BGP Overview

1
BGP Overview

• Joe Walter
• Enterprise Solutions Center
• ESG
• Verizon
• November 20, 2002

2
NJEDge - Border Gateway Protocol (BGP)

• AGENDA
• What is BGP Why is it needed?
• Connectivity and Addressing Choices
• BGP Best Practices
• Q A
• ------(For Reference)_----------
• MPLS (Multiprotocol Label Switching) Overview
• MPLS and Traditional BGP
• BGP/MPLS VPN Overview

3
Border Gateway Protocol

• Evolved from EGP (Exterior Gateway Protocol) -
  RFC 827 from the early 1980s
• BGP is now in Version 4 and has evolved from RFC
  1105, RFC 1163, RFC 1267 which is BGP -1, BGP-2
  and BGP-3 respectively
• BGP-4 was introduced in 1995 in RFC 1771
• Key Difference is that BGP-4 is classless versus
  classful
• Classless - the prefix length is carried along
  with the routing update

4
BGP defined continued

• Path vector protocol. PV is an ordered set of
  values that represent AS numbers.
• PV is contained within the routing update as
  AS_PATH attribute
• determines the shortest path to the destination
• prevents routing loops
• Uses TCP port 179
• Uses Incremental updates - only when network
  topology changes
• Internal and External BGP
• Internal (within the same AS)
• External (between different ASs)

5
CIDR (Classless Interdomain Routing)

• CIDR is an IP address summarization technique
• BGP supports CIDR
• Makes routing scalable
• Takes advantage of hierarchical nature of the
  Internet
• Route summarization is used to cut down the
  number of routes in routing tables

6
CIDR Example
192.168.0.0/24
192.168.1.0/24
CIDR
192.168.0.0/22
192.168.2.0/24
192.168.3.0/24
7
BGP Message Types

• Open - identify itself and specify its BGP
  operational parameters
• Keepalive - 60 seconds default on Cisco
• Update - advertises feasible routes, withdrawn
  routes or both
• Notification - sent whenever an error is detected
  and always causes the BGP connection to close

8
BGP attributes

• Below is a partial list
• Origin
• AS-PATH
• NEXT_HOP
• LOCAL_PREF
• COMMUNITY
• MULTI_EXIT_DISC (MED)
• ORIGINATOR_ID

9
BGP and IGP

• IGP (Interior Gateway Protocol) - OSPF, IS-IS,
  EIGRP and RIP
• BGP relies on IGP for peer reachability
• BGP is synchronized with IGP
• BGP will not announce a route if it is NOT known
  by IGP
• Routers running IGP must know where to send
  packets for BGP destinations.
• MPLS significantly simplifies BGP and IGP
  interaction (to be discussed later)

10
Why is BGP needed?

• In General
• For its ability to deliver routing information
  between routers that are not directly attached
• For its flexible and powerful route advertisement
  and filtering policies
• It runs the Internet
• For Service Providers
• Protocol of choice to exchange routing
  information between SPs in a scalable and
  controlled manner
• For Customers
• To provide dynamic routing connectivity to the
  Internet
• Redundancy
• Routing granularity

11
Addressing and Connectivity Choices

• Addressing Choices
• Private versus Public AS number (ASN)
• Provider Assigned (PA) versus Provider
  Independent (PI) IP addressing
• Connectivity Choices
• Single Homed to Single Autonomous System
• Dual Homed to Single Autonomous System
• Dual Homed to Dual Autonomous Systems
• Split Connection ISP and Extranet

12
Addressing Private vs. Public ASN

• Autonomous System (AS) - A set of routers under a
  common technical administrative control
• AS number (ASN) - an AS identifier
• decimal number in the range of 1 - 65535
• Private ASN
• 64512 ? private ASN ? 65535
• Usually assigned by ISP
• ISP removes Private ASN from AS_PATH before
  sending route to the Internet
• Public ASN
• 1 ? public ASN ? 64511
• Assigned by ARIN

13
Addressing PA vs. PI

• Provider Assigned (PA) IP addresses
• assigned by ISP to the customer from ISP address
  block
• usually comes with the Private ASN
• If customer changes provider then addresses are
  taken back
• Provider Independent (PI) IP addresses
• assigned by ARIN
• usually comes with public ASN
• Stays with the customer if the provider changes

14
Connectivity Single-Homed Autonomous System
Public ASN and PI Addresses
Private ASN and PA Addresses
AS 65100
AS 100
X.X.X.4/30
Y.Y.Y.4/30
X.Y.Z.0/22
A.B.C.0/22
AS 1
AS 1
X.0.0.0/8
Y.0.0.0/8
Internet
Network A.B.C.0/22 Path 1,100
Internet
Network X.0.0.0/8 Path 1
15
Single-Homed Autonomous System Config

• Public ASN and PI Addresses
• Router bgp 100
• network A.B.C.0 mask 255.255.252.0
• Neighbor Y.Y.Y.5 remote-as 1
• Only partial configuration shown

• Private ASN and PA Addresses
• Router bgp 65100
• network X.Y.Z.0 mask 255.255.252.0
• Neighbor X.X.X.5 remote-as 1

16
Connectivity Dual-Homed Autonomous System to
single Autonomous System
Public ASN and PI Addresses
Private ASN and PA Addresses
AS 100
AS 65100
Y.Y.Y.4/30
X.X.X.4/30
A.B.C.0/22
X.Y.Z.0/22
AS 1
Y.0.0.0/8
AS 1
X.0.0.0/8
Y.Y.Y.8/30
X.X.X.8/30
Internet
Network A.B.C.0/22 Path 1,100
Internet
Network X.0.0.0/8 Path 1
17
Dual-Homed Autonomous System to single
Autonomous System Config

• Public ASN and PI Addresses
• Top router
• Router bgp 100
• network A.B.C.0 mask 255.255.252.0
• Neighbor Y.Y.Y.9 remote-as 1
• Bottom router
• Router bgp 100
• network A.B.C.0 mask 255.255.252.0
• Neighbor Y.Y.Y.5 remote-as 1
• Only partial configuration shown

• Private ASN and PA Addresses
• Top router
• Router bgp 65100
• network X.Y.Z.0 mask 255.255.252.0
• Neighbor X.X.X.5 remote-as 1
• Bottom router
• Router bgp 65100
• network X.Y.Z.0 mask 255.255.252.0
• Neighbor X.X.X.9 remote-as 1

18
Connectivity Dual-Homed Autonomous System to
Dual Autonomous Systems
Public ASN and PI Addresses
AS 100
A.B.C.0/22
X.X.X.4/30
Y.Y.Y.4/30
AS 3
AS 1
X.0.0.0/8
Y.0.0.0/8
ISP 1
ISP 2
Internet
Network A.B.C.0/22 Path 1 1,100 Path 2
3,100
19
Dual-Homed Autonomous System to Dual Autonomous
Systems Config

• Public ASN and PI Addresses
• Router on the Left
• router bgp 100
• network A.B.C.0 mask 255.255.252.0
• Neighbor Y.Y.Y.5 remote-as 1
• Only partial configuration shown

• Router on the right
• router bgp 100
• network A.B.C.0 mask 255.255.252.0
• Neighbor X.X.X.5 remote-as 3

20
Split - Dual connection - NJEDge
AS 100
A.B.C.0/22 - PI Internet Space
10.10.0.0/22 - NJEdge Extranet Space
TX A.B.C.0/22 Only RX Default/Partial/Full
TX 10.10.0.022 Only RX NJEdge Extranet Routes
10.X.X.4/30
Y.Y.Y.4/30
AS 1
Y.0.0.0/8
AS 19262
ISP
VIS
Network A.B.C.0/22 Path 1 1,100
Internet
NJEDge
21
Split - Dual connection - NJEDge

• CE- Router Config
• router bgp 100
• network A.B.C.0 mask 255.255.252.0
• network 10.10.0.0 mask 255.255.252.0
• neighbor Y.Y.Y.5 description Tier1 ISP
• neighbor Y.Y.Y.5 remote-as 1
• neighbor Y.Y.Y.5 prefix-list INTERNET-RANGE out
• neighbor Y.Y.Y.5 prefix-list DEFAULT in
• neighbor 10.X.X.5 description NJEdge Extranet
  PE
• neighbor 10.X.X.5 remote-as 19262
• neighbor 10.X.X.5 prefix-list NJEDGE-RANGE out
• neighbor 10.X.X.5 prefix-list NJEDGE-EXTRANET in
• ip prefix-list INTERNET-RANGE seq 5 permit
  A.B.C.0/22
• ip prefix-list DEFAULT seq 5 permit 0.0.0.0/0
• ip prefix-list NJEDGE-RANGE seq 5 permit
  10.10.0.0/22
• ip prefix-list NJEDGE-EXTRANET seq 5 deny
  10.10.0.0/22
• ip prefix-list NJEDGE-EXTRANET seq 10 permit
  10.0.0.0/8 le 24

The configuration DEPENDS on your local
environment Need to setup filtering for policy
IN and OUT bound
22
BGP Best Practices

• Full BGP routing table requires minimum of 128
  Mbytes memory for router
• Keep it Simple!
• Be careful of use of no synchronization
• Be careful of use of no auto-summary
• Avoid redistribution of IGP
• Try to Summarize
• Set inbound policy
• Set outbound policy

23
More on BGP

• Ciscos BGP support link
• http//www.cisco.com/pcgi-bin/Support/browse/psp_v
  iew.pl?pInternetworkingBGP
• Routing TCP/IP Volume II, Jeff Doyle Cisco
  Press
• Internet Routing Architectures, Bassam Halabi,
  Cisco Press
• BGP/MPLS VPNs
• http//www.ietf.org/rfc/rfc2547.txt

24
Q A

• THANK YOU

25
For Reference - MPLS Overview

• MPLS - Multiprotocol Label Switching - a
  technology that combines benefits of Layer 2
  switching and Layer 3 routing
• MPLS Architecture
• Control Plane
• IP Routing Protocols (OSPF, IS-IS, etc). Used to
  obtain Layer 3 network reachability information.
• Label Distribution Protocol (LDP). Used to
  distribute bindings between learned Layer 3
  prefixes and MPLS labels
• Forwarding Plane
• Label Switching
• Basic MPLS Network Components
• P-routers Provider Backbone or Label Switch
  Routers (LSR)
• PE-routers Provider Edge or Label Edge Routers
• CE-routers Customer Edge Routers

26
MPLS Overview - Labels

• Labels are numerical values that are inserted
  into frames
• ATM uses VPI/VCI
• Frame Relay uses DLCI
• Ethernet uses a special shim header
• Labels can be stacked (important for MPLS VPNs)

6 bytes
6 bytes
2 bytes
20 bits
3 bits
1
8 bits
4 bytes
Data
Src. MAC
Type/Length
Label 1
EXP
TTL
FCS
Label 2
E
T
S
Dest MAC
Shim Header - 4 bytes min.
For Ethernet MPLS uses Type value of 0x8847 -
MPLS Unicast
27
MPLS and Traditional BGP

• MPLS significantly simplifies packet forwarding
  to BGP destinations
• Traditionally BGP had to be run on every router
  in the core of an ISP network to enable proper
  packet forwarding
• MPLS allows to forward packets to BGP
  destinations by simply label-switching traffic to
  a BGP next-hop address
• BGP next-hop addresses must be reachable via IGP,
  which allows them to be associated with MPLS
  labels
• This allows ISP core routers to run only an IGP
  (IS-IS). ISP PE routers are the only ones that
  need to run BGP

28
MPLS and Traditional BGP (contd)
Pre-MPLS
Peering ISP or Customer
Peering ISP or Customer
CORE
IBGP
IGP
EBGP
MPLS
LDP
LDP
LDP
Peering ISP or Customer
Peering ISP or Customer
MPLS CORE
P
P
PE
PE
29
BGP/MPLS VPN - RFC 2547

• Implemented within a Service Provider network
• This is not a Tunnel technology (like IPSec)
• Provides Peer-to-Peer Full Mesh Native IP
  connectivity (eliminates the N2 issue)
• VPN Routing Information is Distributed using
  MP-BGP (Multiprotocol BGP)
• Each VPN is assigned a unique Route Distinguisher
  (RD). RD is used by MP-BGP as an Extended
  Community attribute
• VPN routes are constructed by pre-pending a
  32-bit IPv4 prefix with a unique 64-bit Route
  Distinguisher (RD)
• Overlapping IP Addresses are Allowed

30
BGP/MPLS VPN (contd)

• VPN Packet Forwarding is done by MPLS using a
  two-label stack
• An outer label is used to deliver a packet across
  MPLS network
• An inner label is used to forward a packet to an
  appropriate VPN interface
• Uses Virtual Routing and Forwarding (VRF) Tables
  on (Cisco) Routers
• A VRF is a set of physical interfaces and logical
  routing information
• VRFs effectively partition a router into a set of
  independent Virtual Routers

31
BGP/MPLS VPN (contd)
CUST1
CUST1
MPLS/MP-BGP
PE
PE
P
CUST2
CUST2
P
CUST1 10.0.0.0/8 RD 100100
P
CUST2 10.0.0.0/8 RD 200200
PE
CUST1
CUST2
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BGP/MPLS VPN - CE to PE Connectivity

• Physical Connectivity
• ATM
• Frame Relay
• Ethernet
• Point-to-Point (PPP, HDLC)
• Logical Connectivity
• Static Routing
• OSPF
• RIPv2
• BGP
• Single Homed to Single AS with Public/Private ASN
  and PA/PI Address Space
• Dual Homed to Single AS with Public/Private ASN
  and PA/PI Address Space