Slides

IP Network Configuration for Traffic Engineering

Anja FeldmannJennifer Rexford

AT&T Labs - Research

Presented by Zihui Ge

11/21/2000

Outline

Introduction Network model Router configuration Netdb examples Conclusion

Nightmare!-Configure IP Router

Configuring IP routers is extremely complicated Diversity of network equipment Large number of configuration options Interaction of configuration parameters across

multiple routers Rapid changes to network Limited configuration tools

Severe consequences of error by manual configuration of individual routers

HELP!-An Abstract model

Need an ACCURATE network-wide CONSISTENT ABSTRACT view of network configuration. Guide router configuration. Help in error detection.

What need to be modeled? Physical components

• Routers, interfaces Physical and logical connectivity

• Links, BGP sessions Routing protocols

• Static routes, OSPF, BGP Access control

• Packet filters, route filters

A peek on author’s model.

Object AttributesRouter router name, {loopback IP address}, location, {interface},

{global setting}Interface router, interface name, {(IP address, IP prefix)}, capacity,

OSPF weight, queuing strategy, status (up/down), {access list}, {static route}

Link IP prefix, link type (backbone/edge), OSPF area, {interface}

Access list IP prefix, permit/deny, {(interface, packet/route, in/out)}

Static route IP prefix, tag (administrative weight), {interface}

BGP router, remote peer (IP address), remote AS, iBGP/eBGP, {filter policy}, {interface},{session attribute}

Think about router.

Router Typically consists of a route processor, a

switching fabric, a collection of interfaces. Route processor construct forwarding table

based on information of intradomain and interdomain routing protocol

Route processor is identified by loopback IP addresses

Router name, {loopback IP address}, location, {interface}, {global setting}

Think about interface. Position in the router(indicated by interface

name). Each interface has a primary IP address and may

have one or more secondary IP addresses; each IP address is associated with a particular prefix. 12.34.56.77 in prefix 12.34.56.76/30

Could associate with static routes , filters. Router, interface name, {(IP address, IP prefix)},

capacity, queuing strategy, status (up/down),{access list}, {static route},OSPF weight

Think about link. Link is a physical media. Addressing: IP prefix.

12.34.56.76/30 Two ends 12.34.56.77 & 12.34.56.78

Ethernet or FDDI may have smaller mask length Backbone link VS. edge link

All interfaces in the same AS? Participate in intradomain or interdomain routing?

IP prefix, link type (backbone/edge),{interface}, OSPF area

Think about OSPF. Link state protocol. OSPF routers exchange weight information and

compute shortest path Use flooding to propagate link-state update. Length of path is defined as sum of weight

Extension for scalability: Routing hierarchy(OSPF areas) Each link belongs to SAME OSPF area

Attribute “OSPF weight” in interface object Attribute “OSPF area” in link object

Think about static route. Provides a simple way to associate destination

prefixes with edge interfaces. The router knows to direct packets destined to some

prefix to the appropriate next-hop interface. Doesn’t ensure that the rest of the network knows how

to reach this destination prefix. Advertise static route via OSPF or iBGP.

administrative weight IP prefix, tag (administrative weight),{interface}

Think about BGP.

BGP Distance Vector. Filter policy contains import and export policies. BGP object corresponds to one end point of a

BGP session(Addressing the session peer). How to reach the remote end? (Interface) Confederation, route reflectors? router, remote peer (IP address), remote AS,

iBGP/eBGP, {filter policy}, {interface},{session attribute}

Think about filter. Packet filters

firewall detect spoofed source IP address.

Route filter Custom-Provider avoids problem caused by misconfigured BGP

policies in downstream routers. Access list IP prefix, permit/deny, {(interface,packet/route,

in/out)}

Cool! We did it!

Object AttributesRouter router name, {loopback IP address}, location, {interface},

{global setting}Interface router, interface name, {(IP address, IP prefix)}, capacity,

OSPF weight, queuing strategy, status (up/down), {access list}, {static route}

Link IP prefix, link type (backbone/edge), OSPF area, {interface}

Access list IP prefix, permit/deny, {(interface, packet/route, in/out)}

Static route IP prefix, tag (administrative weight), {interface}

BGP router, remote peer (IP address), remote AS, iBGP/eBGP, {filter policy}, {interface},{session attribute}

Where to get information?

Potential Data Sources SNMP MIBs

• Basic traffic statistics Topology discovery tools

• traceroute, pathchar• Active measure: topology, link capacity, …

Route monitoring• Passive monitor: BGP, OSPF link state update

Router configuration files• Physical and logical connectivity, link capacity,

routing protocols, access lists, …

Router Configuration Files Cisco’s Internet Operating

System(ISO) serves as a de facto standard for router configuration.

Access configuration files Complete Consistent (snapshot)

A.txt

A sample file

Dependencies Within a File Domain(Section)-Independent

Referencing undefined items• Undefined access-list, route-map, …• Similar to compile error: undefined variable

Unused items• Similar to compile warning: unused variable

Dependencies Within a File Domain-Dependent

Inconsistent definitions• “Speed” in channel-group• “Bandwidth” in interface entry

• Missing “IP classless” will cause the router to discard packets destined to an IP prefix that is not aligned with octet boundaries.

Dependence on default parameters• Missing OSPF area in the router section (router

won’t participate in OSPF)• Missing OSPF weight in interface section (default

weight is set inversely proportional to capacity)

Dependencies Across Files Inconsistent definitions

Router level Significance (will not cause inconsistency)

• Similar to local variable• For example: access-list

Network wide Significance (problematic)• Similar to External variable or function• A backbone link with interfaces on two routers: two

routers should agree on the selection of an OSPF area.

Dependencies Across Files Inconsistent references to remote

nodes(even worse) eBGP session: peer router resides

outside backbone, on different organization

Two routers:• Neighbor 10.1.2.3 remote-as 1• Neighbor 10.1.2.3 remote-as 2

Netdb Netdb is a Perl script that parses

configuration files in Cisco IOS formation

Populates the network model and detects possible configuration errors for AT&T Common IP Backbone. Running time < 2 minutes

Netdb step 1

read configuration files of all routers read keywords for global settings and

section names forall routers {

identify section boundaries parse global variables check global variables

}

Netdb step 2

foreach section in (controllers, access lists, interfaces, other filter sections, static routes, OSPF, BGP) {

read section keywords read customization input files forall routers

parse section and check keywords, network model violations

forall routers perform error checks

}

STEP 2 - implementation Multiple passes.

• Parse and load all file into memory Strict sequence: Controllers < ..< BGP

•Earlier sections do not depend on later sections

•Processing later section requests previous section complete.

STEP 2 -What do we miss? There is no LINK section in configuration

file! Link Object is created for the first

occurrence of a prefix in interface section, and associated with the IP address of the particular interface.

It is extended to include the IP address of other interfaces if the prefix reappears.

Netdb step 3

forall routers { forall objects

report unassigned attributes

forall statements report unused statements

}

Examples

Examples of netdb error messages router1: unknown interface keyword: hold-queue

value: ...... router2: ROUTE-MAP ERROR: community 1000 undefined

ROUTEMAP1: community 1000 1010 router3: OSPF ERROR: ospf network 10.127.6.132/30

should either be in area 14 or 3 router4: OSPF ERROR: network: 10.126.212.0 0.0.0.3

area 2 with only one IP address 10.126.212.2 router5: OSPF ERROR: network: 10.126.12.172 0.0.0.3

area 3 with no IP address router6: BGP ERROR: cannot resolve IP: 10.11.12.56 from

...BGP statement...

Examples-2

Error messages for policy violations. router1: GLOBAL ERROR: missing parameter BGP-

COMMUNITY router2: GLOBAL ERROR: incorrect parameter CEF value:

ip cef router3: ACL WARNING: default acl 6 missing router3: ACL WARNING: default acl 7 differs from

specification: deny 172.0.0.0/8 router4: ACL ERROR: VPN customer needs in and out

ACL; (customer ABCDE) router5: CONTROLLER ERROR: missing clock sync for

interface Serial2/1/0:2 router6: BGP ERROR: wrong rr definition for rr client

10.126.236.3 with peer-group abc

Conclusion A network model for traffic

engineering A method to ensure network

consistency A tool, netdb, to identify

configuration errors

Thanks!

Happy thanks giving!