Border Gateway Protocol eBGP, iBGP, & Configuration
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 multihoming is a requirement (dual ISP
connections to different carriers), when route path manipulation is
needed, and in transit Autonomous Systems.
A Quick Overview
- 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 it 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
Lists all of the configured BGP neighbors
Router# show ip bgp summary
BGP Database
Lists all networks known by BGP along with their attributes.
Lists all networks known by BGP along with their attributes.
Router# show ip bgpBGP 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.
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.
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.
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.
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 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 R3
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-numberBGP 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
BGP 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.
BGP 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 bgp 65300
R1(config-router)# neighbor 10.1.1.1 remote-as 65300
R1(config-router)# neighbor 10.1.2.1 remote-as 65300
R1(config-router)# neighbor 10.1.3.1 remote-as 65300
Peer group configuration example:
R1(config)# router bgp 65300
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
R1(config-router)# neighbor 10.1.2.1 peer-group MINE
R1(config-router)# neighbor 10.1.3.1 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 bgp 65400
R1(config-router)# neighbor 192.168.2.2 remote-as 65400
R1(config-router)# neighbor 192.168.2.2 update-source loopback0
R2(config)# router bgp 65400
R2(config-router)# neighbor 192.168.1.1 remote-as 65400
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 bgp 65300
R1(config-router)# neighbor 10.1.1.1 remote-as 65300
R1(config-router)# network 10.1.1.0 255.255.255.0
R1(config-router)# neighbor 10.1.2.1 remote-as 65300
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:
- Highest weight
- Highest local preference
- Choose routes originated locally
- Path with the shortest AS path
- Lowest origin code ( i < e < ? )
- Lowest MED
- eBGP route over iBGP route
- Route with nearest IGP neighbor (lowest IGP metric)
- Oldest route
- Neighbor with the lowest router ID
- 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 bgp 65100
R2(config-router)# neighbor 10.1.1.1 remote-as 65100
R2(config-router)# neighbor 10.2.2.1 remote-as 65100
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 65100
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 specific 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 bgp 200
R7(config-router)# neighbor 10.10.10.1 remote-as 100
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 bgp 200
R7(config-router)# neighbor 10.10.10.1 remote-as 200
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
BGP 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 bgp command 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.
Sumber : http://www.ccnpguide.com/ccnp-route-642-902-bgp/
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