BGP Routing Protocol Overview

BGP or Border Gateway Protocol is an external, dynamic routing protocol. It is most
often used between ISPs and between enterprises and their service providers. BGP
is literally the routing protocol of the Internet because it connects independent
networks together, enabling end-to-end transport. Scalability and stability are BGP’s
focus, not speed – as a result it behaves very differently than most other routing
protocols.
BGP is recommended whenever multi homing is a requirement (dual ISP connections
to different carriers), when route path manipulation is needed, and in transit
Autonomous Systems.
A Quick Overview of BGP
 Routers running BGP are called BGP speakers.
 BGP uses autonomous system numbers to keep track of different
administrative domains. 1-64511 are public, 64512-65535 are private.
 BGP is used to connect IGPs, interior gateway protocols like OSPF and
EIGRP. Routing between Autonomous Systems is referred to as interdomain
routing.
 The administrative distance for eBGP routes is 20, iBGP is 200.
 BGP neighbors are called “peers” and must be statically assigned.
 Peers receive incremental, triggered updates as well as keepalives using TCP
port 179.
 BGP is sometimes referred to as a “path-vector” protocol because its route to
a network uses AS numbers on the path to the destination.
 BGP uses it’s path-vector attributes to help in loop prevention. When an
update leaves an AS, the AS number is prepended to the update along with all
the other AS numbers that have spread the update.
 When a BGP router receives an update, it first scans through the list of AS
numbers. If it sees its own AS number, the update is discarded.
BGP Databases
Like most modern routing protocols, BGP has two separate databases – a neighbor
database and a BGP-specific database.
Neighbor Database
Lists all of the configured BGP neighbors
Router# show ip bgp summary
BGP Database
Lists all networks known by BGP along with their attributes.

Router# show ip bgp
BGP Message Types
There are four different BGP message types.
Open
After a BGP neighbor is configured, the router sends an open message to establish
peering with the neighbor.
Update
The type of message used to transfer routing information between peers.
Keepalive
BGP peers send keepalive messages every 60 seconds by default to maintain active
neighbor status.
Notification
If a problem occurs and a BGP peer connection must be dropped, a notification
message is sent and the session is closed.
Internal vs. External
iBGP, or internal BGP is a peering relationship between BGP routers within the same
autonomous system. eBGP, or external BGP describes a peering relationship between
BGP routers in different autonomous systems. It is an important distinction to make.
In the diagram below, R1 and R2 are eBGP peers. R2 and R3 and iBGP peers.
BGP Next-Hop Self
When you have BGP neighbors peering between autonomous systems like R1 and R2
above, BGP uses the the IP address of the router the update was received from as its

“next hop”. When a router receives an update from an eBGP neighbor, it must pass
the update to its iBGP neighbors with-out modifying the next hop attribute.
The next-hop IP address is the IP address of the edge router belonging to the
next-hop autonomous system.
For example, let’s say R1 sends an update to R2 from its 10.1.1.1 serial interface. R2
must use keep the next-hop IP set as 10.1.1.1 when it passes the update along to R3,
its iBGP peer. The problem is that R2 does not know about 10.1.1.1 and so it cannot
use it as its next hop address.
The neighbor [IP address] next-hop-self command solves the problem by advertising
itself as the next-hop address. In this example, it would be applied to R2 so any
updates passed along to R3 would use an R2 address as the next-hop.
R2(config)# router bgp 65300
R2(config-router)# neighbor 10.2.2.2 next-hop-self
R2(config)# exit
BGPs Synchronization Rule
The BGP synchronization rule states that a BGP router cannot use or forward new
route updates it learns from iBGP peers unless it knows about the network from
another source, like an IGP or static route.
The idea is to prevent using or forwarding on information that is unreliable and
cannot be verified. Remember, BGP prefers reliability and stability over using the
newest, fastest route.
This means that iBGP peers will not update each other unless an IGP is running under
the hood. To remove the limitation, use the no synchronization command under BGP
configuration mode. Recent versions of IOS have it disabled by default, but it is
important topic to understand.
Resetting BGP Sessions
Internet routers running BGP have enormous routing tables. When a filter is applied,
like a route map, changes to BGP attributes occur. Those changes could affect many
of the routes already in the routing table from BGP. Because BGP’s network list is
usually very long, applying a route map or prefix list after BGP has converged can be
disastrous. The router would have to check the filter against every possible route
and attribute combination.
To make matters worse, if it were to apply the filters and pull routes back from
neighbors, those changes could then cause another reconvergence – and on and

on. In an effort to avoid that scenario (BGP loves stability), BGP will only apply
attribute and network changes to routes AFTER the filter has been applied. All
existing routes stay unchanged.
If the network administrator decides that the filter needs to be applied to all routes,
then the BGP instance must be reset – forcing the entire BGP table to pass through
the filter. There are three ways to do this:
 Hard reset
 Soft reset
 Route refresh
The hard and soft reset options aren’t discussed here because they are not directly
relevant to the exam. You should know though, that both options are extremely
memory-taxing on the router as all the routes must be recomputed. Route refresh
was developed to solve the high memory problems, while still forcing a reset.
The following command performs the BGP route refresh:
Router# clear ip bgp [ * | neighbor-address]
BGP Configuration
Enabling BGP
Like other routing protocols, BGP must be enabled with the router command. Make
sure to include the AS number.
R1(config)# router bgp autonomous-system-number
BGP Peering
Each neighbor must be statically assigned using the neighbor command. If the AS
number matches the local router’s, it is an iBGP connection. If the AS number is
different, it is an eBGP connection.
R1(config-router)# neighbor ip-address remote-as autonomous-system-number
If a router has a long list of directly connected neighbors, the BGP configuration can
start to get long and difficult to follow – especially as neighbor policies are
applied. Peer groups solve that.
Peer groups are groups of peer neighbors that share a common update
policy. Updating an entire group of neighbor statements can then be done with one
command. Much easier for large BGP networks. Think of a peer group as a logical
grouping of routers that are grouped under a single name to make changes faster
and configurations shorter. Like OUs in Active Directory.
Peer groups not only reduce the number of lines of configuration, but they reduce

the ease the overhead of the router. A BGP update process normally runs for each
neighbor. If a peer group is configured, a single update process runs for all routers in
the group. Notice that this means that all of the router inside a peer group must be
either all iBGP or eBGP neighbors.
Basic neighbor configuration example:
R1(config-router)# neighbor 10.1.1.1 remote-as 65300
Peer group configuration example:
R1(config-router)# neighbor MINE peer-group
R1(config-router)# neighbor MINE remote-as 65300
R1(config-router)# neighbor 10.1.1.1 peer-group MINE
BGP Source Address
R1 in the diagram below has two different options when it comes to peering to
R2. It can peer to the physical interface IP address, 10.1.1.2 or it can peer to R2′s
loopback interface, 192.168.2.2.
If a peer relationship is made using the physical interface as the source address,

problems can occur if the interface goes down. In this scenario, even if R2′s 10.1.1.2
interface drops, it still has connectivity to R2′s networks via R3 and R2′s other
physical interface. Even though an IGP would still show R2′s network as accessible,
the BGP peer relationship would drop because R1 cannot reach its peering address
with R2.
Most implementations recommend using a loopback address as the BGP source
address for this reason. Remember that the loopback address must be added to the
IGP running for this to work.
This way, if R2′s 10.1.1.2 interface fails, R2 will still be reachable.
The update-source command accomplishes this.
Here’s an example:
R1(config-router)# neighbor 192.168.2.2 update-source loopback0
R2(config-router)# neighbor 192.168.1.1 update-source loopback0
Defining Networks
Network statements in BGP are used differently than in other routing protocols like
EIGRP or OSPF. EIGRP and OSPF use the network statements to define which
interfaces you want to participate in the routing protocol process.
BGP uses network statements to define which networks the local router should
advertise. Each network doesn’t have to be originating from the local router, but the
network must exist in the routing table. The optional mask keyword is often
recommended as BGP supports subnetting and supernetting.
Example:
R1(config-router)# network 10.1.1.0 255.255.255.0
R1(config-router)# network 10.1.2.0 255.255.255.0
Understand that by default a BGP router will not advertise a network learned from
one iBGP peer to another. This is why iBGP is not a good replacement for an IGP like
EIGRP and OSPF.
BGP Path Selection

Unlike most other routing protocols, BGP is not concerned with using the fastest path
to a given destination. Instead, BGP assigns a long list of attributes to each
path. Each of these attributes can be administratively tuned for extremely granular
control of route selections.
BGP also does not load balance across links by default. To select the best route, BGP
uses the criteria in the following order:
1. Highest weight
2. Highest local preference
3. Choose routes originated locally
4. Path with the shortest AS path
5. Lowest origin code ( i < e < ? )
6. Lowest MED
7. eBGP route over iBGP route
8. Route with nearest IGP neighbor (lowest IGP metric)
9. Oldest route
10. Neighbor with the lowest router ID
11. Neighbor with the lowest IP address
Controlling Path Selection
The most common method of controlling the attributes listed above is to use route
maps. This allows specific attributes to be changed on specific routes. Before we get
into route maps, let’s first discuss the three prominent attributes: weight, local
preference, and MED.
Weight
On Cisco routers, weight is the most influential BGP attribute. The weight attribute
is proprietary to Cisco and is normally used to select an exit interface when multiple
paths lead to the same destination. Weight is local and is not sent to other
routers. It can be a value between 0-65,535. 0 is the default. In the example below,
if you want R2 to prefer to use R1 when sending traffic to 192.168.20.0 then the
weight attribute could raised on R2 for R1.

R2(config-router)# neighbor 10.1.1.1 weight 100
Local Preference
Local preference is not proprietary to Cisco and can be used in a similar fashion to
weight. It can be set for the entire router or for a specific prefix. Local preferences
can range from 0-4,294,967,295, with 100 being the default value. Unlike weight,
local preference is propagated to iBGP neighbors.
Using the diagram above, if an administrator wanted R2 to use R1 when sending
traffic to 192.168.20.0, the configuration would look something like this:
R1(config-router)# bgp default local-preference 500
After the local preference is raised on R1, it will be shared with R2 and R2 will begin
using it as its best path to the distant network (assuming the weight is the same of
course). If you want to set the local preference on specif prefixes, route maps are
usually the best option. Below is an example of the local preference being set using
a route map:
R7(config-router)# neighbor 10.10.10.1 route-map lp_example in
R2(config-router)# exit
R7(config)# access-list 7 permit 10.30.30.0 0.0.0.255
R7(config)# route-map lp_example permit 10
R7(config-rmap)# match ip address 7
R7(config-rmap)# set local-preference 300

R7(config-rmap)# exit
R7(config)# route-map lp_example permit 20
R7(config-rmap)# set local-preference 100
MED
The MED attribute, or multi-exit discriminator is used to influence which path
external neighbors use to enter an AS. MED is also much farther down on the
attribute list, so attributes like weight, local preference, AS path length, and origin
are used first. The default MED value is 0 and a lower value is preferred. A common
scenario for MED is when a company has two connections to the same ISP for
internet.
Weight or local preference could be used to send outgoing traffic on the higher
bandwidth link, but local preference is not shared with routers outside an AS. MED
could be set on one router so ISP routers prefer that path in.
To set the MED on all routes:
R1(config-router)# default-metric value
Here’s an example using a route map to influence incoming paths to
10.30.30.0/24 using MED:
R7(config-router)# neighbor 10.10.10.1 route-map med_example out
R2(config-router)# exit
R7(config)# access-list 7 permit 10.30.30.0 0.0.0.255
R7(config)# route-map med_example permit 10
R7(config-rmap)# match ip address 7
R7(config-rmap)# set metric 50
R7(config-rmap)# exit
R7(config)# route-map med_example permit 20
R7(config-rmap)# set metric 150
Verification
It’s important that you understand and are able to interpret to results of the show ip
bgp command output. It displays the contents of the local BGP topology database-
including the attributes assigned to each network. It is perhaps the most important
BGP verification and troubleshooting tool!
Because BGP uses many attributes and sources routes in a number of ways, the
output of the show ip bgpcommand can be a bit overwhelming if you don’t know
what you are looking for.

R1# show ip bgp
BGP table version is 21, local router ID is 10.0.22.24
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.1.0.0 0.0.0.0 0 32768 ?
* 10.2.0.0 10.0.22.25 10 0 25 ?
*> 0.0.0.0 0 32768 ?
* 10.0.0.0 10.0.22.25 10 0 25 ?
*> 0.0.0.0 0 32768 ?
*> 192.168.0.0/16 10.0.22.25 10 0 25 ?
Attributes
Here’s a breakdown of some important fields you should consider remembering:
* - An asterisk in the first column means that the route has a valid next hop.
s (suppressed) – BGP is not advertising the network, usually because it is part of a
summarized route.
> - Indicates the best route for a particular destination. These will end up in the
routing table.
i (internal) - If the third column has an i in it, it means the network was learned from
an iBGP neighbor. If it is blank, it means the network was learned from an external
source.
0.0.0.0 - The fifth column shows the next hop address for each route. A 0.0.0.0
indicates the local router originated the route (examples include a network
command entered locally or a network an IGP redistributed into BGP on the router)
Metric (MED value) – The column titled Metric represents the configured MED
values. Recall that 0 is the default and if another value exists, lower is preferred.
i/?- The last column displays information on how BGP originally learned the
route. In the example above is used for each route meaning they were all
redistributed routes into BGP from an IGP. The other option is a question mark,
which indicates that network commands were used to configure the route.
More Related Topics:
Routing Information Protocol & RIP Configuration
How to Configure IGRP (Interior Gateway Routing Protocol)?
CCNP SWITCH 642-813 Guide: Configuring IP SLA
How to Configure Cisco IP SLA?
Basic Information of Configuring HSRP on a Cisco Router

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