Transcript Real, Relevant, Surprising and Fresh: Cisco Brand

Insight into BGP
Mike Smith – Anglia Ruskin University
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Logical view of Internet Architecture
Internet as a collection of Autonomous Systems
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Definition: Autonomous System
“a set of routers under a single technical administration, using an
Interior Gateway Protocol (IGP) and common metrics to determine
how to route packets within the autonomous system, and using an
inter-autonomous system routing protocol to determine how to route
packets to other autonomous systems” (RFC 4271)
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Top Ten AS’s by CIDR blocks
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Unadvertised are not visible on the Internet
ASN allocation by RIR
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Growth in the number of ASs
Growth rate = 3500 new AS per year
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ISP AS Infrastructure
AS’s joined through Internet Packet Exchange (IPX) Gateways
Approximately 100 gateways worldwide
UK has three main gateways
London Internet Exchange (LINX)
Peak Traffic – 1092 Gbps
Average Traffic – 743 Gbps
Have access to 0.5 of Internet Routing Table
190,000 CIDR blocks
388 members
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IXP World Map
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IXP Europe Map
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IXP UK Map
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Peering
In networking, peering is a voluntary interconnection of
Autonomous Systems for the purpose of exchanging traffic.
The pure definition of peering is settlement free – peers
exchange traffic on the basis that I will forward your traffic if you
will forward mine.
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Peering types
There are generally two types of peering: public and private
Public Peering
Public peering is accomplished using a shared fabric (usually a L2
switch technology). At these locations (mainly IXPs), multiple
carriers interconnect with one or more other carriers across a single
physical port.
Private Peering
Private peering is the direct connection between two networks
across a Layer 1 or Layer 2 medium that provides dedicated
network capacity and is not shared. Private peering is commonly
used between very large networks.
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Janet (AS748) Peering Partners
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BGP Concepts and Basic
Configuration
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Routing Protocol Taxonomy
Routing protocols fit into two main categories: Interior Gateway
Protocols (IGPs) and Exterior Gateway Protocols (EGPs)
Interior Gateway Protocol
A routing protocol that exchanges routing information within an
Autonomous System – RIP, EIGRP, OSPF and ISIS
Exterior Gateway Protocol
A routing protocol that exchanges routing information between
different Autonomous Systems - BGP
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Goal of BGP is to provide an inter-domain routing system that
guarantees the loop-free exchange of routing information between
autonomous systems
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Key features of BGP
• Policy based routing
• Uses TCP for connectivity
• Incremental updates
• Rich metrics
• Designed to scale to huge internetworks (for example, the Internet)
• EBGP – used to route between ASs
• IBGP – used to route BGP routing information within an AS
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Comparison of Scalable Routing Protocols
Protocol
IGP/EGP Type
Hierarchical
Metric
OSPF
IGP
Link state
Yes
Cost
IS-IS
IGP
Link state
Yes
Metric
EIGRP
IGP
Advanced
Distance
Vector
No
Composite
(bandwidth,
delay)
BGP
EGP
Path
Vector
No
Path vectors
(attributes)
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Why use BGP?
BGP is an appropriate routing protocol to use when:
• An AS allows packets to transit through to reach other ASs (for
example, it is a service provider)
• An AS has multiple connections to other ASs (multi-homed)
• Routing policy and route selection for traffic entering and leaving the
AS must be manipulated
When not to use BGP?
• Limited understanding of route filtering and BGP path-selection
process
• A single connection to the Internet or another AS
• Lack of memory or processor power to handle constant updates on
BGP routers
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BGP Message Types
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UPDATE message
UPDATE messages contain the routing information for BGP
Unfeasible Routes Length (2 bytes)
Withdrawn Routes (variable)
Total Path Attributes Length (2 bytes)
Path Attributes (variable)
Network Layer Reachability Information (variable)
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UPDATE message - Path Attributes
•The Path Attributes field is of variable length and contains a
sequence of attributes about a path. The Path Attributes field will
be present in every UPDATE message
•The information contained in the Path Attribute field is used to
track specific route information and is also used for routing
decisions and filtering
• The Path Attribute field consists of: attribute type, attribute
length and attribute value
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Well known – attribute must be recognised by all implementations of BGP
Optional – attribute not recognised by an implementation of BGP
Mandatory – attribute must be present in an UPDATE message
Discretionary – attribute does not need to be present in an UPDATE message
Transitive – attribute forwarded to another BGP peer that may not be recognised by this
peer
Non-transitive – attribute not forwarded to another BGP peer
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Commonly used BGP attributes
Type Code
Name
Category
Description
1
ORIGIN
Well-known mandatory
The AS that originated the routing information
2
AS_PATH
Well-known mandatory
A list of all ASs the routing information has transited
3
NEXT_HOP
Well-known mandatory
Specified the IP address of the next-hop router to the specified destination
4
MULTI_EXIT_DISC
Optional non-transitive
Used to determine the best exit/entry point to the same AS if multiple points exist
5
LOCAL_PREF
Well-known discretionary
Is used to set a preference of a route to a iBGP peer
6
ATOMIC_AGGREGATE
Well-known discretionary
Used to choose a less specific (shorter mask) route rather than a more specific (longer
mask) when receiving overlapping routes from a BGP speaker
7
AGGREGATOR
Optional transitive
When a BGP peer perform route aggregation, it will include in the AGGREGATOR attribute
its AS number and BGP Id
8
COMMUNITY
Option transitive
Specifies the communities a route belongs to. A community is a group of destinations that
have a common attribute
9
ORIGINATOR_ID
Optional non-transitive
A BGP speaker that has the role of Route reflector creates this attribute. A route reflector
can advertise iBGP learned routes to other iBGP peers. This is not normally allowed
10
CLUSTER_LIST
Optional non-transitive
Used by a route reflector to specify the BGP peers that a part of its clients.
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Local_Preference Attribute – used to prefer a exit path from the AS
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Multi_Exit_Disc attribute
path
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Basic BGP Configuration
BORDER
router bgp 100
Specifies the AS this router belongs to
Specifies the IP address of the remote
AS and its AS number
neighbor 200.200.200.2 remote-as 200
network 192.168.1.0
Specifies the network to be
advertised
ISP
router bgp 200
neighbor 200.200.200.1 remote-as 100
network 194.82.46.0
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BGP Concepts and
Advanced Configuration
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router bgp 300
neighbor 192.168.1.5 remote-as 100
neighbor 172.24.1.18 remote-as 65000
network 10.2.2.0 mask 255.255.255.0
router bgp 65000
router bgp 100
neighbor 192.168.1.6 remote-as 300
neighbor 172.24.1.17 remote-as 300
network 10.3.3.0 mask 255.255.255.0
network 10.1.1.0 mask 255.255.255.0
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Modify default BGP action using AS_PATH attribute
In this example, the provider router, ISP, is to be configured so that it
does not propagate routes that originate from AS 100 to the customer
router CustRtr
X
AS 100 routes not to be advertised to AS
65000
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ISP
router bgp 300
neighbor 192.168.1.5 remote-as 100
neighbor 172.24.1.18 remote-as 65000
network 10.2.2.0 mask 255.255.255.0
ip as-path access-list 1 deny ^100$
ip as-path access-list 1 permit .*
router bgp 300
neighbor 172.24.1.18 filter-list 1 out
Using the AS_PATH attribute to prevent AS100 routes being advertised to CustRtr
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Configuring Local_Preference Attribute
Preferred exit path
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Basic steps
1. Configure IP interfaces
2. Configure EIGRP (in this case) as an IGP in AS 64512
3. Configure IBGP between SanJose1 and SanJose2
4. Configure BGP Local Preference attribute on SanJose1 and
SanJose2
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SanJose 1
router eigrp 64512
network 172.16.0.0
no auto-summary
router bgp 64512
neighbor 172.16.32.1 remote-as 64512
neighbor 172.16.32.1 update-source lo0
neighbor 192.168.1.5 remote-as 200
network 172.16.0.0
route-map PRIMARY_T1_IN permit 10
set local_preference 150
router bgp 64512
neighbor 192.168.1.5 route_map PRIMARY_T1_IN in
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SanJose 2
router eigrp 64512
network 172.16.0.0
no auto-summary
router bgp 64512
neighbor 172.16.64.1 remote-as 64512
eighbor 172.16.64.1 update-source lo0
neighbor 192.168.1.1 remote-as 200
route-map SECONDARY_T1_IN permit 10
set local_preference 125
router bgp 64512
neighbor 192.168.1.1 route_map SECONDARY_T1_IN in
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Thank you.