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.
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| Basic Topology of BGP |
Basics of BGP
- BGP is the path-vector protocol that provides routing information for autonomous systems on the Internet via its AS-Path attribute.
- 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.
- 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.
- Medium-sized businesses usually get into BGP for the purpose of true multi-homing for their entire network.
- 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
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| Types of BGP |
There are different terms
used when describing BGP. These including:
- Internal BGP (iBGP) operates inside an autonomous System (AS).
- 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.
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:
- It is an advanced distance-vector protocol.
- BGP sends full routing updates at the start of the session, trigger updates are sent afterward.
- BGP maintains connection by sending periodic keepalives.
- It creates and maintains connections between peers, using TCP port 179.
- BGP sends a triggered update when a keepalive, an update, or a notification is not received
- It has its own routing table, although it is capable of both sharing and inquiring of the interior IP routing table.
- 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.
- It selects the route based on the AS Path.
- It selects that route, which provides a network with least AS.
- Max AS length 65535
- 1 to 64511 Public
- 64512 to 65535 Private
- BGP doesn’t discover neighbour automatically. In BGP we have to define neighbours. They are called BGP peers.
- BGP supports only one type of authentication that is MD-5
How to advertise Routes in BGP?
Using two ways
- Using Network Command
- Redistribution
BGP Messages
- Open- BGP sends Open message using TCP Port no 179.
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)
- Keep alive- BGP sends periodic keep alive. Default time 60 sec.
- Update- When two routers become BGP neighbour they send update message.
Contents of update
- Routes
- Route Attributes- These are those criteria’s which are used to select the best path.
- Notification- When a neighbour is reset then it sends notification message. It contains the cause of resetting.
BGP Tables
- Neighbour
- BGP Table
- Routing Table
BGP States
- Idle- Searching for Neighbour.
- Connect- TCP three way handshake done.
- Open sent- Open message has been sent.
- Open confirm- Open message has been received.
- Established- Connection Done.
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| BGP States |
BGP Terminology
- Next Hop Self- When a BGP edge router learns the external routes then it advertise those 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.)
- Route Reflector Client- Normally an IBGP neighbour never exchanges the routes of one 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.
- EBGP-Multihop- When a BGP router wants to establish EBGP neighbourship then it set TTL value 1 in open message. If your neighbour is not directly connected, then you have to change the TTL Value using EBGP Multihop command.
- Max-Path- By default BGP selects one best path using route attributes. If you want to implement load balancing then you have to change max path value.
- BGP- By default, the redistribution of iBGP into IGP isn’t allowed on Cisco IOS.
- Update Source- When we want to establish neighbourship via loopback then we have to use update source command.
- Aggregation- In BGP Summarization is called Aggregation.
BGP Attributes/Rich Metric/Path Attributes
- Weight- Cisco Proprietary Attributes.
Directly Connected – 32768
Indirectly Connected – 0
Preferred – Directly Connected.
- Local Preference
Default 100 < 4.2 billion
Preferred – Higher
- Self-Originate- a router gives preference to self-originate routes.
- AS-Path- Lowest Path will be Preferred.
- Origin- ebgp >ibgp >Redistributed
- MED- Multi Exit Discriminator
By default – 0
Lowest will be preferred.
- eBGP > iBGP- eBGP will be preferred.
- IGP Cost- Select Lowest Cost.
- Ebgp Peer- Old neighbour will be preferred.
- Router ID- Lower Router ID will be preferred.
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:
- If the path specifies a next hop that is inaccessible, drop the update.
- Prefer the path with the largest weight.
- If the weights are the same, prefer the path with the largest local preference.
- If the local preferences are the same, prefer the path that was originated by BGP running on this router.
- If no route was originated, prefer the route that has the shortest AS_path.
- 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).
- If the origin codes are the same, prefer the path with the lowest MED attribute.
- If the paths have the same MED, prefer the external path over the internal path.
- If the paths are still the same, prefer the path through the closest IGP neighbour.
- Prefer the path with the lowest IP address, as specified by the BGP router ID.
Message Header Format
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| Message Header Format |
- Marker- Included for compatibility, must be set to all ones.
- Length- Total length of the message in octets, including the header.
- Type- Type of BGP message. The following values are defined:
Open (1)
Update (2)
Notification (3)
KeepAlive (4)
Route-Refresh (5)
BGP Example
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| BGP Example Topology |
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) #no
shut
R1 (config-if) #int
s0/1
R1 (config-if) #ip
add 192.168.4.2 255.255.255.0
R1 (config-if) #no
shut
R1 (config-if) #do sh ip int br
R2 (config) #int
fa0/0
R2 (config-if) #ip
add 172.168.102.1 255.255.255.0
R2 (config-if) #no
shut
R2 (config-if) #int
s0/0
R2 (config-if) #ip
add 192.168.1.2 255.255.255.0
R2 (config-if) #no
shut
R2 (config-if) #int
s0/1
R2 (config-if) #ip
add 192.168.2.1 255.255.255.0
R2 (config-if) #no
shut
R2 (config-if) #do sh ip int br
R3 (config) #int
fa0/0
R3 (config-if) #int
172.168.103.1 255.255.2550
R3 (config-if) #no
shut
R3 (config-if) #int
s0/0
R3 (config-if) #ip
add 192.168.2.2 255.255.255.0
R3 (config-if) #no
shut
R3 (config-if) #int
s0/1
R3 (config-if) #ip
add 192.168.3.1 255.255.255.0
R3 (config-if) #no shut
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) #int
lo 2
R3 (config-if) #ip
add 172.30.2.1 255.255.255.0
R3 (config-if) #int
lo 3
R3 (config-if) #ip
add 172.30.3.1 255.255.255.0
R3 (config-if) #int
lo 4
R3 (config-if) #ip
add 172.30.4.1 255.255.255.0
R3 (config-if) #int
lo 5
R3 (config-if) #ip
add 172.30.5.1 255.255.255.0
R3 (config-if) #do sh ip int br
R4 (config) #int
fa0/0
R4 (config-if) #ip
add 172.168.104.1 255.255.255.0
R4 (config-if) #no
shut
R4 (config-if) #int
s0/0
R4 (config-if) #ip
add 192.168.3.2 255.255.255.0
R4 (config-if) #no
shut
R4 (config-if) #int
s0/1
R4 (config-if) #ip
add 192.168.4.1 255.255.255.0
R4 (config-if) #no
shut
R4 (config-if) #do sh ip int br
Here we will Perform BGP Routing
R1 (config) #router
bgp 100
R1 (config-router)
#network 192.168.1.0
R1 (config-router)
#network 192.168.4.0
R1 (config-router)
#network 172.168.101.0 mask 255.255.255.0
R1 (config-router)
#neighbour 192.168.1.2 remote-as 100
R1 (config-router) #neighbour
192.168.4.1 remote-as 100
R2 (config) #router
bgp 100
R2 (config-router)
#network 192.168.1.0
R2 (config-router)
#network 192.68.2.0
R2 (config-router)
#network 172.168.102.0 mask 255.255.255.0
R2 (config-router)
#neighbour 192.168.1.1 remote-as 100
R2 (config-router) #neighbour
192.168.2.2 remote-as 200
R3 (config) #router
bgp 200
R3 (config-router)
#network 192.168.2.0
R3 (config-router)
#network 192.68.3.0
R3 (config-router)
#network 172.168.103.0 mask 255.255.255.0
R3 (config-router)
#network 172.30.1.0 mask 255.255.255.0
R3 (config-router)
#network 172.30.2.0 mask 255.255.255.0
R3 (config-router)
#network 172.30.3.0 mask 255.255.255.0
R3 (config-router)
#network 172.30.4.0 mask 255.255.255.0
R3 (config-router)
#network 172.30.5.0 mask 255.255.255.0
R3 (config-router)
#neighbour 192.168.2.1 remote-as 100
R3 (config-router) #neighbour
192.168.3.2 remote-as 100
R4 (config) #router
bgp 100
R4 (config-router)
#network 192.168.3.0
R4 (config-router)
#network 192.68.4.0
R4 (config-router)
#network 172.168.104.0 mask 255.255.255.0
R4 (config-router)
#neighbour 192.168.3.1 remote-as 200
R4 (config-router)
#neighbour 192.168.4.2 remote-as 100
BGP Creates Three Tables
- Neighbour Table
R1#sh ip bgp neighbours
R1#sh ip bgp neighbours 192.68.1.2
R1#sh ip bgp
neighbours 192.68.4.1
- BGP Table
R1#sh ip bgp
- Routing Table
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
bgp 100
R2 (config-router) #neighbour
192.168.1.1 next-hop-self
R4 (config) #router
bgp 100
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
bgp 100
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
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
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