF process
with ID 10, whereas "router bgp 3" indicates a BGP process
in AS 3.
- network
-- This command
indicates the networks or, in the case of OSPF, the interfaces
that will run under a specific router process.
- wildcard bits
-- In the RTA
network command, you will notice a representation of the form
0.0.255.255, or basically a number of 0s followed with a number
of 1s. These are wildcard bits (sometimes known as an inverse
mask). Any place there is a 0 in the wildcard bits, the ip
address compared to it match; 1s are do-not-care bits. For
example, 172.16.0.0 0.0.255.255 indicates any IP address or
network of the form 172.16.x.x. Inverse masks are used when
defining access lists as well.
- area area-number
-- This
defines which OSPF area any interfaces falling within the range
of IP addresses specified in the network statement will be a
part of.
- neighbor
-- This command is
used to define the BGP neighbor connection parameters and
policies between this router and this peer. In the RTA
configuration, "neighbor 172.16.1.2 remote-as 3," for
example, is an indication that a BGP peer session is to be
established between RTA and peer 172.16.1.2 in AS 3.
- no synchronization
-- This
command turns the synchronization off between BGP and IGP.
- no auto-summary
-- This
command will turn off the BGP automatic summarization at major
net boundaries. Without this command, BGP will not send the
subnets of a major net that are redistributed into BGP; that is,
updates about 172.16.1.0/24, 172.16.2.0/24, and so on will be
sent as a single prefix: 172.16.0.0/16. Summarization at the
major net boundary should be done only if the AS is the owner of
the whole major net.
- update-source interface
--
Specifies the interface to be used as a source IP address of the
BGP session with the neighbor. In RTA's configuration, for
example, the second neighbor statement indicates that Loopback 0
is to be used as a source interface.
- remote-as
-- Specifies the
AS number of the remote BGP peer. In RTA's configuration, the
first neighbor statement indicates that the internal BGP
neighbor 172.16.1.2 belongs to the AS3, which also happens to be
the local AS. The third neighbor statement indicates that the
external BGP peer 172.16.20.1 belongs to AS1.
In RTF's configuration, you can see
the ebgp-multihop 2
command being used as part of the neighbor configuration. This is an
indication that the exterior BGP peer is not directly connected and
can be reached at a maximum of two hops away. Remember that
ebgp-multihop is applicable only with EBGP, and not with IBGP.
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Lab
Activity |
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In
this lab, you will learn how to establish a
BGP connection between routers via a router
that is not running BGP. |
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The example also shows how the peer
connection will look after the neighbors are in an established
state. From RTF's point of view, neighbor 172.16.2.254 is an
internal neighbor that belongs to AS3. The neighbor connection is
running BGP Version 4 with a table version of 2. The table version
changes every time the BGP table gets updated.
The other RTF neighbor, 192.68.12.1,
is also in an established state. This is an external neighbor that
belongs to AS2. Note that the display indicates that this neighbor
is two hops away (as configured using ebgp-multihop).
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