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Rip protocol

The RIP metric is based on hop count and can be between 1 and 15. The metric 16 is used for infinity, which means that if the route is unreachable, a metric of 16 is displayed. The question is, why was the metric chosen as 16? Why not 17 or 18? The metric filed in RIP-1 packet format clearly shows that it is 32 bits long. This means that, theoretically, RIP can support 232 hops. Although this is a large number, the metric of 15 was chosen to avoid a count to infinity problem. (This is also referred to as a routing loop.) In a large network with a few hundred routers, a routing loop results in a long time for convergence if the metric for infinity has a large value. The number 16 was chosen to get a shorter convergence time.

The 15-hop limit was chosen also because RIP was intentionally designed for small networks. It was not intended for the large networks that potentially can have more than 15 hop

DEfault routes

**OSPF NOTES**

Types of OSPF Areas
Normal Areas: These areas can either be standard areas or transit (backbone) areas. Standard areas are defined as areas that can accept intra-area, inter-area and external routes. The backbone area is the central area to which all other areas in OSPF connect.

Note: Intra-area routes refer to updates that are passed within the area. Inter-area routes refer to updates that are passed between areas. External routes refer to updates passed from another routing protocol into the OSPF domain by the Autonomous System Border Router (ASBR).

Stub Areas: These areas do not accept routes belonging to external autonomous systems (AS); however, these areas have inter-area and intra-area routes. In order to reach the outside networks, the routers in the stub area use a default route which is injected into the area by the Area Border Router (ABR), they also accept type 3. A stub area is typically configured in situations where the branch office need not know about all the routes to every other office, instead it could use a default route to the central office and get to other places from there. Hence the memory requirements of the leaf node routers is reduced, and so is the size of the OSPF database.

To define an area as a stub area, use the OSPF router configuration command, area stub

Totally Stub Areas: Cisco Proprietary. These areas do not allow routes other than intra-area and the default routes to be propagated within the area. The ABR injects a default route into the area and all the routers belonging to this area use the default route to send any traffic outside the area. ABR ONLY PASSES 0.0.0.0 ROUTES.

To define a totally stub area, use the OSPF router configuration command, area stub no-summary, on the ABR.

NSSA: This type of area allows the flexibility of importing a few external routes into the area while still trying to retain the stub characteristic. Assume that one of the routers in the stub area is connected to an external AS running a different routing protocol, it now becomes the ASBR, and hence the area can no more be called a stub area. However, if the area is configured as a NSSA, then the ASBR generates a NSSA external link-state advertisement (LSA) (Type-7) which can be flooded throughout the NSSA area. These Type-7 LSAs are converted into Type-5 LSAs at the NSSA ABR and flooded throughout the OSPF domain

To define a NSSA, use the OSPF router configuration command, area nssa no-summary.

OSPF LSA Types
Type 1: Router link advertisements generated by each router
for each area it belongs to. Flooded to a single area only.

Type 2: Network link advertisements generated by designated
routers describing the set of routers attached to a particular
network. Flooded to the area that contains the network.

Type 3/4: Summary link advertisements generated by ABRs
describing inter-area routes. Type 3 describes routes to
networks and is used for summarization. Type 4 describes
routes to the ASBR.

Type 5: Generated by the ASBR and describes links external
to the Autonomous System (AS). Flooded to all areas except
stub areas.

Type 6: Group membership link entry generated by multicast
OSPF routers.

Type 7: NSSA external routes generated by ASBR. Only
flooded to the NSSA. The ABR converts LSA type 7 into LSA
type 5 before flooding them into the backbone (area 0).

OSPF Route Types
O - OSPF
IA – OSPF Inter Area
N1 – OSPF NSSA External type 1
N2 – OSPF NSSA External type 2
E1 – OSPF external type 1
E2 – OSPF external type 2

To send 0.0.0.0 default route into OSPF [default-information originate always]
During convergence the slave will request a LSU during LOADING stage.
Show ip ospf interface to tell DR/BDR/DROTHER
On serial interfaces non-broadcast is the default network type.
Topology table does a full exchange every 30 minutes.
DR’s flood type 2 LSA’s to neighbor routers on the segment.
Stub areas don’t accept E1 or E2 routes
OSF does not summarize along classful boundaries
Feature similar to spoofing = Demand circuits. They filter periodic updates once the LSD is in sync; also avoid WAN OSPF overhead and avoid DDR links from dialing for no reason.
LSA1 = Router link “O” in route table
[Passive-interface e 0/0] suppress routing updates out
Show ip ospf virtual-links will show Virtual links
[area 0 virtual link 192.168.1.1]
DBD = Database Description
DDP Database Description Packet And they carry LSA entries from OSPF master L-S Database
100mbps = a cost of 1
10mbps = a cost of 10
[area 1 range 192.168.8.0 255.255.248.0] to summarize networks
An OSPF DBD\DDP contains LSA entries from OSPF master router’s link-state DB

**BGP Notes**

BGP Attributes

A BGP attribute, or path attribute, is a metric used to describe the characteristics of a BGP path. Attributes are contained in update messages passed between BGP peers to advertise routes. There are four categories of BGP attributes. These are:

1) Well-known Mandatory 3

-AS-path
-Next-hop
-Origin
2) Well-known Discretionary 2
-Local Preference
-Atomic Aggregate
3) Optional Transitive 2
-Aggregator
-Community
4) Optional Non-transitive 1
-Multi-exit-discriminator (MED)

Attribute Type
1-origin
2-AS-path
3-Next-hop
4-MED
5-Local preference
6-Atomic aggregate
7-aggregator (AS + IP = 48 bit value)
8-community
9-originator-ID
10-cluster

BGP Route Types
i – IGP (OSPF, EIGRP)
e – EGP (other BGP AS)
? – Incomplete or unknown

Optional transitive attributes may also be marked as partial.

Well-known attributes are attributes that all BGP implementations must recognize and are propagated to all BGP neighbors. Mandatory well-known attributes must appear in the route description. Discretionary attributes do not have to appear in the route description (they are not mandatory).

Optional attributes may not be supported by all BGP implementations. The transitive bit determines if an optional attribute is passed to BGP neighbors. If an optional attribute is transitive and not supported by a neighbor, it is passed and marked as partial. If it is non-transitive, a neighbor that does not implement the attribute deletes it.

The attributes that are commonly used in the Cisco Implementation of BGP are as follows:

WELL-KNOWN, MANDATORY

AS-path: A list of the Autonomous Systems (AS) numbers that a route passes through to reach the destination. As the update passes through an AS the AS number is inserted at the beginning of the list. The AS-path attribute has a reverse-order list of AS passed through to get to the destination.

Next-hop: The next-hop address that is used to reach the destination.

Origin: Indicates how BGP learned a particular route. There are three possible types — IGP (route is internal to the AS), EGP (learned via EBGP), or Incomplete (origin unknown or learned in a different way).

WELL-KNOWN, DISCRETIONARY

Local Preference: Defines the preferred exit point from the local AS for a specific route. Will choose the higher number.

Atomic Aggregate: Set if a router advertises an aggregate causes path attribute information to be lost.

OPTIONAL, TRANSITIVE

Aggregator: Specifies the router ID and AS of the router that originated an aggregate prefix. Used in conjunction with the atomic aggregate attribute.

Community: Used to group routes that share common properties so that policies can be applied at the group level.

OPTIONAL, NON-TRANSITIVE

Multi-exit-discriminator (MED): Indicates the preferred path into an AS to external neighbors when multiple paths exist.

A list of path attributes is contained in BGP update messages. The attribute is variable length and consists of three fields: Attribute type consisting of a 1-byte attribute flags field and a 1-byte attribute code field, Attribute length field that is 1 or 2 bytes, and a variable length attribute value field. The attribute type codes used by Cisco are: 1-origin, 2-AS-path, 3-Next-hop, 4-MED, 5-Local preference, 6-Atomic aggregate, 7-aggregator, 8-community, 9-originator-ID, and 10-cluster list.

This article discussed the BGP attributes with the intent of preparing you for those confusing questions on the BSCN or BSCI exams. Hopefully, there is no more confusion on the various attributes that are used in the Cisco implementation of BGP.

Local Preference Attribute
The local preference attribute is used to prefer an exit point from the local autonomous system (AS). Unlike the weight attribute, the local preference attribute is propagated throughout the local AS. If there are multiple exit points from the AS, the local preference attribute is used to select the exit point for a specific route. In Figure 39-3, 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.

Multi-Exit Discriminator Attribute
The multi-exit discriminator (MED) or metric attribute is used as a suggestion to an external AS regarding the preferred route into the AS that is advertising the metric.

The term suggestion is used because the external AS that is receiving the MEDs may be using other BGP attributes for route selection. We will cover the rules regarding route selection in the next section. In Figure 39-4, Router C is advertising the route 172.16.1.0 with a metric of 10, while Route D is advertising 172.16.1.0 with a metric of 5. The lower value of the metric is preferred, so AS 100 will select the route to router D for network 172.16.1.0 in AS 200. MEDs are advertised throughout the local AS.

Origin Attribute
The origin attribute indicates how BGP learned about a particular route. The origin attribute can have one of three possible values:

IGP—The route is interior to the originating AS. This value is set when the network router configuration command is used to inject the route into BGP.
EGP—The route is learned via the Exterior Border Gateway Protocol (EBGP).
Incomplete—The origin of the route is unknown or learned in some other way. An origin of incomplete occurs when a route is redistributed into BGP.

AS_path Attribute
When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed. Figure 39-5 shows the situation in which a route is passing through three autonomous systems.

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.

Next-Hop Attribute
The EBGP next-hop attribute is the IP address that is used to reach the advertising router. For EBGP peers, the next-hop address is the IP address of the connection between the peers. For IBGP, the EBGP next-hop address is carried into the local AS, as illustrated in

Community Attribute
The community attribute provides a way of grouping destinations, called communities, to which routing decisions (such as acceptance, preference, and redistribution) can be applied. Route maps are used to set the community attribute. Predefined community attributes are listed here:

No-export — do not advertise this route to EBGP peers.
No-advertise — do not advertise this route to any peer.
Internet —Advertise this route to the Internet community; all routers in the network belong to it.
Local AS – Only IBGP neighbors

BGP Path Selection
BGP could possibly receive multiple advertisements for the same route from multiple sources. BGP selects only one path as the best path. When the path is selected, BGP puts the selected path in the IP routing table and propagates the path to its neighbors. BGP uses the following criteria, in the order presented, to select a path for a destination:

If the path specifies a next hop that is inaccessible, drop the update.
Prefer the path with the largest weight. (local to router only)
If the weights are the same, prefer the path with the largest local preference.
If the local preferences are the same, prefer the path that was originated by BGP running on this router.
If no route was originated, prefer the route that has the shortest AS_path.
If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than incomplete).
If the origin codes are the same, prefer the path with the lowest MED attribute.
If the paths have the same MED, prefer the external path over the internal path.
If the paths are still the same, prefer the path through the closest IGP neighbor.
Prefer the path with the lowest IP address, as specified by the BGP router ID.

***************************
Learn to spell synchronization
To exchange AS information with ISP probably use EBGP.
Routes can be injected into BGP table by redist from existing igp, network command and redist from static null 0.
It is not recommended to redistribute dynamic IGP routes into BGP.
[No synchronization] allows BGP to advertise routes not in routing table
Prefix list ge means greater or =, le means less or =, [ip prefix-list 207.0.0.0/8 ge 10 le 18]
Route reflectors and aggregate-address gets around IBGP full mesh requirement.
Route reflectors modify split horizon and reduce TCP sessions
Doesn’t require a hierarchal addressing scheme.
Uses TCP port 179.
[show ip bgp paths] = show bgp topology table.

**IS-IS NOTES**

Routed Protocols
Connection oriented – CONP (the protocol) layer 3, x.25; CMNS (the service) Connection Mode Network Service

Connectionless – CLNP (protocol) layer 3 Connectionless network protocol; CLNS (service) Connectionless Network Service

Routing Protocols

ES-IS – Level 0 = Communication between end stations. Like ARP. MAC to IP.

IS-IS Originally for CLNP, later IP. Integrated IS-IS or Dual IS-IS. IGP. Link state, comparable to OSPF. L1, L2, L1,2. IP mode, CLNS or both DUAL mode. Supports VLSM. Can redistribute.

Level 0 - End system to Intermediate system. Host to host and host to IS ( router)
Level 1 – IS’s in same area (intra-area) Topology table (L1)
Level 2 – Like a ABR, in a single Domain L1 Table and
Layer 3 – Routing between domains.
CE5E-CE8-5091-300-1852 boson

IS-IS Terms
Domain - Single admin unit (Similar to OSPF AS)
Circuit – Same as interface. Circuit ID
PDU- protocol data units (same as IP packets) Link –State PDU (LSP)
SNP - Sequence number packet. Control packets for LSP’s (like DNS version #)
DIS – Designated IS. Pseudonode (Like OSPF DR) Sends LSP. (IS-IS has no backup (BDR) Uses Dykstra Algorithm.

Pay attention to (b) only.

NSAP Network Service Address Point
Broken into (interdomain) IDP and (domain) DSP

49.0001.0000.8635.68c4.00 = NET
49.0001 = area ID
0000.8695.6ac4 = System ID
00 = NSEL

Area ID -
System ID - (SySID) 6 byte endsystem or IS ID (mac address)
N-Selector – Similar to IP port or socket. (00 = NET)
High order DSP – similar to IP subnet

Does not summarize along classful boundaries

The following example illustrates the proper syntax for turning up the ISIS process on interface S1 and E0 of this particular router.
NewYork(config)#router isis
NewYork(config-router)#net 47.0001.1501.0001.0001.00
NewYork(config)#interface ethernet 0
NewYork(config-if)#ip router isis
NewYork(config-if)#exit
NewYork(config)#interface serial 1
NewYork(config-if)#ip router isis

Here is a look at a more complete configuration inclusive of the basic commands necessary for a functional ISIS implementation.

interface Ethernet0
ip address 150.100.1.2 255.255.255.0
ip router isis
!
interface Serial1
ip address 150.100.3.2 255.255.255.0
ip router isis
!
router isis
net 47.0001.1501.0001.0001.00

Ottawa#sh clns is-neighbors

System Id Interface State Type Priority Circuit Id Format
1501.0001.0001 Se0 Up L1 0 00 Phase V
0000.0200.0001 Et0 Up L1 64 1501.0003.0001.0 Phase V

ESH End system hello ES to IS
IIH IS-IS Hello IS to IS
PSNP = Partial SNP used for ack of lsp on p2p links and request missing portions of LS database
CSNP Complete SNP =used for Periodically by DIS on LAN to ensure reliability and P2P link when link comes up
[Show isis routes] [show clns routes] to show clns destinations the router knows
NSAP has two major fields IDP initial domain part and DSP domain specific part.

L1 L2 Packet Types

LSP - Link state packet. Broadcast or P 2 P or P 2 MP contain header
Hello PDU – ESH End System Hello to IS, ISH IS Hello to host, IIH IS to IS
CNSP – (Version) Complete number Sequence PDU (whole database)
PSNP – Partial SEQ number PDU (updates)

Metric
Cost – 6 bit, 1-63, default 10 on outbound Interfaces.
Hello default 10 seconds. Hold timer 20 seconds.

SNP make sure LSA are sent reliable.
PSNP – Ack’s Single LSP description

2 databases
Link State – Like OSPF All routers will have same DB
Forwarding – SPF algorithm. (routing table) best routes.
Hello = 10 seconds
Hold Timer = 30 Seconds

**EIGRP NOTES**

EIGRP uses DUAL algorithm.
Rip uses Bellman-Ford
Max hop count 100
Summarize an eigrp route in AS# 3
[ip summary-address eigrp 3 192.168.0.0 255.25.0.0]

[Distribute-list 20 out s 0/0] = suppress sending updates
EIGRP and IGRP support load balancing across unequal cost path metric with variance command
Alter AD distance = [Distance eigrp internal-distance external-distance].
When manual summarization is configured the summary address is entered into the route table and shown to be sourced from null 0, it is done on a per interface basis.
In order for igrp and eigrp to automatically redistribute they must have the same AS number

**MISC NOTES**

Route Maps

Introduced in IOS 11.0
Complex Access List
Use standard or extended access list
#route-map ‘map-tag’ permit | deny
#match condition command
#set condition command
implicit deny at end
[ip policy route-map chicago] = Route map on an interface

Redistribute EIGRP AS# 5 into OSPF with metric of 1
redistribute eigrp 5 ospf metric-type 1
NAT
Inside Local
Configured IP address assigned to a host on the inside network
Inside Global
The IP address of an inside host as it appears to the outside network, “Translated IP Address”.
Outside Local
The IP address of an outside host as it appears to the inside network. These addresses can be allocated from the RFC 1918 space if desired.
Outside Global
The configured IP address assigned to a host in the outside network.
Simple Translation Entry
A translation entry which maps one IP address to another.
Extended Translation Entry
A translation entry which maps one IP address and port pair to another.

224.0.0.1 All hosts on a subnet
224.0.0.2 All routers on a subnet
224.0.0.5 All OSPF routers (DR Others)
224.0.0.6 All OSPF Designated Routers
224.0.0.9 All RIPv2 routers
224.0.0.10 All EIGRP routers

IPV6
Unicast: A packet sent to a unicast address is delivered to the interface identified by that address.
Multicast: The packet will be processed by all members of the multicast group.
Anycast (new to IPv6): The packet will be delivered to only one of the interfaces configured for the anycast address — usually to the nearest one.
The notation of IPv6 is simplified with the following rules:
Rule one: Leading zeroes can be skipped.

For example, this line: FE80:0000:0000:0000:0202:B3FF:FE1E:8329

Can be written as: FE80:0:0:0:202:B3FF:FE1E:8329

Rule two: A consecutive number of zeroes within an address can be replaced by a double colon - but only once within an address.

For example: FE80:0:0:0:202:B3FF:FE1E:8329

becomes: FE80::202:B3FF:FE1E:8329
Ipv6 has larger address space, mobility and security and simpler header
Address Purpose
IPV6 128 bit long, header contains next header fields, sends RA messages

SHOW COMMANDS

[show ip ospf 76] show all areas in process 76
[show ip interface] show if access-lists is on an interface
[show ip ospf interface] to tell DR/BDR/DROTHER
[show ip ospf virtual-links] will show Virtual links
[show ip ospf neighbor] not plural!
[show ip ospf neighbor detail[ detailed OSPF info
[show ip bgp paths] = show bgp topology table.
[show clns routes] to show clns destinations the router knows
[show clns neighbors] show isis neighbors
[show ip eigrp topology all-links] show entire topology database.

Summary Commands
EIGRP
[ip summary-address eigrp 3 192.168.1.0 255.255.255.0]
OSPF
[area 1 range 192.168.8.0 255.255.248.0] to summarize networks
[summary-address 192.231.0.0 255.255.0.0] summarize external routes

Policy based routing can match by next hop and match by IP address.
A domain is a collection of connected areas.
Hello packets are Multicast.
[ip forward-protocol 161] forwards SNMP traffic
ip helper address forwards DNS 53, TFTP 69 and 49 TACACS by default.

Router redist features to ensure optimal; paths = Admin dist, Route maps, Distribute-list, passive interface

Show ip policy displays which route map is associated with an interface.
Route filters use distribute lists
Area 6 stub no-summary = totally stubby area
Static routes must be redistributed into a dynamic routing protocol
Preferred route distribution Use one way dist when there are multiple paths and static routes with multiple paths.

IPV6 uses Unicast, Multicast and Anycast
Policy based routing can match by next hop and match by IP address.
A domain is a collection of connected areas.
Hello packets are Multicast.

Router redist features to ensure optimal; paths = Admin dist, Route maps, Distribute-list, passive interface

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