Bel's Blog

ROUTE SELECTION DECISION PROCESS

beautbelsblog — Sun, 28 Nov 2010 19:34:03 +0000

The following process summarizes how BGP chooses the best route on a Cisco router:

Prefer the route with the highest weight. (The weight attribute is proprietary to Cisco and is local to the router only.)
If multiple routes have the same weight, prefer the route with the highest local preference value. (The local preference is used within an autonomous system.)
If multiple routes have the same local preference, prefer the route that the local router originated. A locally originated route has a next hop of 0.0.0.0 in the BGP table.
If none of the routes were locally originated, prefer the route with the shortest autonomous system path.
If the autonomous system path length is the same, prefer the lowest origin code (IGP < EGP < incomplete).
If all origin codes are the same, prefer the path with the lowest MED. (The MED is exchanged between autonomous systems.) The MED comparison is made only if the neighboring autonomous system is the same for all routes considered, unless the bgp always-compare-med command is enabled.
If the routes have the same MED, prefer external paths to internal paths.
If synchronization is disabled and only internal paths remain, prefer the path through the closest IGP neighbor, which means that the router prefers the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next hop).
For EBGP paths, select the oldest route to minimize the effect of routes going up and down (flapping).
Prefer the route with the lowest neighbor BGP router ID value.
If the BGP router IDs are the same, prefer the router with the lowest neighbor IP address.

Only the best path is entered in the routing table and propagated to the BGP neighbors of the router.

BGP ATTRIBUTES

beautbelsblog — Fri, 26 Nov 2010 05:22:41 +0000

ATTRIBUTE TYPE CODE	ATTRIBUTE CATEGORY	ATTRIBUTE DESCRIPTION
1	Well-known mandatory	ORIGIN
2	Well-known mandatory	AS_PATH
3	Well-known mandatory	NEXT_HOP
4	Option non-transitive	MULTI_EXIT_DISC
5	Well-known discretionary	LOCAL_PREF
6	Well-known discretionary	ATOMIC_AGGREGATE
7	Optional transitive	AGGREGATOR
8	Optional transitive (Cisco defined)	COMMUNITY
9	Optional non-transitive (Cisco defined)	ORIGINATOR_ID
10	Optional non-transitive (Cisco defined)	CLUSTER_LIST

ESTABLISHING A BGP SESSION

beautbelsblog — Fri, 26 Nov 2010 04:03:18 +0000

When establishing a BGP session, BGP goes through the following states:

IDLE - Router is searching router table to see whether a route exists to reach the neighbor.

CONNECT- waiting for the TCP connection to be completed with the peer. If successful, system sends an OPEN message and moves to the Open Sent state. If TCP connection fails, system resets the ConnectRetry timer, proceeds to the Active state and listens for a TCP connection to be initialized by a BGP peer

ACTIVE - Router is trying to acquire a peer by initiating a TCP connection.

(OPEN SENT) – Open message sent with the parameters for the BGP session.

OPEN CONFIRM – Router received agreement on the parameters for establishing session. Alternatively the router goes into ACTIVE state if no response to OPEN message.

ESTABLISHED – Peering is established – routing begins.

CHAPTER SIX – BGP

beautbelsblog — Fri, 26 Nov 2010 03:00:46 +0000

The main goal of BGP is to provide an interdomain routing system that guarantees loop-free exchange of routing information between autonomous systems. BGP is a successor of the Exterior Gateway Protocol, which was developed to isolate networks from each other as the Internet grew. BGP4 has many enhancements over earlier protocols, the Internet uses BGP4 exclusively to connect enterprises to ISPs and to connect ISPs to each other.

DEFINITIONS

AS PATH - is a well-known mandatory attribute, Whenever a route update passes through an autonomous system, the autonomous system number is prepended (added) to that update when it is advertised to the next EBGP neighbor. The AS PATH attribute is actually the list of autonomous system numbers that a route has traversed to reach a destination, with the number of the autonomous system that originated the route at the end of the list.

HARD RESET – means that the router issuing either of these commands closes the appropriate TCP connections, reestablishes those TCP sessions as appropriate, and resends all information to each of the neighbors affected by the particular command that is used. The clear ip bgp * command causes the BGP forwarding table on the router that issued this command to be completely deleted, and all networks must be relearned from every neighbor. If a router has multiple neighbors, this action is a very dramatic event. This command forces all neighbors to resend their entire tables simultaneously. You should use this command with extreme caution, and it is not normally used in a production environment.

LOCAL PREFERENCE ATTRIBUTE – is a well-known discretionary attribute that provides an indication to routers in the autonomous system about which path is preferred to exit the autonomous system. A path with a higher LOCAL PREFERENCE is preferred. The LOCAL PREFERENCE is an attribute that is configured on a router and exchanged among routers within the same autonomous system only. The default value for LOCAL PREFERENCE on a Cisco router is 100.

MED ATTRIBUTE (also called the metric) - is an optional nontransitive attribute. The MED is an indication to EBGP neighbors about the preferred path into an autonomous system. The MED ATTRIBUTE is a dynamic way to influence another autonomous system about which path that it should choose to reach a certain route in their autonomous system when multiple entry points exist. A lower metric is preferred. Unlike LOCAL PREFERENCE, the MED is exchanged between autonomous systems.

MULTIHOMING – is when an autonomous system has more than one connection to the Internet. Two typical reasons for using MULTIHOMING is to increase the reliability of the connection to the Internet, if one connection fails, the other connection remains available and to increase the performance of the connection, better paths can be used to certain destinations. An organization can be MULTIHOMED to either a single ISP or to multiple ISPs.

NETWORK LAYER REACHABILITY INFORMATION (NLRI) – BGP sends routing update messages containing NLRI to describe a route and how to get there. In this context, an NLRI is a prefix. A BGP update message carries one or more NLRI prefixes and the attributes of a route for the NLRI prefixes; the route attributes include a BGP next hop gateway address, community values, and other information.

NEXT-HOP ATTRIBUTE – is a well-known mandatory attribute that indicates the next-hop IP address that is to be used to reach a destination. BGP routes autonomous system by autonomous system, not router by router. The NEXT-HOP ATTRIBUTE defines the IP address of the border router that should be used as the next hop to the destination.

ORIGIN - is a well-known mandatory attribute and specifies how the advertising router learned about the route listed in the NLRI field. BGP uses the ORIGIN attribute in its decision-making process, preferring routes with a lower ORIGIN value. VALUES; i IGP – the route was learned through an IGP routing protocol, e EGP – the route was learned through an EGP routing protocol and ? Incomplete – the route was learned by some other means.

RESETTING – is a method of informing neighbors of a policy change. If BGP sessions are reset, all information received on those sessions is invalidated and removed from the BGP table. Also, the remote neighbor detects a BGP session down state and invalidates the routes that were received. After 30 to 60 seconds, the BGP sessions are reestablished automatically, and the BGP table is exchanged again but through the new filters. However, RESETTING the BGP session disrupts packet forwarding.

SOFT RESET – the router issuing the command does not reset the BGP session. Instead, the router creates a new update and sends the entire table to the specified neighbors.

WEIGHT ATTRIBUTE - is a Cisco attribute for path selection. The WEIGHT is configured locally on a router and is not propagated to any other routers. This attribute applies when you are using one router with multiple exit points in autonomous system, as opposed to the LOCAL PREFERENCE ATTRIBUTE, which is used when two or more routers provide multiple exit points. The WEIGHT can have a value from 0 to 65535. By default, paths that the router originates have a WEIGHT of 32768, and other paths have a WEIGHT of 0. Routes with a higher WEIGHT are preferred when multiple routes exist to the same destination.

PASSIVE INTERFACE SUMMARY

beautbelsblog — Fri, 12 Nov 2010 05:00:22 +0000

A passive interface essentially makes a router a silent host on a network. Identifying an interface as passive prevents routing updates for a routing protocol from being sent through a router interface.

ROUTING PROTOCOL	SUPPRESSES OUTGOING ROUTING UPDATES	SUPPRESSES INCOMING ROUTING UPDATES	STOPS NEIGHBOUR ADJACENCY
RIP	Yes	No	No
EIGRP	Yes	Yes	Yes
OSPF	Yes	Yes	Yes
IS-IS	Yes	Yes	Yes

DEFAULT SEED METRICS

beautbelsblog — Thu, 21 Oct 2010 06:47:43 +0000

Metric value determines the shortest or “best” path to an IP network.

Seed metric artificially sets the distance, cost, and so on, to each external (redistributed) network from the redistribution point. When a router redistributes routes from one routing domain to another, the metric cannot be translated from one routing protocol to another.

PROTOCOL	DEFAULT SEED METRICS
RIP	Infinity (unreachable)
EIGRP	Infinity (unreachable)
OSPF	20 for all except BGP, which is 1
IS-IS	0
BGP	BGP metric is set to IGP metric value

**The exception to this rule is OSPF E2 routes, which hold their initial metric regardless of how far they are propagated across an autonomous system.

CHAPTER FIVE – ROUTE OPTIMIZATION

beautbelsblog — Thu, 21 Oct 2010 06:21:36 +0000

Routing can be optimized in a network by controlling when a router exchanges routing updates and what those updates contain. To ensure that the network operates efficiently, you must control and tune routing updates. Information about networks must be sent where it is needed and filtered from where it is not needed. Factors that have the most impact on redistribution include: Metrics, Administrative Distance and classful/classless capabilities of the protocols.

DEFINITIONS

AUTOMATIC DHCP –

BOUNDARY ROUTER - is the router that interconnects the autonomous systems and must be running all the routing protocols that are exchanging routes.

DYNAMIC DHCP –

MANUAL DHCP -

ROUTE MAPS –

ROUTE REDISTRIBUTION - allows a routing protocol to advertise routes that were not learned through that routing protocol. These REDISTRIBUTED ROUTES could have been learned via a different routing protocol, such as when REDISTRIBUTING between EIGRP and OSPF, from static routes, or by a direct connection to a network. REDISTRIBUTION is always performed outbound. The router doing REDISTRIBUTION does not change its routing table.

ADMINISTRATIVE DISTANCES

beautbelsblog — Wed, 20 Oct 2010 09:48:46 +0000

The administrative distance is a value between 0 and 255. The lower the administrative distance value, the higher the believability of the protocol.

ROUTE SOURCE	DEFAULT DISTANCE
Connected Interface	0
Static Route	1
EIGRP Summary Route	5
External BGP	20
Internal EIGRP	90
IGRP	100
OSPF	110
IS-IS	115
RIPv1, RIPv2	120
External EIGRP	170
Internal BGP	200
Unknown	255

NSAP ADDRESSING

beautbelsblog — Wed, 20 Oct 2010 02:52:47 +0000

The simplest NSAP format, which is used by most companies running IS-IS as their IGP, includes the following:

Area address: It must be at least 1 byte, separated into two parts:
- The AFI, which is set to 49 (private), signifying that the AFI is locally administered and, therefore, individual addresses can be assigned by the company.
- The area ID, which are the octets of the area address after the AFI.

System ID
- Cisco routers compliant with the U.S. Government OSI Profile (GOSIP) version 2.0 standards require a 6-byte system ID.
- System IDs must remain unique across the domain to avoid conflict at Level 1 or Level 2 .

N-Selector (NSEL)
- It must always be set to 0 for a router.
- The NSAP is called the NET when it has an NSEL of 0.

For example, you might assign 49.0001.0000.0c12.3456.00, which represents the following:

- AFI of 49

- Area ID of 0001

- System ID of 0000.0c12.3456, the MAC address of a LAN interface

- NSEL of 0

The following three rules apply:

The NET must begin with a single octet (for example, 49.xxxx …).

The NET must end with a single octet (for example xxx.00).

On Cisco routers, the System ID must be six octets (for example, xxxx.0000.0c12.3456.xx)

COMPARISON BETWEEN OSPF AND IS-IS

beautbelsblog — Wed, 20 Oct 2010 02:06:50 +0000

Although OSPF and IS-IS are very similar, there are some differences between the two routing protocols. OSPF runs on top of IP, whereas IS-IS runs through CLNS. Both routing protocols have the following characteristics:

Open-standard link-state routing protocols

Support VLSM

Support route summarization between areas

Similar mechanisms to maintain the health of the LSDB (link-state advertisements [LSAs], link-state aging timers, and LSDB synchronization)

Use the shortest path first (SPF) algorithm, with similar update, decision, and flooding processes

Successful in the largest and most demanding ISP networks

Converge quickly after network changes

The table below summarizes the differences between OSPF and Integrated IS-IS.

OSPF	INTERGRATED IS-IS
Area borders inside routers	Area borders on links
Each link in only one area	Each router in only one area
More complex to extend back bone	Simple extension of backbone
Many small LSAs sent	Fewer LSPs sent
Runs on top of IP	Runs on top of Data Link Layer
Requires IP addresses	Requires IP and CLNS addresses
Default metric is scaled by interface bandwidth	Default metric is 10 for all interfaces
Not easy to extend	Easy to support new protocols with new TLV tuples
Equipment, personnel and information more available	Equipment, personnel and information not as accessible.

Bel's Blog

ROUTE SELECTION DECISION PROCESS

The following process summarizes how BGP chooses the best route on a Cisco router:

BGP ATTRIBUTES

ATTRIBUTE TYPE CODE

ATTRIBUTE CATEGORY

ATTRIBUTE DESCRIPTION

1

Well-known mandatory

ORIGIN

2

Well-known mandatory

AS_PATH

3

Well-known mandatory

NEXT_HOP

4

Option non-transitive

MULTI_EXIT_DISC

5

Well-known discretionary

LOCAL_PREF

6

Well-known discretionary

ATOMIC_AGGREGATE

7

Optional transitive

AGGREGATOR

8

Optional transitive (Cisco defined)

COMMUNITY

9

Optional non-transitive (Cisco defined)

ORIGINATOR_ID

10

Optional non-transitive (Cisco defined)

CLUSTER_LIST

ESTABLISHING A BGP SESSION

When establishing a BGP session, BGP goes through the following states:

CHAPTER SIX – BGP

DEFINITIONS

PASSIVE INTERFACE SUMMARY

A passive interface essentially makes a router a silent host on a network. Identifying an interface as passive prevents routing updates for a routing protocol from being sent through a router interface.

DEFAULT SEED METRICS

Metric value determines the shortest or “best” path to an IP network.

Seed metric artificially sets the distance, cost, and so on, to each external (redistributed) network from the redistribution point. When a router redistributes routes from one routing domain to another, the metric cannot be translated from one routing protocol to another.

PROTOCOL

DEFAULT SEED METRICS

CHAPTER FIVE – ROUTE OPTIMIZATION

DEFINITIONS

ADMINISTRATIVE DISTANCES

The administrative distance is a value between 0 and 255. The lower the administrative distance value, the higher the believability of the protocol.

ROUTE SOURCE

DEFAULT

DISTANCE

NSAP ADDRESSING

The simplest NSAP format, which is used by most companies running IS-IS as their IGP, includes the following:

The following three rules apply:

COMPARISON BETWEEN OSPF AND IS-IS

Although OSPF and IS-IS are very similar, there are some differences between the two routing protocols. OSPF runs on top of IP, whereas IS-IS runs through CLNS. Both routing protocols have the following characteristics:

The table below summarizes the differences between OSPF and Integrated IS-IS.

OSPF

INTERGRATED IS-IS

Area borders inside routers

Area borders on links

Each link in only one area

Each router in only one area

More complex to extend back bone

Simple extension of backbone

Many small LSAs sent

Fewer LSPs sent

Runs on top of IP

Runs on top of Data Link Layer

Requires IP addresses

Requires IP and CLNS addresses

Default metric is scaled by interface bandwidth

Default metric is 10 for all interfaces

Not easy to extend

Easy to support new protocols with new TLV tuples

Equipment, personnel and information more available