Transcript T4 Presentation - African Internet Exchange Point

Technical Aspects
of Peering
Session 4
Overview
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Peering checklist/requirements
Peering step by step
Peering arrangements and options
Exercises
Peering requirements
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BGP V4 router with enough memory to
receive all routes:
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256MB minimum for the global routing table
32MB for all African routes
Less for just your country
Portable address space (ie not received
from your upstream ISPS; this space is
already being advertised to peers
upstream)
AS Number
Both obtainable from AfriNIC
Additional requirements
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List of prefixes that will be advertised
and received from peers
IP Address of each peer (including
your own border router)
Maximum number of hops between
the BGP routers if they are not
adjacent to each other
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eBGP multihop is not recommended, and
not included in the handout
Peering step by step: Step 1
John Doe Communications wants to peer with Expert Networks.
Step 1: Write down all the necessary information for each party:
A:
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A:
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B:
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Company Name:
AS number:
Address space:
Border router:
BGP peer address:
John Doe Communications
AS100
12.1.1.0/24, 196.25.0.0/16
Cisco 2621
12.1.1.10
Company Name:
AS number:
Address space:
Border router:
BGP peer address:
Expert Networks
AS200
150.200.54.0/23
Linux PC running Quagga
150.200.54.125
Step 2.1
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Configure a loopback interface on the
router. This is necessary in order to have an
BGP peer with an interface that will always
be up even if some of the physical
interfaces on the router go down.
(Especially useful with iBGP.)
interface Loopback0
ip address 12.1.1.10 255.255.255.255
Step 2.3
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Define filters to advertise and receive only the routes
we know about. This is very important. If this is
omitted any peer can flood your routing table with
bogus entries. It can also cause your router to crash
if too many prefixes are accepted by your router.
! accept all prefixes smaller or equal to /24,
! but only from the address space that we know
! belongs to each AS. AS 100 is our own AS.
ip prefix-list AS100 seq 5 permit 12.1.1.0/24
ip prefix-list AS100 seq 10 permit 196.25.0.0/16 le 24
! AS200 is our peer
ip prefix-list AS200 seq 5 permit 150.200.54.0/23 le 24
Step 2.4
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The rest of the settings all reside in the BGP section
of the configuration. Specify your AS number here.
! configure BGP sessions
router BGP 100
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By default BGP does not advertise a route until all
routes within the AS have learned of the route
through the IGP. This command enables BGP to
advertise routes to peers without synchronizing the
route to the IGP.
no synchronization
Step 2.5
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Log all changes such as BGP connections going down.
These changes can be monitored by exporting the
router logs to a syslog server. Most ISPs have a
central log server and have technicians monitoring all
events.
bgp log-neighbour-changes
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Do not use “redistribute” commands to get routes
into BGP. They make it too easy for unwanted routes
to appear in your BGP tables.
Step 2.5
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Add a network statement for each route that you will
announce. Also add a null route for aggregates that
might not already be in your IGP routing table.
Without these commands no routes in our routing
table would be advertised to any peers.
! ensure that the aggregate route is always present
ip route 196.25.0.0 255.255.0.0 null0 254
! add your own networks to BGP
router bgp 100
network 12.1.1.0 mask 255.255.255.0
network 196.25.0.0 mask 255.255.0.0
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Do the above on only one router, or only a few
routers in your AS, not on every router.
Step 2.6
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Do not attempt to summarize routes. This command
is necessary if we want to exchange classless routes
(i.e. routes other than Class A, B, or C routes).
no auto-summary
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We now set up a peering session with Expert
Networks (AS 200). If there was more than one peer
we would have entered similar commands for each
peer. The first command specifies the AS number of
the peer (also known as a neighbour).
neighbour 1.2.3.4 remote-as 200
Step 2.6
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Add a description. If there are many neighbours
defined, it is useful to find the appropriate neighbour
when configuration changes have to be made by
looking at these descriptions.
neighbour 1.2.3.4 description Expert Networks
Step 2.7
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This command instructs the router to set the
gateways for all routes added to the routing table to
itself. Always enable this when peering with other
autonomous systems.
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neighbour 1.2.3.4 next-hop-self
Step 2.8
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Instruct the router to store received
updates. This allows us to update a BGP
session without having to restart the
session.
neighbour 1.2.3.4 soft-reconfiguration inbound
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This uses extra memory. In IOS 12 or
later, you can get the same benefits using
the BGP route refresh capability instead of
using memory. Use “show ip bgp neighbor
x.x.x.x”
Step 2.8
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Only advertise and accept routes allowed
by our filters to prevent flooding of our
routing table.
neighbour 1.2.3.4 prefix-list AS100 out
neighbour 1.2.3.4 prefix-list AS200 in
Step 3: Verify
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The following commands can be used to diagnose
problems with your BGP configuration:
! show a summary of peering sessions
show ip bgp summary
! show neighbour details
show ip bgp neighbours
! show routes received from neighbours
show ip bgp
! show routes received from neighbour 192.168.4.1
show ip bgp neighbours 192.168.4.1 received-routes
show ip bgp neighbours 192.168.4.1 routes
! show routes advertised to neighbour 192.168.4.3
show ip bgp neighbours 192.168.4.3 advertised-routes
! show all routes known via all protocols
show ip route
Option 1: Mandatory
multi-lateral peering
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All IXP participants peer
with a central route server.
This forces all to peer with
all and reduces the
number of peering
sessions that has to be
maintained by each peer.
A central route server != a
route reflector!! Route
reflectors used within iBGP
to eliminate the need for a
fully meshed network.
Option 1: Mandatory
multi-lateral peering
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Advantages
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Automatic peering
with all - easy
Complexity is
centralised – easy
for ISPs
Easy to connect –
only one BGP
session
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Disadvantages
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Forced peering
with all - inflexible
Complexity is
centralised – hard
for IXP operator
Complex policies
are impossible
Option 2: Bilateral peering
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Option 1 doesn’t scale
well: Some IXPs let all
participants negotiate
their own
arrangements.
This mesh network
scales well, but it takes
a lot more work by each
ISP.
If some participants
choose not to peer with
each other, then there
will be a partial mesh
instead of a full mesh.
Option 2: Bilateral peering
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Advantages
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Choose who to
peer with or not
All routers are
managed by ISPs,
not by IXP
operator
Complex policies
are possible
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Disadvantages
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Non-peering may
cause inefficient
routing
ISP router config
becomes complex
Difficult for new
participant to
connect
Option 3: Hybrid
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It is possible to have both models operating
simultaneously at an IXP, with some ISPs peering
with the central route server and the others manually
configuring their routers for bilateral peering with
selected peers.
Not the most desirable!
But can develop eg. if very large and very small ISPs
are part of the same IXP – gives control over
business relationships.
One option is to start with a single central route
server and multi-lateral peering, and allow bi-lateral
peering to be added later.
Exercise 1
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Several ISPs
Each ISP has a router at HQ, linked to
upstream provider
Each ISP adds a new router at
exchange point (XP), connected to HQ
router
Start using iBGP between HQ and XP
Not yet peering with other ISPs
HQ Routers
XP Routers
AS w
R
a
y
z
Rb
z
AS w
R
a
y
x
AS w
R
a
x
y
Rb
z
Upstream
Exchange Point
x
Rb
Exercise 2
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Follows on from exercise 1
Each ISP starts BGP peering with all
others (bi-lateral peering)
HQ Routers
XP Routers
p
R
a
y
x
Rb
z
Rb
z
AS w
p
R
a
y
x
AS w
p
R
a
x
y
Rb
z
Upstream
Exchange Point Switch
AS w
Exercise 3
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Follows on from exercise 1
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Undo part of exercise 2 first
Each ISP starts BGP peering with a
route server (multilateral peering)
Route Server
R
AS w
p
R
a
y
x
Rb
z
Rb
z
AS w
p
R
a
y
x
AS w
p
R
a
x
y
Rb
z
Upstream
Exchange Point Switch
HQ Routers
XP Routers