SVC-EDINET
Learning Materials
BGP – Basic Routing and Route Optimization
Version
1.1.2
Abstract
Students will be able to configure basic BGP on Cisco routers, understand difference between
Internal and External BGP and be able to interpret routing information passed between BGP
routers.
Students will also be able to optimize routes using Local Preference, AS-path and weight
attributes and summarize prefixes propagated from some AS to the another AS.
SVC-EDINET Learning materials
This learning material is supported by VŠB - Technical University Ostrava
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1. BGP – Basic Routing and Route Optimization
Learning Objectives
Students will learn




How to configure External BGP with loopback interfaces
How to configure Internal BGP
AS-path, weight and local preference attribute usage
How to summarize IP prefixes propagated to neighbor AS
Prerequisites



good IP routing knowledge (including OSPF)
basic knowledge of BGP routing protocol and BGP attributes processing
Cisco IOS CLI knowledge
Required Time
8 hours
Aditional Learning Sources
Basic BGP overview:
http://en.wikipedia.org/wiki/Border_Gateway_Protocol
BGP attributes:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm
BGP configuration on Cisco routers:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm
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Scenario
You have to configure BGP routing protocol among three autonomous systems AS100, AS200 and
AS300. Routers RA and RG in AS100 use OSPF as internal routing protocol and also establish IBGP
session. Among all autonomous systems are EBGP relations established.
After the basic configuration (i.e. neighbor and network configuration) we will make some additional
steps :
Reconfigure AS100 as non-transit AS.
Change AS100 configuration that way traffic to AS200 and AS300 will flow by RG-RD link.
Try to accomplish the same goal (RG-RD link preference) by configuration change in AS200 instead in
AS100.
Propagate only summarized route 172.16.0.0 from AS100 to neighboring AS, no individual subnets.
Make second line between RG and RD operational. Use weight BGP attribute and set second line as
preferred. First line will function as a backup now.
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Topology Diagram
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Addressing scheme
Router name / Interface
Router RA / Loopback 1
Router RA / Loopback 2
Router RA / Loopback 0
Router RA / Ethernet 0
Router RA / Serial 0
Router RG / Loopback 1
Router RG / Loopback 2
Router RG / Loopback 0
Router RG / Ethernet 0
Router RG / Serial 0
Router RG / Serial 1
Router RC / Loopback 1
Router RC / Ethernet 0
Router RC / Serial 0
Router RD / Loopback 1
Router RD / Ethernet 0
Router RD / Serial 0
Router RD / Serial 1
Table 1.
IP address
172.16.10.1
172.16.11.1
172.16.100.1
172.16.1.1
200.1.1.1
172.16.20.1
172.16.21.1
172.16.100.2
172.16.1.2
200.1.2.1
200.1.2.5
192.168.10.1
200.1.3.1
200.1.1.2
192.168.20.1
200.1.3.2
200.1.2.2
200.1.2.6
Network Mask
255.255.255.0
255.255.255.0
255.255.255.255
255.255.255.0
255.255.255.252
255.255.255.0
255.255.255.0
255.255.255.255
255.255.255.0
255.255.255.252
255.255.255.252
255.255.255.0
255.255.255.252
255.255.255.252
255.255.255.0
255.255.255.252
255.255.255.252
255.255.255.252
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Configuration Guide
Step 1 (basic IP addressing, will be pre-configured in all routers before beginning)
We use loopback interfaces for local LAN simulation, because Cisco 2501 routers used in this lab has
insufficient number of interfaces. Second line between RG and RD will not be used initially.
Router RA
int loo0
ip address
int loo1
ip address
int loo2
ip address
int e0
ip address
int s0
ip address
clock rate
172.16.100.1 255.255.255.255
172.16.10.1 255.255.255.0
172.16.11.1 255.255.255.0
172.16.1.1 255.255.255.0
200.1.1.1 255.255.255.252
2000000
Router RG
int loo0
ip address
int loo1
ip address
int loo2
ip address
int e0
ip address
int s0
ip address
int s1
ip address
shutdown
172.16.100.2 255.255.255.255
172.16.20.1 255.255.255.0
172.16.21.1 255.255.255.0
172.16.1.2 255.255.255.0
200.1.2.1 255.255.255.252
200.1.2.5 255.255.255.252
Router RC
int loo1
ip address 192.168.10.1 255.255.255.0
int e0
ip address 200.1.3.1 255.255.255.252
int s0
ip address 200.1.1.2 255.255.255.252
Router RD
int loo1
ip address 192.168.20.1 255.255.255.0
int e0
ip address 200.1.3.2 255.255.255.252
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int s0
ip address
clock rate
int s1
ip address
clock rate
shutdown
200.1.2.2 255.255.255.252
125000
200.1.2.6 255.255.255.252
125000
Step 2 (OSPF, basic BGP neighbor and advertised network configuration)
OSPF routing protocol will be configured inside AS100.
IBGP session between RA and RG will be configured on loopback interface addresses, so you have to
use ...update-source command. Using loopback interfaces is useful if there is more redundant paths
between RA and RG.
Because we do not advertise lines between autonomous systems into OSPF, we have to use …nexthop-self command in BGP. Otherwise router inside AS100 will not be able to find path to next hop in
neighboring AS.
IGP synchronization and route summarization are probably turned off by default (it is IOS version
dependent). If not, turn it off manually.
Router RA
router ospf 1
network 172.16.0.0 0.0.255.255 area 0
router bgp 100
no synchronization
network 172.16.1.0 mask 255.255.255.0
network 172.16.10.0 mask 255.255.255.0
network 172.16.11.0 mask 255.255.255.0
network 172.16.20.0 mask 255.255.255.0
network 172.16.21.0 mask 255.255.255.0
neighbor 172.16.100.2 remote-as 100
neighbor 172.16.100.2 update-source loo0
neighbor 172.16.100.2 next-hop-self
neighbor 200.1.1.2 remote-as 200
no auto-summary
Router RG
router ospf 1
network 172.16.0.0 0.0.255.255 area 0
router bgp 100
no synchronization
network 172.16.1.0 mask 255.255.255.0
network 172.16.10.0 mask 255.255.255.0
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network 172.16.11.0 mask 255.255.255.0
network 172.16.20.0 mask 255.255.255.0
network 172.16.21.0 mask 255.255.255.0
neighbor 172.16.100.1 remote-as 100
neighbor 172.16.100.1 update-source loo0
neighbor 172.16.100.1 next-hop-self
neighbor 200.1.2.2 remote-as 300
no auto-summary
Router RC
router bgp 200
network 192.168.10.0
neighbor 200.1.1.1 remote-as 100
neighbor 200.1.3.2 remote-as 300
no auto-summary
Router RD
router bgp 300
network 192.168.20.0
neighbor 200.1.2.1 remote-as 100
neighbor 200.1.3.1 remote-as 200
no auto-summary
Step 3 (function verification)
Use these commands to verify proper routing protocol function. The same commands you will use in
all subsequent steps :
sh
sh
sh
sh
sh
ip
ip
ip
ip
ip
debug
debug
debug
debug
bgp summary
bgp neighbors
bgp
bgp 172.16.20.0
route
ip
ip
ip
ip
verify BGP neighbor relationship
verify entire BGP table
verify details for one BGP prefix
verify IP routing table
bgp
bgp <neighbor_ip>
bgp events
bgp updates
After each configuration modification it is necessary to reset BGP relationship between routers by
issuing command
clear ip bgp *
or
clear ip bgp <neighbor_ip>
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Step 4 (make AS100 non-transit)
Only routes with empty AS_path attribute will be advertised from AS100 to both neighboring
autonomous systems (it is filtered by regular expression ^$).
Router RG
router bgp 100
neighbor 200.1.2.2 filter-list 10 out
ip as-path access-list 10 permit ^$
Router RA will need similar configuration.
You can verify this by sh ip bgp 192.168.20.0 on RC.
Step 5 (change AS100 configuration so that traffic to both AS200 and AS300 will flow preferably by
RG-RD link)
We will make RG-RD link more attractive by setting local preference attribute for prefixes advertised
from RD to 200, which is better then default value 100.
Router RG
router bgp 100
neighbor 200.1.2.2 route-map setloc in
route-map setloc permit 10
set local-preference 200
Verify by sh ip bgp 192.168.10.0 or sh ip route 192.168.10.0 or trace
on RA.
192.168.10.1
Step 6 (try to accomplish the same goal by configuration change in AS200, not in AS100)
We will make AS-path thru RC artificially longer then path thru RD by prepending AS200 two times.
Result is that way through AS300 (RG-RD) has shorter AS-path now and will be preferred.
Don’t forget to undo changes made in step 5. Verification steps are the same as in Step 5.
Router RC
router bgp 200
neighbor 200.1.1.1 route-map appendas out
route-map appendas permit 10
set as-path prepend 200 200
If you want to prepend as-path only to 192.168.10.0 prefix, you can modify route-map this way :
access-list 1 permit 192.168.10.0 0.0.0.255
route-map appendas permit 10
match ip address 1
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set as-path prepend 200 200
route-map appendas permit 20
Note the permit 20 line. If it is missing, only 192.168.10.0 prefix will be propagated.
Step 7 (Propagate only summarized route 172.16.0.0 from AS100 to neighboring AS, no individual
subnets)
Undo changes made in step 6.
Both router RA and RG
router bgp 100
aggregate-adress 172.16.0.0 255.255.0.0 summary-only
Note the value of Aggregator and Atomic-aggregate attributes on RC and RD routers in sh ip bgp
172.16.0.0 listing.
You can accomplish the same result by modifying prefixes advertised to neighboring BGP routers.
Other network commands and also aggregate-address from router bgp 100 should by
deleted. Static route directed to null 0 is necessary, because BGP will not advertise prefixes, which it
does not have in local routing table.
Both router RA and RG
router bgp 100
network 172.16.0.0
ip route 172.16.0.0 255.255.0.0 null0
Step 8 (make second line between RG and RD operational; use weight BGP attribute and set second
line as preferred)
Issue no shutdown command on both sides of second RA-RG line. Establish second BGP session
between them and use weight BGP attribute to set it as a preferred line. Higher weight will be
preferred.
Router RG
router bgp 100
neighbor 200.1.2.6 remote-as 300
neighbor 200.1.2.6 weight 100
neighbor 200.1.2.2 weight 10
Router RD
router bgp 300
neighbor 200.1.2.5 remote-as 100
neighbor 200.1.2.5 weight 100
neighbor 200.1.2.1 weight 10
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Again, verify by sh ip bgp 172.16.0.0 and sh ip route 172.16.0.0.
Knowledge-assesment
1) Q: In which order will following BGP attributes be used in the decision process ? Weight,
MED, AS path, Local Preference
A: Weight, Local Preference, AS Path, MED
2) Q: What is the purpose of MED BGP attribute ? Where is it propagated to?
A: MED is a hint for a neighbor AS, which path I prefer for traffic routed
into my AS.
It is propagated only into neighbor AS,not further.
3) Q: What is the type the following attributes:(well-known mandatory, well-known
discretionary, optionall transitive, optional nontransitive) ?
a)
b)
c)
d)
e)
Next Hop
Local Preference
AS Path
MED
Origin
A:a)
b)
c)
d)
e)
well-known mandatory
well-known discretionary
well-known mandatory
optional nontransitive
well-known mandatory
4) Q: What does it mean BGP synchronization and why is it used ?
A: Synchronization means, that BGP will not use nor propagate network
prefixes, if they are not known in IGP.
Purpose is to avoid situation, when IGP-only routers will receive
packets with destination address not known in IGP.
5) Q: Why is it necessary to make a full-mesh of IBGP peers ?
A: It is necessary because prefixes learnt by IBGP are not propagated by
IBGP.
6) Q: How we can make information passing from internal routing protocols to the BGP ?
A: We have several possibilities:
- statically propagate network prefix via "network" command
- propagate summarized prefixes as "aggregate-address" from IGP to BGP
- redistribute prefixes directly from IGP to BGP
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Final Configuration (after all steps has been completed)
Router RA
int loo0
ip address 172.16.100.1 255.255.255.255
int loo1
ip address 172.16.10.1 255.255.255.0
int loo2
ip address 172.16.11.1 255.255.255.0
int e0
ip address 172.16.1.1 255.255.255.0
int s0
ip address 200.1.1.1 255.255.255.252
clock rate 2000000
router ospf 1
network 172.16.0.0 0.0.255.255 area 0
router bgp 100
no synchronization
network 172.16.1.0 mask 255.255.255.0
network 172.16.10.0 mask 255.255.255.0
network 172.16.11.0 mask 255.255.255.0
network 172.16.20.0 mask 255.255.255.0
network 172.16.21.0 mask 255.255.255.0
neighbor 172.16.100.2 remote-as 100
neighbor 172.16.100.2 update-source loo0
neighbor 172.16.100.2 next-hop-self
neighbor 200.1.1.2 remote-as 200
neighbor 200.1.1.2 filter-list 10 out
no auto-summary
aggregate-adress 172.16.0.0 255.255.0.0 summary-only
ip as-path access-list 10 permit ^$
Router RG
int loo0
ip address
int loo1
ip address
int loo2
ip address
int e0
ip address
int s0
ip address
clock rate
int s1
ip address
clock rate
router ospf 1
172.16.100.2 255.255.255.255
172.16.20.1 255.255.255.0
172.16.21.1 255.255.255.0
172.16.1.2 255.255.255.0
200.1.2.1 255.255.255.252
128000
200.1.2.5 255.255.255.252
128000
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network 172.16.0.0 0.0.255.255 area 0
router bgp 100
no synchronization
network 172.16.1.0 mask 255.255.255.0
network 172.16.10.0 mask 255.255.255.0
network 172.16.11.0 mask 255.255.255.0
network 172.16.20.0 mask 255.255.255.0
network 172.16.21.0 mask 255.255.255.0
neighbor 172.16.100.1 remote-as 100
neighbor 172.16.100.1 update-source loo0
neighbor 172.16.100.1 next-hop-self
neighbor 200.1.2.2 remote-as 300
neighbor 200.1.2.2 filter-list 10 out
neighbor 200.1.2.2 route-map setloc in
neighbor 200.1.2.2 weight 10
neighbor 200.1.2.6 remote-as 300
neighbor 200.1.2.6 weight 100
no auto-summary
aggregate-adress 172.16.0.0 255.255.0.0 summary-only
ip as-path access-list 10 permit ^$
route-map setloc permit 10
set local-preference 200
Router RC
int loo1
ip address 192.168.10.1 255.255.255.0
int e0
ip address 200.1.3.1 255.255.255.252
clock rate 2000000
int s0
ip address 200.1.1.2 255.255.255.252
router bgp 200
network 192.168.10.0
neighbor 200.1.1.1 remote-as 100
neighbor 200.1.3.2 remote-as 300
no auto-summary
Router RD
int loo1
ip address 192.168.20.1 255.255.255.0
int e0
ip address 200.1.3.2 255.255.255.252
int s0
ip address 200.1.2.2 255.255.255.252
int s1
ip address 200.1.2.6 255.255.255.252
router bgp 300
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network 192.168.20.0
neighbor 200.1.2.1 remote-as 100
neighbor 200.1.2.1 weight 10
neighbor 200.1.3.1 remote-as 200
neighbor 200.1.2.5 remote-as 100
neighbor 200.1.2.5 weight 100
no auto-summary
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