NXLD41 BGP border gateway routing protocol kho tài liệu bách khoa

BGP Border Gateway Routing ProtocolBGP Border Gateway Routing Protocol is a standardized exterior gateway protocol designed to exchange routing and reachability information between auton

Trang 1

BGP (Border Gateway Routing Protocol)

BGP (Border Gateway Routing Protocol) is a standardized exterior gateway protocol designed to exchange routing and reachability information between autonomous systems (AS) on the Internet The Border Gateway Protocol makes routing decisions based on paths, network policies or rule-sets configured by a network administrator, and are involved in making core routing decisions

BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing protocol employed on the Internet

Basics of BGP

1 BGP is the path-vector protocol that provides routing information for autonomous systems on the Internet via its AS-Path attribute

2 BGP is a Layer 4 protocol that sits on top of TCP It is much simpler than OSPF, because it doesn’t have to worry about the things TCP will handle

3 Peers that have been manually configured to exchange routing information will form a TCP connection and begin speaking BGP There is no discovery in BGP

4 Medium-sized businesses usually get into BGP for the purpose of true multi-homing for their entire network

Figure 1 Basic Topology of BGP

Trang 2

5 An important aspect of BGP is that the AS-Path itself is an anti-loop mechanism Routers will not import any routes that contain themselves in the AS-Path

Current Version

The current version of BGP is version 4 (BGP4) codified in RFC 4271 since 2006 Early versions of the protocol are widely considered obsolete and are rarely supported

Types of BGP

There are different terms used when describing BGP These including:

1 Internal BGP (iBGP) operates inside an autonomous System (AS)

2 External BGP (eBGP), which is also known as an interdomain routing protocol, operates outside

an AS and connects one AS to another These terms are just used to describe the same protocol just the area of operation is what differs

Figure 2 Types of BGP

Trang 3

Uses

Most Internet service providers must use BGP to establish routing between one another (especially if they are multihomed) Compare this with Signaling System 7 (SS7), which is the inter-provider core call setup protocol on the PSTN

Very large private IP networks use BGP internally An example would be the joining of a number of large OSPF (Open Shortest Path First) networks where OSPF by itself would not scale to size Another reason

to use BGP is multihoming a network for better redundancy, either to multiple access points of a single ISP or to multiple ISPs

BGP AD

 EBGP-20

 IBGP-200

Characteristics of BGP-4

The key features of BGP-4 include and not limited to these:

1 It is an advanced distance-vector protocol

2 BGP sends full routing updates at the start of the session, trigger updates are sent afterward

3 BGP maintains connection by sending periodic keepalives

4 It creates and maintains connections between peers, using TCP port 179

5 BGP sends a triggered update when a keepalive, an update, or a notification is not received

6 It has its own routing table, although it is capable of both sharing and inquiring of the interior IP routing table

7 BGP uses a very complex metric, and is the source of its strength The metric, referred to as attributes, allows great flexibility in path selection

8 It selects the route based on the AS Path

9 It selects that route, which provides a network with least AS

10 Max AS length 65535

11 1 to 64511 Public

12 64512 to 65535 Private

13 BGP doesn’t discover neighbour automatically In BGP we have to define neighbours They are called BGP peers

14 BGP supports only one type of authentication that is MD-5

Trang 4

How to advertise Routes in BGP?

Using two ways

 Using Network Command

 Redistribution

BGP Messages

Content of Open Message

Version – 4 (Right now we are using BGP V4)

MY-AS – Itself AS number

Router ID

Hold Time – 180 sec (Default)

Contents of update

a) Routes

b) Route Attributes- These are those criteria’s which are used to select the best path

cause of resetting

BGP Tables

1 Neighbour

2 BGP Table

3 Routing Table

BGP States

Trang 5

BGP Terminology

routes with default next hop to IBGP neighbour

To solve this drawback we have a solution that is called next hop self

This command tells a router give your own IP address as a next hop to your IBGP neighbor

(Note: Always applied on edge routers.)

neighbour with another neighbour

To solve this problem we have a solution that is called route reflector client

This command tells a router do exchange the routes of one neighbor with another neighbor

Figure 3 BGP States

Trang 6

value 1 in open message If your neighbour is not directly connected, then you have to change the TTL Value using EBGP Multihop command

implement load balancing then you have to change max path value

5 BGP- By default, the redistribution of iBGP into IGP isn’t allowed on Cisco IOS

update source command

BGP Attributes/Rich Metric/Path Attributes

Directly Connected – 32768

Indirectly Connected – 0

Preferred – Directly Connected

Default 100 < 4.2 billion

Preferred – Higher

6 MED- Multi Exit Discriminator

By default – 0

Lowest will be preferred

Trang 7

By default BGP selects single path, for load balancing we use maximum path command

BGP Path Selection

BGP could possibly receive multiple advertisements for the same route from multiple sources BGP selects only one path as the best path When the path is selected, BGP puts the selected path in the IP routing table and propagates the path to its neighbours BGP uses the following criteria, in the order presented, to select a path for a destination:

1 If the path specifies a next hop that is inaccessible, drop the update

2 Prefer the path with the largest weight

3 If the weights are the same, prefer the path with the largest local preference

4 If the local preferences are the same, prefer the path that was originated by BGP running on this router

5 If no route was originated, prefer the route that has the shortest AS_path

6 If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP

is lower than EGP, and EGP is lower than incomplete)

7 If the origin codes are the same, prefer the path with the lowest MED attribute

8 If the paths have the same MED, prefer the external path over the internal path

9 If the paths are still the same, prefer the path through the closest IGP neighbour

10 Prefer the path with the lowest IP address, as specified by the BGP router ID

Message Header Format

1 Marker- Included for compatibility, must be set to all ones

2 Length- Total length of the message in octets, including the header

3 Type- Type of BGP message The following values are defined:

a) Open (1)

b) Update (2)

c) Notification (3)

Figure 4 Message Header Format

Trang 8

d) KeepAlive (4)

e) Route-Refresh (5)

BGP Example

R1 (config) #int fa0/0

R1 (config-if) #ip add 172.168.101.1 255.255.255.0

R1 (config-if) #no shut

R1 (config-if) #int s0/0

R1 (config-if) #ip add 192.168.1.1 255.255.255.0

R1 (config-if) #ip add 192.168.4.2 255.255.255.0

R1 (config-if) #do sh ip int br

Figure 5 BGP Example Topology

Trang 9

R2 (config-if) #ip add 172.168.102.1 255.255.255.0

R2 (config-if) #ip add 192.168.1.2 255.255.255.0

R2 (config-if) #ip add 192.168.2.1 255.255.255.0

R3 (config-if) #int 172.168.103.1 255.255.2550

R3 (config-if) #ip add 192.168.2.2 255.255.255.0

R3 (config-if) #ip add 192.168.3.1 255.255.255.0

On Router 3, we will create some loopback also

R3 (config-if) #int lo 1

R3 (config-if) #ip add 172.30.1.1 255.255.255.0

R3 (config-if) #ip add 172.30.2.1 255.255.255.0

R3 (config-if) #ip add 172.30.3.1 255.255.255.0

R3 (config-if) #ip add 172.30.4.1 255.255.255.0

R3 (config-if) #ip add 172.30.5.1 255.255.255.0

R4 (config-if) #ip add 172.168.104.1 255.255.255.0

R4 (config-if) #ip add 192.168.3.2 255.255.255.0

Trang 10

R4 (config-if) #ip add 192.168.4.1 255.255.255.0

Here we will Perform BGP Routing

R1 (config) #router bgp 100

R1 (config-router) #network 192.168.1.0

R1 (config-router) #network 172.168.101.0 mask 255.255.255.0

R1 (config-router) #neighbour 192.168.1.2 remote-as 100

BGP Creates Three Tables

Trang 11

R1#sh ip bgp neighbours

R1#sh ip bgp neighbours 192.68.1.2

R1#sh ip bgp neighbours 192.68.4.1

R1#sh ip bgp

R1#sh ip route bgp

Now if we talk about the network 172.30.0.0, we can see this network on R1 via 2.2 Because it advertises routes with default next hop which is 2.2 Now on edge router we run a command next hop self

Here edge router is R2 & R4

R2 (config-router) #neighbour 192.168.1.1 next-hop-self

R4 (config-router) #neighbour 192.168.4.2 next-hop-self

Here we tell router, provide your own IP add as a next hop

In BGP a router will share the information with the directly connected router only Router 1 would not share the information with the Router 3 For that here we will run route reflector client command

R2 #sh ip route bgp

We won’t see any routes of 104 networks

R4 #sh ip route bgp

We won’t see any routes of 102 networks

R1 (config-router) #neighbour 192.168.1.2 rout-reflector-client

R1 (config-router) #neighbour 192.168.4.1 rout-reflector-client

R4#sh ip route bgp

Now we can see here 102 routes

R2#sh ip route bgp

Trang 12

We can see 104 routes here

R1#sh ip bgp

First rich metric is weight

For indirectly connected 0

Next local preference 100 Always prefer higher

Next AS path 200

Next self-originate

MED is by default 0

We have two ID 4.1 & 1.2 1.2 will prefer (lower will prefer)

R1#sh ip route bgp

Here we can see 172.30.0.0 route learn via 192.168.1.2

R1#traceroute 172.30.1.1 via 1.2

R1#sh ip protocols

By default it selects only one path

If we want to implement load balance then we need to change max path value

R1 (Config) #router bgp 100

R1 (config-router) #maximum-paths?

1 to 16

Định dạng
Số trang	12
Dung lượng	445,1 KB