Dell Force10 BGP Configuration

Module 9

9-2

Objectives BGP Overview

BGP Implementation in FTOS

Basic BGP Configuration

Troubleshooting

9-3

BGP Overview An IGP (like ISIS or OSPF) is usually

used to optimize routing within the AS

BGP is an External Gateway Protocol Path Vector (AS# and direction)

BGP is more concerned with the flow of data between ASs Establishes communication between

two different Autonomous Systems

BGP has a distinct focus on routing policy Unlike OSPF, BGP does not

dynamically learn routes

BGP is a simple protocol, but complex to configure

Mistakes can impact the Internet

BGP must be explicitly configured to make routing decisions

AS-1 AS-2

AS-4 AS-3

iBGP peering eBGP peering

External route

9-4

OSPF vs. BGP OSPF BGP

Version: v2 v4

RFC: 2328 1771

IGP - EGP: (1) IGP EGP

Type: Link state Path vector

Algorithm: Dijkstra Best Path Selection

Convergence: (2) Fast Slow

Administrative distance: 110 20

OSI level / port: IP / 89 TCP / 179

Configuration complexity: Medium Hard to very hard

Routing granularity: Router Autonomous System (3)

Scalable for: < 100 routers > 100 routers

Resource consumption: Low to Medium High

1. IGP stands for Interior Gateway Protocols, EGP for Exterior Gateway Protocols. EGPs are used between independent administrative entities, often called autonomous systems and IGPs are used inside them.

2. Convergence time: time needed for the routers to update their routing table after a change in the network topology.

3. When several paths are available for a specific network, BGP chooses the route crossing the smallest number of autonomous systems.

9-5

Policy with BGP

BGP provides capability for enforcing various policies

BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other ASs

Import policy What to do with routes learned from neighbors?

Select the best path

Export policy What routes to announce to neighbors?

It depends on the relationship with the neighbor

9-6

More About BGP

BGP uses TCP as a reliable transport mechanism over port 179

Two Types of BGP Sessions eBGP

BGP session between BGP talkers with different AS numbers

I-BGP

BGP session between BGP talkers with the same AS number Usually relies on IGP unless fully meshed

Routes in BGP are also known as prefixes

Routers configured for BGP are called BGP speakers

9-7

Receiving a BGP Prefix

The path passes through two steps before being chosen as a candidate to populate the RIB.

Step 1 - Check if the path is valid: The prefix must not been marked as not-synchronized

There must be a route in the RIB to reach the next-hop For prefixes learned through eBGP sessions, the local AS number must

not be in the AS-PATH of the prefix

If the path is valid, the prefix will get installed into the BGP table

Step 2 - Route selection will start.

Route selection is a multi-step process to determine the best path available in BGP.

9-8

How BGP Builds The Route Table

IP Unicast Route Table

Inbound Policy Engine

BGP Table

BGP Updates BGP Updates

Everything

Discarded

Outbound Policy Engine

Hardware Forwarding Table

Prefixes

RIB-Out

RIB-In

Prefix Used Forwarding

Entries Best Path

RIB-Local

9-9

What is Route Selection?

This is how BGP determines and selects the best path to reach the prefix

If there is only one path, no comparison needed

If there are many paths to reach the prefix, there is a special algorithm that BGP uses to select the best path

Additionally, BGP uses attributes and route maps to help in a Best Path Route Selection process

9-10

Best Path Selection Criteria

Lowest Cluster ID

List

From Lowest

Router ID

From Lowest

Neighbor Address

Highest Weight

Highest Local Preference

Locally Originated Path

Shortest AS Path

Lowest Origin Code

Lowest MED

Learned via EBGP

Lowest NEXT-HOP Cost

A Single Route Is Selected & Installed In The Routing Table

Tie Breakers

9-13

BGP Implementation in FTOS

BGP process runs in RP1 Maximum of 1,536 BGP peers

Deterministic MED default, missing MED is treated as worst-path

Confederations and Route Reflectors

Peer Groups (maximum of 64 peers per group)

Route Refresh

Graceful Restart (IETF-IDR-RESTART-08)

Common BGP attributes: AS-path

MED

Local Preference

Origin

9-14

Force10(conf)# router bgp 65111

Force10(conf-router_bgp)# bgp router-id

192.168.1.11

Force10(conf-router_bgp)# network

10.10.91.0/24

Force10(conf-router_bgp)# neighbor

192.168.1.12 remote-as 65111

Force10(conf-router_bgp)# neighbor

192.168.1.12 no shutdown

Force10(conf-router-bgp)# neighbor

192.168.1.12 update-source Loopback 0

Force10(conf-router_bgp)#show conf

!

router bgp 6501 network 10.10.91.0/24 neighbor 192.168.1.12 remote-as 6501 neighbor 192.168.1.12 update-source Loopback 0 neighbor 192.168.1.12 no shutdown

A

B

C

A. Enable global BGP routing process by assigning an AS number

Force10(conf)# router bgp

B. Assign a BGP router-id Force10(conf-router_bgp)# bgp

router-id

C. Configure the network to the peer Force10(conf-router_bgp)#

network < mask>

[route-map ]

D. Configure any neighbors you wish to peer with Force10(conf-router_bgp)#

neighbor

update-

source loopback

E. Enable the peering session Force10(conf-router_bgp)#

neighbor no

shutdown

Basic BGP Configuration Commands

D

E

9-15

Configuring iBGP vs eBGP

eBGP Neighbors remote AS number is different

Neighbor must be on same subnet

iBGP Neighbors remote AS number is the same

Neighbors can be across the network Reachable via an IGP (OSPF, RIP, IS-IS) etc.

Configure iBGP sessions between loopback interface addresses Make sure loopback interfaces are described by the IGP Make sure to use the update-source option when configuring a

neighbor using loopback interfaces

Force10 (conf-router bgp) # neighbor

update-source loopback

9-16

Answer:

Compare iBGP with eBGP Configuration

Which one is eBGP and why?

Force10(conf)# router bgp 65111

Force10(conf-router_bgp)# bgp router-id 192.168.1.11

Force10(conf-router_bgp)# network 10.10.91.0/24

Force10(conf-router_bgp)# neighbor 10.10.91.9 remote-as 65222

Force10(conf-router_bgp)# neighbor 10.10.91.9 no shutdown

Force10(conf)# router bgp 65111

Force10(conf-router_bgp)# bgp router-id 192.168.1.11

Force10(conf-router_bgp)# network 10.10.91.0/24

Force10(conf-router_bgp)# neighbor 192.168.1.12 remote-as 65111

Force10(conf-router_bgp)# neighbor 192.168.1.12 update-source Loopback 0

Force10(conf-router_bgp)# neighbor 10.10.91.9 no shutdown

The neighbor with a different AS number

9-17

Route Redistribution

You can add routes from other routing instances or protocols to the OSPF process

With the redistribute command, you can include IGP, static, or directly connected routes

Include, directly connected or user-configured (static) routes in BGP: redistribute {connected | static} [route-map map-name]

Include specific OSPF routes redistribute ospf process-id [match external {1 | 2} |

match internal] [metric-type {external | internal}]

[route-map map-name]

9-18

Verifying BGP Route Redistribution

Redistributed routes should be injected into BGP as a result of configuration BGP.

The 144.254 static routes were configured for redistribution and appear as BGP routes.

2/22/2013

A0(conf-router_bgp)# do show ip route bgp Destination Gateway Dist/Metric Last Change ----------- ------- ----------- ----------- B IN 144.254.11.0/24 via 192.168.1.11 200/0 00:06:16 B IN 144.254.12.0/24 via 192.168.1.12 200/0 00:06:21

A0(conf-router_bgp)# redistribute static

9-19

Verifying BGP Configuration

Verify BGP adjacencies are formed with neighboring routers after the initial configuration.

Example (abbreviated) output:

Note: the ideal state for a BGP neighbor is "ESTABLISHED"

A1(conf-router_bgp)# do show bgp neighbors

BGP neighbor is 192.168.1.11, remote AS 6501, internal link BGP version 4, remote router ID 192.168.1.11 BGP state ESTABLISHED, in this state for 00:12:06

9-20

Verifying BGP Configuration

The command is used to examine the results of configuring BGP. The output below shows a correct BGP configuration with BGP neighbors peering correctly and exchanging BGP update messages.

A1(conf-router_bgp)# do show ip bgp summary BGP router identifier 192.168.1.10, local AS number 6501 BGP table version is 3, main routing table version 3 3 network entrie(s) using 470 bytes of memory 6 paths using 408 bytes of memory BGP-RIB over all using 414 bytes of memory 1 BGP path attribute entrie(s) using 64 bytes of memory 1 BGP AS-PATH entrie(s) using 39 bytes of memory 2 neighbor(s) using 12288 bytes of memory

Neighbor AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/Pfx

192.168.1.11 6501 21 34 3 0 0 00:18:08 2 192.168.1.12 6501 14 32 3 0 0 00:10:51 2

9-21

Additional BGP Verification Commands

show ip bgp neighbor advertised-routes

show ip bgp neighbor

show ip bgp prefix-list

show ip bgp flap-statistics

show ip bgp dampened-paths

show ip bgp inconsistent-as

show ip bgp filter-list

show ip bgp community-list

show ip route

ping

traceroute

9-22

Force10#debug ip bgp updates

BGP: 15.0.0.1 rcvd UPDATE w/ attr: origin i, path (5000), nexthop 15.0.0.1, metric 0,

local pref 100, BGP: 15.0.0.1 rcvd 25.0.0.0/8

Force10#debug ip bgp out

BGP: 15.0.2.1 sending NOTIFICATION 6/0 (cease)

BGP: 15.0.2.1 went from Established to Idle

Force10#BGP: 15.0.2.1 went from Start to Connect

BGP: 15.0.2.1 went from Connect to Active

BGP: 15.0.3.2 sending UPDATE w/ empty path attr

BGP: 15.0.3.2 send UPDATE 18.0.0.0/8 -- unreachable

BGP: 15.0.3.2 sent message type 2, length (incl. header) 6

BGP: 15.0.2.1 sending OPEN, version 4, my as: 23

BGP: 15.0.2.1 went from Active to OpenSent

BGP: 15.0.2.1 went from OpenSent to OpenConfirm

BGP: 15.0.2.1 went from OpenConfirm to Established

BGP: all prefix 17.0.0.0/8 DENIED due to: outbound filter-list deny3

BGP: 15.0.2.1 sending UPDATE w/ attr: origin i, path 23 , nexthop 15.0.2.2, metric 0,

BGP: 15.0.2.1 send UPDATE 16.0.0.0/8, next 15.0.2.2, path 23

BGP: 15.0.2.1 sent message type 2, length (incl. header) 31

BGP: 15.0.2.1 sending KEEPALIVE

Useful Debug Commands

debug ip bgp updates

debug ip bgp [in/out]

9-23

Troubleshooting: BGP Are BGP peer sessions up?

show ip bgp summary show ip bgp neighbors debug ip bgp events

Are you receiving the routing information you expect? show ip bgp neighbors routes to see what route

prefixes you are receiving from the neighbor Verify that you are receiving the prefixes you expect.

Check that a prefix list, filter list or route-map applied to the neighbor is not blocking prefixes

Verify there is not a better path available This should never happen if you are singly homed

Verify the path for the next-hop is available show ip route to verify the next hop

is known If it is not in the routing table, review how the next hop should have

been learned From an eBGP neighbor, if the local AS number appears in the path, the

path will be denied upon ingress and not installed into the BGP routing-information base

9-24

BGP Neighbor not Established

Verify the configuration of the BGP peer and if an eBGP neighbor, contact the support group for that neighbor to compare: AS number

Neighbor IP address BGP version

EBGP Multi-hop (if needed)

Try: debug ip bgp events and then clear ip bgp while connected to the console Type undeb all before this command so you can easily disable it by

using the up arrow

Verify that there is no access list entry that is preventing the BGP session from being established

The neighbor initiating the BGP session uses TCP/UDP port 179 to establish the connection

9-25

BGP Routes not Advertised

Verify the routes you are advertising to the neighbor using show ip bgp neighbors advertised-routes

to see what route prefixes you are advertising to the neighbor

Verify the route is in the BGP routing table using\: show ip bgp

If the next-hop is unavailable, you will see it here

Verify the route is in the local routing table using: show ip route

How is the route learned: BGP, connected, static, OSPF?

9-26

Module Summary

In this module we learned

Overview of BGP

Implementing BGP in FTOS

Basic BGP Configuration

Troubleshooting