CCIE L3 Ep.6 r0uting-c0nc3pts.gz

IP ROUTING PROCESS OVERVIEW — Dot the I’s
So, I like to separate “The routing process” in two or three ways:

• Routing,
  I refer to the routing process when two devices or more speaks/exchange routes or commom information (for RIP), prefix (NRLI for BGP) and SLA (for OSPF). And, I understood that routing isn’t the process to send “user” traffic between network. Keyword here is Control plane and/or Managmet plane. well, find the outgoing interface for the packet.
• Packet-Switching, and;
  Yes, I know that packet switching is the process to send data between network. That’s it, packet ingrees on router interface, remove and make a new L2 header and send to the next hop on engress interface. Normally using data plane. Well, move the packet between interfaces.
• Encapsulation
  Rebuild the packet’s layer 2 header

Are you think about? Do you agree or not? Share your point of view with me :).

Packet switching methods

Great article about Cisco process switch, Fast Switching and CEF:

Packet Switching Methods on Cisco Networks

THE ROUTING PROCESS

FIRST STEP — THE ROUTING PROCESS
“What does the router do when a packet enters?”

Find the longest match to the destination

+ show ip route 1.2.3.4
+1.0.0.0/8
+1.2.0.0/16
+1.2.3.0/24
+1.2.3.0/25 ← choosen

If necessary, recurse to the outgoing interface
+ 1.2.3.4 via 5.6.7.8
+ 5.6.7.8 via 9.0.1.2
+ 9.0.1.2 via 3.4.5.6
+ 3.4.5.6 directly connected, GigabitEthernet1

A route cannot be installed in the routing table unless the recursive looup is sucessful.

METRIC VS. DISTANCE
And what if there are multiple longest prefix matches?

Same protocol:
Choose lowest metric ( Rules vary per protocol)
Different protocols
Choose lowest administrative Distance

SECOND STEP — THE ROUTING PROCESS

• Move the packet between interface. This is called the “switching path” (Process, Fast, CEF, etc.);
• Where load balancing occurs (per flow is better than per packet)

show ip interface
show ip cef [address][detail]
show ip cef exact-route [source][destination]

THIRD STEP — THE ROUTING PROCESS

• Build the layer 2 header based on egress interface type (E.g Ethernet VS. PPP);
• For multipoint interfaces, L3 to L2 resoluition is required (ethernet, Frame relay multipoint, ATM Multipoint and mGRE (DMVPN) but links point-to-point not;

ENCAPSULATION
ETHERNET

• On Ethernet, next-hop IP to next-hop MAC. Address resolution Protocol (ARP)

show ip arp
debug arp

mGRE (DMVPN)

• Resolve the tunnel IP to the NMBA IP. Next-Hop Resolution Protocol (NHRP)

show ip nhrp
!If no match, ask the next-hop server (NHS)
debug nhrp
debug dmvpn

Realiable Static Routing

Only criteria for installing a static route is the route can find the outgoing interfaace. next hop may be reachable or may be down.

Reliable static route combine static routing with IP SLA feature and track objects. If tracked object is down, static route is removed from the routing table.

Floating static route

This is or these are static route with hisghesrt Ad than principal route. floating route are backup routes.

Default static route is 1

R4(config)#ip route 150.1.5.5 255.255.255.255 150.0.45.5 
R4(config)#ip route 150.1.5.5 255.255.255.255 150.0.0.5 10(backup AD 10)

INE CCIE Enterprise Infrastructure

Configuring IP SLA , Track and float route

! Configuring TRACK and IP SLA
R4(config)#ip sla 4
R4(config-ip-sla)#icmp-echo 155.1.45.5 source-ip 155.1.45.4
R4(config-ip-sla-echo)#timeout 2000
R4(config-ip-sla-echo)#threshold 2000
R4(config)#ip sla schedule 4 life forever start-time now
R4(config)#track 4 ip sla 4 reachability
!Configuring route float and applie track on route
R4(config)#ip route 150.1.5.5 255.255.255.255 150.0.45.5 track 4
R4(config)#ip route 150.1.5.5 255.255.255.255 150.0.0.5 name dmvpn 2
Show commands
R4#show ip sla configuration
R4#show ip sla statistics

For more information

IP SLAs Configuration Guide, Cisco IOS Release 15M&T

POLICY BASED ROUTING

• Normal IP routing is destination based. so, find de longest macth to the destination and route the packet towards the next-hop.
• PBR allows fowarding decision to be based on source, destination, Incoming interface and protocol (FTP, HTTP, etc.)

PBR can appy to two types of traffic:

Incoming traffic
PBR applied at the link leval
ip policy route-map [route-map name]

Locally originated
PBR applied glabally
ip local policy royte-map [route-map name]

PBR Policy configuration

Lab reference

!Route R6 default route
R6(config)# ip route 0.0.0.0 0.0.0.0 155.1.146.1
!Traffic criteria
R1(config)#ip access_list extended R6_Loopback
R1(config-ext-nacl)#permit ip any host 150.1.6.6
!Policy sets and matches
R1(config-ext-nacl)#route-map POLICY-CHANGE-R6DEFAULT
R1(config-route-map)#match ip address R6_Loopback
R1(config-route-map)#set ip next-hop 155.1.146.4
!Policy applied
R1(config-route-map)#interface g0/1.146
R1(config-if)#ip policy route-map POLICY-CHANGE-R6DEFAULT

Notes: deny in extended ACL used in route-map is equal “route normally RIB based”.

Here in medium there are two great articles about these topics from Giulianos Barros’ profile

My journey to CCIE RS — IP Routing 1

My journey to CCIE RS — IP Routing 1

My journey to CCIE RS — IP Routing 2

My journey to CCIE RS — IP Routing 2

in this weeek I learned about :

1.2 Routing Concepts

• 1.2.a Administrative distance
• 1.2.b VRF-lite
• 1.2.c Static routing
• 1.2.d Policy Based Routing

4.6 Network optimization

• 4.6.a IP SLA
• 4.6.a i ICMP probes
• 4.6.a ii UDP probes
• 4.6.a iii TCP probes
• 4.6.b Tracking object

What’s next? IPv4 header and PATH MTU

CCIE L3 Pck3t.v4

About me:

Rafael Alves — Network Analyst — Solor | LinkedIn