Border Gateway Protocol (BGP)
BGP is a protocol which performs routing information
exchange among routers to determine the optimal paths for the traffic flow. A
BGP router forms a neighbor relationship by connecting to its neighbors and
exchanging the routes, once the connection is established.
For those interested, some ISP’s allow the ability to telnet into their edge BGP routers to view the BGP routing tables (Check out ‘route-server.ip.att.net’).
BGP's purpose is not only to
exchange its information, but also to exchange network reach-ability and
availability information for the Autonomous Systems (AS) paths with other BGP
systems on the network. This process allows all systems to construct topology
graphs of the entire network infrastructure on both sides of the BGP link.
This process also allows these
systems to identify loops and other issues that may affect network performance
and availability.
BGP Overview
· BGP uses the concept of autonomous systems (AS). An autonomous system is a group of networks under a common administration. The Internet Assigned Numbers Authority (IANA) assigns AS numbers: 1 to 64511 are public AS numbers and 64512 to 65535 are private AS numbers.
·
Autonomous systems run Interior Gateway Protocols (IGP) within the
system. They run an Exterior Gateway Protocol (EGP) between them. BGP version 4
is the only EGP currently in use.
·
Routing between autonomous systems is called interdomain routing.
·
The administrative distance for EBGP routes is 20. The administrative
distance for IBGP routes is 200.
·
BGP neighbors are called peers and must be statically configured.
·
BGP uses TCP port 179. BGP peers exchange incremental, triggered route
updates and periodic keepalives.
·
Routers can run only one instance of BGP at a time.
·
BGP is a path-vector protocol. Its route to a network consists of a list
of autonomous systems on the path to that network.
· BGP's loop prevention mechanism is an autonomous system number. When an update about a network leaves an autonomous system, that autonomous system's number is prepended to the list of autonomous systems that have handled that update. When an autonomous system receives an update, it examines the autonomous system list. If it finds its own autonomous system number in that list, the update is discarded.
BGP Databases
BGP uses three databases. The first two listed are BGP-specific; the third is shared by all routing processes on the router:- Neighbor database: A list of all configured BGP neighbors. To view it, use the show ip bgp summary command.
- BGP database, or RIB (Routing Information Base): A list of networks known by BGP, along with their paths and attributes. To view it, use the show ip bgp command.
- Routing table: A list of the paths to each network used by the router, and the next hop for each network. To view it, use the show ip route command.
BGP Message Types
BGP has four types of messages:- Open: After a neighbor is configured, BGP sends an open message to try to establish peering with that neighbor. Includes information such as autonomous system number, router ID, and hold time.
- Update: Message used to transfer routing information between peers. Includes new routes, withdrawn routes, and path attributes.
- Keepalive: BGP peers exchange keepalive messages every 60 seconds by default. These keep the peering session active.
- Notification: When a problem occurs that causes a router to end the BGP peering session, a notification message is sent to the BGP neighbor and the connection is closed.
BGP Routing
As with any routing protocol, BGP maintains routing tables, transmits routing updates, and bases routing decisions on routing metrics. The primary function of a BGP system is to exchange network-reach ability information, including information about the list of autonomous system paths, with other BGP systems. This information can be used to construct a graph of autonomous system connectivity from which routing loops can be pruned and with which autonomous system-level policy decisions can be enforced.Each BGP router maintains a routing table that lists all feasible paths to a particular network. The router does not refresh the routing table, however. Instead, routing information received from peer routers is retained until an incremental update is received.
BGP devices exchange routing information upon initial data exchange and after incremental updates. When a router first connects to the network, BGP routers exchange their entire BGP routing tables. Similarly, when the routing table changes, routers send the portion of their routing table that has changed. BGP routers do not send regularly scheduled routing updates, and BGP routing updates advertise only the optimal path to a network.
BGP Operation
BGP performs three types of routing: interautonomous system routing, intra-autonomous system routing, and pass-through autonomous system routing.Interautonomous system routing occurs between two or more BGP routers in different autonomous systems. Peer routers in these systems use BGP to maintain a consistent view of the internetwork topology. BGP neighbors communicating between autonomous systems must reside on the same physical network. The Internet serves as an example of an entity that uses this type of routing because it is comprised of autonomous systems or administrative domains. Many of these domains represent the various institutions, corporations, and entities that make up the Internet. BGP is frequently used to provide path determination to provide optimal routing within the Internet.
Intra-autonomous system routing occurs between two or more BGP routers located within the same autonomous system. Peer routers within the same autonomous system use BGP to maintain a consistent view of the system topology. BGP also is used to determine which router will serve as the connection point for specific external autonomous systems. Once again, the Internet provides an example of interautonomous system routing. An organization, such as a university, could make use of BGP to provide optimal routing within its own administrative domain or autonomous system. The BGP protocol can provide both inter- and intra-autonomous system routing services.
Pass-through autonomous system routing occurs between two or more BGP peer routers that exchange traffic across an autonomous system that does not run BGP. In a pass-through autonomous system environment, the BGP traffic did not originate within the autonomous system in question and is not destined for a node in the autonomous system. BGP must interact with whatever intra-autonomous system routing protocol is being used to successfully transport BGP traffic through that autonomous system.
Figure: In pass-through autonomous system routing, BGP pairs with another intra-autonomous system-routing protocol.
What is the difference between eBGP and iBGP?
1. EBGP is peering between two different AS, whereas IBGP is between same AS (Autonomous System).
2. Routes learned from eBGP peer will be
advertised to other peers (BGP or IBGP); however, routes learned from
IBGP peer will not be advertised to other IBGP peers.
3. By default, EBGP peers are set with
TTL = 1, which means neighbors are assumed to be directly connected,
which is not in the case of IBGP. We can change this behavior for EBGP
by using command “neighbor x.x.x.x ebgp-multihop <TTL>”. Multihop
is the term used in EBGP only.
4. EBGP routes have administrative distance of 20, whereas IBGP has 200.
5. Next hop remains unchanged when route
is advertised to IBGP peer; however, it is changed when it is advertised
to EBGP peer by default.
This default behavior of IBGP can be
changed by the command “neighbor x.x.x.x next-hop-self”; this changes
the next hop, while advertising, as a local route.
Configuring BGP
In its most basic configuration BGP acts very similarly to a distance vector routing protocol. Each network which is advertised is selected by choosing the shortest path. BGP just uses a path (Autonomous Systems – AS) hop count instead of a device hop count.For example, BGP works by routing traffic between AS’s, so if Verizon had AS 12345 and AT&T had AS 54321 then traffic destined for the AT&T network would be routed from Verizon to AT&T with a path of (54321). What this means is in order to reach a specific network on the AT&T network, traffic on the Verizon network would have to be routed to AS 54321.
In the following figure I show three AS’s and their corresponding path metrics:
BGP also has loop prevention built
in (although this is an open debate); this is implemented with a simple
mechanism which disallows routes to be advertised into an AS if the local AS is
part of the path metric.
With the example above, only the
most basic AS_Path metric is shown, however there are many different path
attributes which can be used with BGP to affect path selection along with
AS_Path.
Basic BGP Configuration
The first thing that must be understood is that each BGP device can have both internal and external BGP connections to other devices. Internal BGP connections are within the same AS while external BGP connections are between different AS’s. This is important because the configuration and behavior is slightly different between the two.eBGP Configuration
At its most basic the configuration of eBGP requires only two commands, these include:
- router bgp as-number
- neighbor ip-address remote-as remote-as-number
It must also be known that with eBGP by default there is a direct connection requirement which is enforced by an advertised TTL of 1. Now when configuring BGP using loopback interfaces this can become an issue as the packet actually takes two hops from the remote device to the physical interface and from the physical interface to the loopback interface.
This issue can be resolved by using the neighbor ebgp-multihop command on Cisco equipment.
iBGP Configuration
iBGP configuration is very similar to eBGP configuration but requires a little understanding of iBGP requirements. By default, iBGP requires that all iBGP devices being used are fully meshed (although there are ways of getting around this). This does not however mean that a direct connection is required but that each iBGP peer must neighbor with each other iBGP router.
The following configuration shows that configuration of an iBGP neighbor is the same as with eBGP:
- router bgp as-number
- neighbor ip-address remote-as remote-as-number
In this example, when Level3-2
advertises the eBGP route for the 192.168.128.0/17 network to Level3-1 it will
do this with a next-hop of 10.10.10.1 by default. Now if Level3-1 does not have
a valid route to the 10.10.10.1 address then it will be unable to route traffic
destined for the 192.168.128.0/17 network.
The most common method of resolving
this issue is by using the neighbor neighbor-ip-address next-hop-self-command.
When using this command the local eBGP peer will advertise the next-hop with
its own IP address and not the address configured with the BGP neighbor command.
In this case, Level3-2 could be
configured with the neighbor 10.100.100.2 next-hop-self-command which
would advertise the 192.168.128.0/17 network with a next-hop of 10.100.100.1
instead of 10.10.10.1.
BGP Next-Hop Selection
The next hop for a route received from an EBGP neighbor is the IP address of the neighbor that sent the update.When a BGP router receives an update from an EBGP neighbor, it must pass that update to its IBGP neighbors without changing the next-hop attribute. The next-hop IP address is the IP address of an edge router belonging to the next-hop autonomous system. Therefore, IBGP routers must have a route to the network connecting their autonomous system to that edge router. For example, in Figure, RtrA sends an update to RtrB, listing a next hop of 10.2.2.1, its serial interface. When RtrB forwards that update to RtrC, the next-hop IP address will still be 10.2.2.1. RtrC needs to have a route to the 10.2.2.0 network to have a valid next hop.
To change this behavior, use the neighbor [ip address] next-hop-self-command in BGP
configuration mode. In Figure, this configuration goes on RtrB. After you give
this command, RtrB advertises its IP address to RtrC as the next hop for
networks from AS 65100, rather than the address of RtrA. Thus, RtrC does not
have to know about the external network between RtrA and RtrB (network 10.2.2.0).
How does BGP select the best routing path /
BGP Attributes
BGP
Best Path Selection Algorithm determines the best route by selecting the
shortest path to the destination. An Autonomous System is a single network or a
set of networks and routers, which are under the control of one administrative
entity. Nevertheless, network administrators frequently manipulate such options
as local preference, lowest multi-exit discriminator and weight.
The
list of the selection criteria is presented below in the same order in which
BGP uses them to select the optimal routes to be injected into the IP Routing
table:
1) Weight — weight is the first criterion used by the router and it
is set locally on the user’s router. The Weight is not passed to the following
router updates. In case there are multiple paths to a certain IP address, BGP
always selects the path with the highest weight. The weight parameter can be
set either through neighbor command, route maps or via the AS-path access list.
Figure: BGP Weight Attribute
Figure: BGP Weight Attribute
2) Local Preference — this criterion indicates which route has local preference
and BGP selects the one with the highest preference. Local Preference default
is 100.
Figure: BGP Local Preference Attribute
In Figure: BGP Local Preference Attribute, AS 100 is receiving two advertisements for network 172.16.1.0 from AS 200. When Router A receives the advertisement for network 172.16.1.0, the corresponding local preference is set to 50. When Router B receives the advertisement for network 172.16.1.0, the corresponding local preference is set to 100. These local preference values will be exchanged between routers A and B. Because Router B has a higher local preference than Router A, Router B will be used as the exit point from AS 100 to reach network 172.16.1.0 in AS 200.
Figure: BGP Local Preference Attribute
In Figure: BGP Local Preference Attribute, AS 100 is receiving two advertisements for network 172.16.1.0 from AS 200. When Router A receives the advertisement for network 172.16.1.0, the corresponding local preference is set to 50. When Router B receives the advertisement for network 172.16.1.0, the corresponding local preference is set to 100. These local preference values will be exchanged between routers A and B. Because Router B has a higher local preference than Router A, Router B will be used as the exit point from AS 100 to reach network 172.16.1.0 in AS 200.
3) Network or Aggregate — this criterion chooses the path that was originated
locally via an aggregate or a network, as the aggregation of certain routes in
one is quite effective and helps to save a lot of space on the network.
4) Shortest AS_PATH — this criterion is used by BGP only in case it detects two
similar paths with nearly the same local preference, weight and locally
originated or aggregate addresses.
Figure: BGP AS_PATH Attribute
AS1 originates the route to 172.16.1.0 and advertises this route to AS 2 and AS 3, with the AS_path attribute equal to {1}. AS 3 will advertise back to AS 1 with AS-path attribute {3,1}, and AS 2 will advertise back to AS 1 with AS-path attribute {2,1}. AS 1 will reject these routes when its own AS number is detected in the route advertisement. This is the mechanism that BGP uses to detect routing loops. AS 2 and AS 3 propagate the route to each other with their AS numbers added to the AS_path attribute. These routes will not be installed in the IP routing table because AS 2 and AS 3 are learning a route to 172.16.1.0 from AS 1 with a shorter AS_path list.
Figure: BGP AS_PATH Attribute
AS1 originates the route to 172.16.1.0 and advertises this route to AS 2 and AS 3, with the AS_path attribute equal to {1}. AS 3 will advertise back to AS 1 with AS-path attribute {3,1}, and AS 2 will advertise back to AS 1 with AS-path attribute {2,1}. AS 1 will reject these routes when its own AS number is detected in the route advertisement. This is the mechanism that BGP uses to detect routing loops. AS 2 and AS 3 propagate the route to each other with their AS numbers added to the AS_path attribute. These routes will not be installed in the IP routing table because AS 2 and AS 3 are learning a route to 172.16.1.0 from AS 1 with a shorter AS_path list.
5) Lowest origin type — this criterion assigns higher preference to Exterior
Gateway Protocol (EGP) and lower preference to Interior Gateway Protocol (IGP).
6) Lowest multi-exit discriminator
(MED) — this criterion, representing the
external metric of a route, gives preference to the lower MED value.
Figure: BGP Multi-Exit Discriminator Attribute
7) eBGP over iBGP — just like the “Lowest origin type” criterion, this
criterion prefers eBGP rather than iBGP.
8) Lowest IGP metric — this criterion selects the path with the lowest IGP
metric to the BGP next hop.
9) Multiple paths — this criterion serves as indication whether multiple
routes need to be installed in the routing table.
10) External paths — out of several external paths, this criterion selects the
first received path.
11) Lowest router ID — this criterion selects the path which connects to the BGP
router that has the lowest router ID.
12) Minimum cluster list — in case multiple paths have the same router ID or
originator, this criterion selects the path with the minimum length of the
cluster list.
13) Lowest neighbor address — this criterion selects the path, which originates from
the lowest neighbor address.
Example Configuration of BGP
To start BGP on a Cisco router, the following command is required:router bgp autonomous system number
To define networks to be advertised, apply the following command:network network-number mask network-mask
You must be aware that the network command is not used in
the same way you use it when you apply networks in OSPF or EIGRP. With BGP, you
use the network command to advertise networks that originate
from the router and need to be advertised through BGP.To identify peer routers, apply the following command:
neighbor ip-address | peer-group name remote-as autonomous system number
Next, we will see how to configure IBGP and EBGP among the three routers in below
Figure.
Example-1 displays the IBGP configuration on R1 to R2.
Example-1. IBGP on R1
R1(config)#router bgp ?
<1-65535> Autonomous system number
R1(config)#router bgp 1
R1(config-router)#neighbor 131.108.1.2 remote 1
Example-2 displays the IBGP configuration to R1 and EBGP configuration to
R3.Example-2. IBGP/EBGP on R2
R2(config)#router bgp 1
R2(config-router)#neighbor 131.108.1.1 remote-as 1
R2(config-router)#neighbor 131.108.255.2 remote-as 2
Finally, Example-3 displays the EBGP connection from R3 to R2.Example-3. EBGP on R3
R3(config)#router bgp ?
<1-65535> Autonomous system number
R3(config)#router bgp 2
R3(config-router)#neighbor 131.108.255.1 remote-as 1
At this stage, because no network statements have been
applied, no BGP entries are on any routers. Use some loopback interfaces on R1
and advertise them through BGP to R2 and R3.Example-4 displays the three new loopback addresses on R1, ranging from 131.108.2.0 to 131.108.4.0.
Example-4. Loopback Configuration on R1
R1(config)#interface loopback 0
R1(config-if)#ip address 131.108.2.1 255.255.255.0
R1(config-if)#interface loopback 1
R1(config-if)#ip address 131.108.3.1 255.255.255.0
R1(config-if)#interface loopback 2
R1(config-if)#ip address 131.108.4.1 255.255.255.0
We must next advertise these loopbacks with the network
command. Because these networks are local to R1 and present in R1's IP routing
table as connected routes, you can apply the network command
as displayed in Example-5.Example-5. network Command on R1
R1(config)#router bgp 1
R1(config-router)#network 131.108.2.0 mask 255.255.255.0
R1(config-router)#network 131.108.3.0 mask 255.255.255.0
R1(config-router)#network 131.108.4.0 mask 255.255.255.0
Example-6 displays the BGP table on R1, using the command show ip bgp.
Example-6. show ip bgp on R1
R1#show ip bgp
BGP table version is 4, local router ID is 131.108.1.1
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
*> 131.108.2.0/24 0.0.0.0 0 32768 i
*> 131.108.3.0/24 0.0.0.0 0 32768 i
*> 131.108.4.0/24 0.0.0.0 0 32768 i
The BGP table on R1 displays three local networks (next hop is 0.0.0.0 or local interfaces). Example-6 also displays the path as i, or advertised through BGP. The local router ID is 131.108.1.1.
Example-7 displays the BGP table on R2.
Example-7. show ip bgp on R2
R2#show ip bgp
BGP table version is 7, local router ID is 171.109.3.1
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
* i131.108.2.0/24 131.108.1.1 0 100 0 i
* i131.108.3.0/24 131.108.1.1 0 100 0 i
* i131.108.4.0/24 131.108.1.1 0 100 0 i
R2's local router is 131.108.1.2, and it learns the remote loopbacks on R1
through the next hop address 131.108.1.1, or R1's Ethernet interface. Notice
that R2 has set the local preference to 100 (default value); the origin
attribute is set to i or IGP.Because R1 and R2 are running only IBGP and no other interior gateway protocol, R2's IP routing table does not have the BGP entries inserted because of synchronization.
Example-8 confirms this with only the locally connected routes visible on R2.
Example-8. show ip route on R2
R2#show ip route
131.108.0.0/24 is subnetted, 2 subnets
C 131.108.255.0 is directly connected, Serial1/0
C 131.108.1.0 is directly connected, Ethernet0/0
To enable BGP to insert the routes, you must disable synchronization or
configure an IGP routing protocol. R2, in turn, does not propagate the
loopbacks to R3; therefore, R3 does not have any entries at all, either in the
BGP table or IP routing table.Disable synchronization on R1 and R2.
Example-9 displays the no synchronization command on R1 and R2.
Example-9. Disabling Synchronization on R1/R2
R1(config)#router bgp 1
R1(config-router)#no synchronization
R2(config)#router bgp 1
R2(config-router)#no synchronization
Example-10 displays R2's routing table.Example-10. R2's Routing Table
R2#sh ip route
131.108.0.0/24 is subnetted, 5 subnets
C 131.108.255.0 is directly connected, Serial1/0
B 131.108.4.0 [200/0] via 131.108.1.1, 00:00:43
B 131.108.3.0 [200/0] via 131.108.1.1, 00:00:43
B 131.108.2.0 [200/0] via 131.108.1.1, 00:00:43
C 131.108.1.0 is directly connected, Ethernet0/0
The three remote networks are inserted into the IP routing tables as
BGP-learned networks.Example-11 displays R3's BGP and IP routing table.
Example-11. R3's BGP and IP Tables
R3>show ip bgp
BGP table version is 10, local router ID is 131.108.255.2
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
*> 131.108.2.0/24 131.108.255.1 0 1 i
*> 131.108.3.0/24 131.108.255.1 0 1 i
*> 131.108.4.0/24 131.108.255.1 0 1 i
R3>show ip route
131.108.0.0/24 is subnetted, 5 subnets
C 131.108.255.0 is directly connected, Serial0
B 131.108.4.0 [20/0] via 131.108.255.1, 00:02:09
B 131.108.3.0 [20/0] via 131.108.255.1, 00:02:09
B 131.108.2.0 [20/0] via 131.108.255.1, 00:02:09
C 131.108.1.0 is directly connected, Ethernet0
Notice that the next hop address on R3 is R2. The AS path on R3 indicates
that the remote networks, 131.108.2.0 to 131.108.4.0/24, transverse autonomous
system number 1, as displayed in the BGP table in Example-11.Verifying BGP Operation
This final scenario looks at Cisco IOS mechanisms for monitoring and verifying BGP routing within a Cisco router network.Refer to Figure and the BGP topology to see how to use some common show commands to verify that BGP is operating correctly.
Show and debug commands can be valuable, not only in the real-life networks you come across, but also during your certification exams.
This scenario covers the following commands:
- show ip bgp summary— Displays BGP neighbors in summary mode
- show ip bgp— Displays the BGP topology table
- clear ip bgp *— Clears all BGP TCP sessions
- show tcp brief— Displays all TCP sessions (BGP uses TCP)
- debug ip bgp events— Displays any BGP events, such as neighbor state changes
Example 1. show ip bgp summary on R1
R1#show ip bgp summary
BGP router identifier 131.108.255.13, local AS number 1
BGP table version is 11, main routing table version 11
6 network entries and 10 paths using 854 bytes of memory
3 BGP path attribute entries using 280 bytes of memory
BGP activity 50/44 prefixes, 73/63 paths
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
131.108.1.2 4 1 194 195 11 0 0 00:03:22 2
131.108.255.6 4 1 84 83 11 0 0 00:03:23 3
131.108.255.14 4 1 152 152 11 0 0 00:03:23 3
141.199.1.1 4 1001 0 0 0 0 0 never Idle
Example 1 displays a lot of useful information, including the local router
identifier 131.108.255.13, the local AS of 1, and the BGP table version of 11.
(An increasing version number indicates a network change is occurring; if no
changes occur, this number remains the same.) It also shows six network paths
on R1, using 854 bytes of memory.Memory is important in BGP because in a large network, such as the Internet, memory can be a limiting factor. As more BGP entries populate the IP routing table, more memory is required.
Example 2 show ip bgp
R1>show ip bgp
BGP table version is 11, local router ID is 131.108.255.13
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
*>i131.108.255.0/30 131.108.1.2 0 100 1000 i
* i 131.108.255.14 0 100 1000 i
*> 131.108.255.4/30 0.0.0.0 0 32768 i
* i 131.108.255.6 0 100 1000 i
*>i131.108.255.8/30 131.108.1.2 0 100 1000 i
* i 131.108.255.6 0 100 1000 i
*> 131.108.255.12/30 0.0.0.0 0 32768 i
* i 131.108.255.14 0 100 1000 i
*>i141.108.1.0/24 131.108.255.6 0 100 1000 i
*>i151.108.1.0/24 131.108.255.14 0 100 1000 i
Again, the BGP table version is displayed as 11 and the local router ID is
131.108.255.13. The various networks are listed along with the next hop
address, metric (MED), local preference (Locpref), weight, and the path. On the
left side (part of the status codes) indicates an internal BGP route and the i
on the right side of Example 2 indicates the origin. (i is for IGP, part of the
origin codes.)If a BGP configuration change is completed on Cisco IOS routers, the BGP peer session must be cleared. The command to clear all sessions is clear ip bgp *. To clear a single peer router, use the clear ip bgp peer-ip-address command.
Example 3. clear ip bgp * and show ip bgp on R1
R1#clear ip bgp ?
* Clear all connections
<1-65535> AS number of the peers
A.B.C.D BGP neighbor address to clear
dampening Clear route flap dampening information
flap-statistics Clear route flap statistics
peer-group Clear BGP connections of peer-group
R1#clear ip bgp *
R1#show ip bgp
BGP table version is 11, local router ID is 131.108.255.13
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
*>i131.108.255.0/30 131.108.1.2 0 1000 1000 i
* i 131.108.255.14 0 1000 1000 i
*> 131.108.255.4/30 0.0.0.0 0 32768 i
* i 131.108.255.6 0 1000 1000 i
*>i131.108.255.8/30 131.108.1.2 0 1000 1000 i
* i 131.108.255.6 0 1000 1000 i
*> 131.108.255.12/30 0.0.0.0 0 32768 i
* i 131.108.255.14 0 1000 1000 i
*>i141.108.1.0/24 131.108.255.6 0 1000 1000 i
*>i151.108.1.0/24 131.108.255.14 0 100 1000 i
This tool displays a number of options, including clearing BGP sessions
based on AS numbers or remote peer address. On Cisco IOS routers, you must clear
the BGP sessions if you want a change to take place because BGP does not update
changes after a BGP session is established. You can, however, configure soft
configurations with the neighbor peer address soft-reconfiguration
inbound command, which enables you to make changes and not have to
clear the TCP peer, resulting in no downtime.Example 4 show tcp brief
R1#show tcp brief
TCB Local Address Foreign Address (state)
812CC228 131.108.255.5.11040 131.108.255.6.179 ESTAB
812CF508 131.108.1.1.11039 131.108.1.2.179 ESTAB
812D0054 131.108.255.13.11041 131.108.255.14.179 ESTAB
Router R1, as displayed in Example 4, has three TCP sessions in an
established state. The TCP port numbers are also listed. This command is useful
because you need to be certain that TCP is active at Layer 4 of the OSI model
when troubleshooting to discover why two BGP peers are not sending updates, for
example. The foreign addresses list the TCP port as 179, and the local address
is a number TCP generates. This tells you that R1 has three TCP sessions
active, and you can expect BGP to send updates and keepalives across each TCP
session.Example 5. show tcp brief on R1
R1#sh tcp brief
TCB Local Address Foreign Address (state)
812CF984 131.108.255.5.11042 131.108.255.6.179 ESTAB
812CCB20 131.108.1.1.11044 131.108.1.2.179 ESTAB
812CC6A4 131.108.255.13.11043 131.108.255.14.179 ESTAB
Example 6 debug ip bgp keepalives on R1
R1#debug ip bgp keepalives
BGP keepalives debugging is on
4d01h: BGP: 131.108.255.6 sending KEEPALIVE
4d01h: BGP: 131.108.255.6 KEEPALIVE rcvd
4d01h: BGP: 131.108.255.14 sending KEEPALIVE
4d01h: BGP: 131.108.255.14 KEEPALIVE rcvd
4d01h: BGP: 131.108.1.2 sending KEEPALIVE
4d01h: BGP: 131.108.1.2 KEEPALIVE rcvd
R1 is sending and receiving keepalives to the three remote peers to ensure
that the remote routers are still active. Assume that R1 is reloaded.BGP Attributes At a Glance
